Fred, Just want to thank you for all your great input.Tested the Celestron c130 today @ 400yrds. Used the eye piece that came with unit 32mm,TeleVue click zoom 24-8 and TeleVue 5mm.Could not see anything with the 5mm eye piece probably do to lack of experience.The 24-8 worked the best in the morning on the lowest setting of 24 turning power up adversely affected the clarity and made it harder to see the holes.It seemed to me that the lowest power lens worked the best and gave a very clear picture @ 400yd.Is there a better lens than the Celestron 32mm that I should try? I will test again Monday then decide which lenses to send back.I may test a Swarsky along side the C130.I was very pleased with the C130.Not quite sure I know what I am doing on lens selection.Please advise.Thanks again Pat Cent. Calif.
BEST spotting scope?
- Thread starter patgblue
- Start date
Pat, Fred, & others, used my 130MAK at a 500 yard max shoot today and was very happy with a 40MM Japanese made Meade that I got on EBay for $60. The gent with the Meade 125 telescope was there and also liked my eyepiece the best out of his collection. At this point it seems to me that for 400-600 yards the 40MM is best under most conditions, while a good 32MM would be best on a very good day. The 40MM,50x) was able to spot 22 cal holes at 500 while a $3300 dual 36X Kowa didn’t have the magnification. Not sure if there’s a decent 1¼†45MM or 50MM,40X) made for use in bad conditions. If so I’d think for seeing holes out to 600 a collection of 3 eyepieces,32MM, 40MM, & 50MM???) would be ideal for this spotter. But I don’t think it’ll ever be able to see with the common spotters that run $1200+. Not that it’s too far off at $300 though.
Optics Planet shows the TeleVue 8-24 click zoom as a discontinued product. Wonder if/why they stopped making them?
http://www.opticsplanet.net/televue-click-stop-zoom-8-to-24-eyepieces.html
For the third different shoot I was able to look through a new Nikon ED spotter and some other quality spotters and am just about certain that if I hit the lottery, that’s the baby I’m getting. Used to like the Leicas best. But that’s now changed.
Bill
Optics Planet shows the TeleVue 8-24 click zoom as a discontinued product. Wonder if/why they stopped making them?
http://www.opticsplanet.net/televue-click-stop-zoom-8-to-24-eyepieces.html
For the third different shoot I was able to look through a new Nikon ED spotter and some other quality spotters and am just about certain that if I hit the lottery, that’s the baby I’m getting. Used to like the Leicas best. But that’s now changed.
Bill
Fred Bohl
Gold $$ Contributor
Pat
Sorry to read of your problems at the higher powers. From your descriptions there are two very likely causes:
1. Exit pupil size: the supplied eyepiece at 32mm,63x) will have about 2mm exit pupil; the 24-8mm,83-250x)zoom will have about 1.6-0.5mm exit pupil; and the 5mm,400x) will have about 0.3mm exit pupil. For comparison a Leupold Competition 45x - 45 mm will have about 1mm exit pupil. At or below 1mm exit pupil size it takes concentration and practice to keep your eye aligned with the scope.
2. Telescope eyepieces have very short eye relief and critical rear focus depth compared to telescopic sights. You must therefore also maintain very close,20mm) and critical,+/- 1mm) eye position. The suggested TeleVue eyepieces both have their click-stop, Instadjust eye guard which permits fast, accurate positioning of the eye to the exit-pupil distance with maximum suppression of environmental stray light. Instadjust is topped off by a soft,for comfort and eyeglass protection) fold-down rubber eye guard and protective cap.
Note that at these small exit pupil sizes, using these eyepieces without glasses is usually easier. If you have astigmatism they accept the TeleVue Dioptrx astigmatism corrector which I use instead of my glasses,works great).
You didn't mention trouble with scope aiming at high power but if that is also a problem you may wish to put an slow motion pan and tilt adapter between the scope and scope stand. The fine pitch treads of the adjustments really help. Currently offered units include the Vixen - Piggyback 1/4"-20 DX Fine Adjustment Unit [VX-3562],$90 at OPT) and Orion Precision Slow-Motion Adapter [07033],$35 at Orion). I use the older version of the Vixen but have not used either of the above although they appear very similar.
I probably should have warned you of the issues associated with the use of high magnifications needed for effective target viewing and analysis at long range. However there is no easy way, long range optics with human eyes is not easy.
If you can provide more details of problems you are experiencing, I will try to provide more help. Unless you or Paul objects, I would prefer to do that in this thread so that the other interested readers might also benefit and/or contribute
Sorry to read of your problems at the higher powers. From your descriptions there are two very likely causes:
1. Exit pupil size: the supplied eyepiece at 32mm,63x) will have about 2mm exit pupil; the 24-8mm,83-250x)zoom will have about 1.6-0.5mm exit pupil; and the 5mm,400x) will have about 0.3mm exit pupil. For comparison a Leupold Competition 45x - 45 mm will have about 1mm exit pupil. At or below 1mm exit pupil size it takes concentration and practice to keep your eye aligned with the scope.
2. Telescope eyepieces have very short eye relief and critical rear focus depth compared to telescopic sights. You must therefore also maintain very close,20mm) and critical,+/- 1mm) eye position. The suggested TeleVue eyepieces both have their click-stop, Instadjust eye guard which permits fast, accurate positioning of the eye to the exit-pupil distance with maximum suppression of environmental stray light. Instadjust is topped off by a soft,for comfort and eyeglass protection) fold-down rubber eye guard and protective cap.
Note that at these small exit pupil sizes, using these eyepieces without glasses is usually easier. If you have astigmatism they accept the TeleVue Dioptrx astigmatism corrector which I use instead of my glasses,works great).
You didn't mention trouble with scope aiming at high power but if that is also a problem you may wish to put an slow motion pan and tilt adapter between the scope and scope stand. The fine pitch treads of the adjustments really help. Currently offered units include the Vixen - Piggyback 1/4"-20 DX Fine Adjustment Unit [VX-3562],$90 at OPT) and Orion Precision Slow-Motion Adapter [07033],$35 at Orion). I use the older version of the Vixen but have not used either of the above although they appear very similar.
I probably should have warned you of the issues associated with the use of high magnifications needed for effective target viewing and analysis at long range. However there is no easy way, long range optics with human eyes is not easy.
If you can provide more details of problems you are experiencing, I will try to provide more help. Unless you or Paul objects, I would prefer to do that in this thread so that the other interested readers might also benefit and/or contribute
Forum Boss
Administrator
Bill,
Very nice report that does give us some solid comparison info. I've been thinking that about 65x would be good for 600 yards, but then my range,in the mountains) usually has very good seeing conditions,low humidity, low pollution).
I think we are establishing that these low-end Mirror scopes are a viable option for the price, though, as you say, they won't run with a Zeiss 85,$1500 just for body).
Interesting note on the new Nikon ED. For those that say that the ED/Flourite doesn't matter, well all I can say is your eyes can see the difference--but that might be because of better polishing, and overall better quality glass in the ED/Flourite models, not just the lower chromatic dispersion.
Re the Leica. I've never been particularly impressed with the Televue at the price. Every Zeiss I've looked through was better than every Televid. I do like the Leica's focusing system though, with course and fine focus in line on the same axis:
Both the Televid 77 and Zeis 85 are up to around $1500 now for body only !!
Will be testing Kowa 821-82mm with 27 LER this week against a Celestron C130 MAK with 24mm and 19mm TeleVue lenses.Found that the quality of single power lens seems to be much better than the 8-24mm click vue veritable lens.That is my personal opinion.Kowa with lens 795.00.Celestron 550.00 with lens.Have not found anything better than the Celestron downfall BIG 130mm lens with 2000mm focal length.Will post results.Anyone used the TeleVue 85mm said to be the ultimate.Anyone used the TeleVue 60 degree ever Britte adapter instead of the 45 degree adapter?Will post results.Fred your input on TeleVUE 88mm or anything is greatly appreciated. No spell check?
Fred Bohl
Gold $$ Contributor
Pat
Have you checked the two links from my 6/20/07 post,repeated here)?
I invite you to review and of course comment on my related paper at http://www.twincityrodandgun.com/PDF%20files/ScopeRes-Mag1.pdf and the calculator at http://www.twincityrodandgun.com/ScopeResCalc/ScopeResolution1.htm
I think I may have failed to adequately communicate the importance of the concept of Matching Magnification. Match Magnification means the Dawes Limit equals the magnified normal visual acuity of the human eye. At less magnification the resolution is limited by the user's eye. Above this magnification the resolution is diffraction limited by the objective diameter of the telescope. Put another way, below the Match Magnification the telescope and eyepiece are delivering images at higher resolution content than your eye can resolve so increasing magnifications increase perceived resolution. Above the Match Magnification increasing the magnification further does not increase perceived resolution. The image will be bigger and occupy a larger percentage of the field of view,will be easier to analyze for pattern) but edges of features will become more blurred and no more detail will be added as magnification increases. This leads to the perception that image quality deteriorates as the magnification increases above the Match Magnification.
As you evaluate other eyepieces, they should be compared for image quality below the Match Magnification. Above the Match Magnification the image quality is controlled by the objective size. Above the Match Magnification, suitability of the eyepiece and the magnification it yields should be based on how well you can evaluate your target pattern and not on the image clarity. The differences between eyepieces that yield the same magnification will be much less apparent when used at magnifications above the Match Magnification.
Note that the Match Magnification for your Kowa 821 is 42.4x versus the Celestron C130 at 67.2x for your next comparisons.
Have you checked the two links from my 6/20/07 post,repeated here)?
I invite you to review and of course comment on my related paper at http://www.twincityrodandgun.com/PDF%20files/ScopeRes-Mag1.pdf and the calculator at http://www.twincityrodandgun.com/ScopeResCalc/ScopeResolution1.htm
I think I may have failed to adequately communicate the importance of the concept of Matching Magnification. Match Magnification means the Dawes Limit equals the magnified normal visual acuity of the human eye. At less magnification the resolution is limited by the user's eye. Above this magnification the resolution is diffraction limited by the objective diameter of the telescope. Put another way, below the Match Magnification the telescope and eyepiece are delivering images at higher resolution content than your eye can resolve so increasing magnifications increase perceived resolution. Above the Match Magnification increasing the magnification further does not increase perceived resolution. The image will be bigger and occupy a larger percentage of the field of view,will be easier to analyze for pattern) but edges of features will become more blurred and no more detail will be added as magnification increases. This leads to the perception that image quality deteriorates as the magnification increases above the Match Magnification.
As you evaluate other eyepieces, they should be compared for image quality below the Match Magnification. Above the Match Magnification the image quality is controlled by the objective size. Above the Match Magnification, suitability of the eyepiece and the magnification it yields should be based on how well you can evaluate your target pattern and not on the image clarity. The differences between eyepieces that yield the same magnification will be much less apparent when used at magnifications above the Match Magnification.
Note that the Match Magnification for your Kowa 821 is 42.4x versus the Celestron C130 at 67.2x for your next comparisons.
Fred Bohl
Gold $$ Contributor
Pat
In your 6/25/07 post you said "Will be testing Kowa 821-82mm with 27 LER this week against a Celestron C130 MAK with 24mm and 19mm TeleVue lenses." The 27x LER on the Kowa will be below its 42.4 Match Magnification so the image resolution will be dependent on your visual resolution/acuity. Where as the TeleVue 24mm at 83.3x and 19mm at 105x on the Celestron C130 will be above its 67.2x Match Magnification where the image resolution will be diffraction limited by the objective size.
Therefore the Kowa at 27x should deliver a clearer and sharper image than the C130 at either 83x or 100x. However, the important features within the images will be 3.1 times larger at 83x and 3.9 times larger at 105x than at 27x and therefore should be easier to analyze even though not as clear or sharp.
As to you assessment that fixed eyepieces deliver higher quality images than variables, this is very noticeable when compared below the Match Magnification but less noticeable when compared above the Match Magnification where image resolution is diffraction limited by the objective size.
In your 6/25/07 post you said "Will be testing Kowa 821-82mm with 27 LER this week against a Celestron C130 MAK with 24mm and 19mm TeleVue lenses." The 27x LER on the Kowa will be below its 42.4 Match Magnification so the image resolution will be dependent on your visual resolution/acuity. Where as the TeleVue 24mm at 83.3x and 19mm at 105x on the Celestron C130 will be above its 67.2x Match Magnification where the image resolution will be diffraction limited by the objective size.
Therefore the Kowa at 27x should deliver a clearer and sharper image than the C130 at either 83x or 100x. However, the important features within the images will be 3.1 times larger at 83x and 3.9 times larger at 105x than at 27x and therefore should be easier to analyze even though not as clear or sharp.
As to you assessment that fixed eyepieces deliver higher quality images than variables, this is very noticeable when compared below the Match Magnification but less noticeable when compared above the Match Magnification where image resolution is diffraction limited by the objective size.
HI Fred, Have not tested Kowa and Celestron will test Thursday.Now have the televue click stop 8-24mm lens,19mm lens,24mm fixed lens and Celestron 32mm lens to test.Also now have the Kowa with 27mm LER to test.Any suggestions? Really appreciate your input.Thanks Pat PS.still not shure on what above or below Match Magnification means.Not really understanding Match Magnification.
Fred Bohl
Gold $$ Contributor
Pat
The normal or corrected to normal,20/20 feet in USA 6/6 meter elsewhere) human visual resolution/acuity,smallest resolvable feature) is 1 MOA. We use telescopes to improve that,reduce the angle) by magnifying the features.
Magnified Eye Resolution,MOA) = 1MOA/Magnification,x)
However, the telescope used to magnify the image has its own resolution limit do to diffraction cause by the finite diameter of the objective lens.
Dawes Resolution Limit,MOA) = 1.933/Objective Diameter,mm)
The Match Magnification is the magnification at which the two limits are equal.
Match Magnification,x) = 1MOA/Dawes Resolution Limit,MOA)
For your Kowa 821 with its 82mm objective its resolution limit is 0.024MOA above a 42.4x Match Magnification. Below 42.4x resolution is proportional to magnification. Progressively higher magnification eyepieces will produce better perceived resolution up to a maximum of 42.4x and 0.024MOA.
For your Celestron C130 with its 130mm objective its resolution limit is 0.015MOA above a 67.2x Match Magnification. Below 67.2x resolution is proportional to magnification. Progressively higher magnification eyepieces will produce better perceived resolution up to a maximum of 67.2x and 0.015MOA.
Above the Match Magnification increasing the magnification further does not increase perceived resolution. The image will be bigger and occupy a larger percentage of the field of view,will be easier to analyze for pattern) but edges of features will become more blurred and no more detail will be added as magnification increases.
The advantage the C130 offers is to let you use higher magnifications for easier image analysis without feature blur or with less feature blur at even higher magnifications.
The normal or corrected to normal,20/20 feet in USA 6/6 meter elsewhere) human visual resolution/acuity,smallest resolvable feature) is 1 MOA. We use telescopes to improve that,reduce the angle) by magnifying the features.
Magnified Eye Resolution,MOA) = 1MOA/Magnification,x)
However, the telescope used to magnify the image has its own resolution limit do to diffraction cause by the finite diameter of the objective lens.
Dawes Resolution Limit,MOA) = 1.933/Objective Diameter,mm)
The Match Magnification is the magnification at which the two limits are equal.
Match Magnification,x) = 1MOA/Dawes Resolution Limit,MOA)
For your Kowa 821 with its 82mm objective its resolution limit is 0.024MOA above a 42.4x Match Magnification. Below 42.4x resolution is proportional to magnification. Progressively higher magnification eyepieces will produce better perceived resolution up to a maximum of 42.4x and 0.024MOA.
For your Celestron C130 with its 130mm objective its resolution limit is 0.015MOA above a 67.2x Match Magnification. Below 67.2x resolution is proportional to magnification. Progressively higher magnification eyepieces will produce better perceived resolution up to a maximum of 67.2x and 0.015MOA.
Above the Match Magnification increasing the magnification further does not increase perceived resolution. The image will be bigger and occupy a larger percentage of the field of view,will be easier to analyze for pattern) but edges of features will become more blurred and no more detail will be added as magnification increases.
The advantage the C130 offers is to let you use higher magnifications for easier image analysis without feature blur or with less feature blur at even higher magnifications.
Fred Bohl
Gold $$ Contributor
milanuk
Mirage particularly when combined with wind always adds to the difficulty of target image analysis. Generally speaking, increasing magnification will make the apparent target motion seem worse so we tend to reduce magnification to make things look mare stable. This of course does not reduce the mirage but does reduce our ability to interpret its effects.
To a point the higher magnification & resolution together with the narrower field of view provided by the large objective and long focal length of the C130MAK will enhance our ability to interpret the mirage effects. However their is a practical limit to that benefit when the mirage reaches a level that the confusion it causes exceeds our ability to analyze the effect. That point is dependent on the visual capabilities and practice level of the observer not the telescope but should be at a higher magnification for the C130MAK than the others.
For the purpose of analyzing the target with a spotting scope but not for aiming a shot, I find 250x usable for most mirage conditions and even 400x usable with low mirage and wind. Others under the same conditions found 200x and 300x to be their own limits of usability with the same equipment.
I have no experience in relative results for watching trace.
Mirage particularly when combined with wind always adds to the difficulty of target image analysis. Generally speaking, increasing magnification will make the apparent target motion seem worse so we tend to reduce magnification to make things look mare stable. This of course does not reduce the mirage but does reduce our ability to interpret its effects.
To a point the higher magnification & resolution together with the narrower field of view provided by the large objective and long focal length of the C130MAK will enhance our ability to interpret the mirage effects. However their is a practical limit to that benefit when the mirage reaches a level that the confusion it causes exceeds our ability to analyze the effect. That point is dependent on the visual capabilities and practice level of the observer not the telescope but should be at a higher magnification for the C130MAK than the others.
For the purpose of analyzing the target with a spotting scope but not for aiming a shot, I find 250x usable for most mirage conditions and even 400x usable with low mirage and wind. Others under the same conditions found 200x and 300x to be their own limits of usability with the same equipment.
I have no experience in relative results for watching trace.
memilanuk
Gold $$ Contributor
Fred,
What I was referring to w/ regards to the mirage... for HP it is not uncommon to intentionally pull the focus back 1/2-2/3 of the way to the target, and watch the mirage across the middle of the range, rather than at the target. A scope like my old fixed 22x Champion's Choice,hah!) CC45 hardly shows anything. My Kowa TSN-661,no fluoride coatings) shows mirage OK. A Leica Televid 77 APO,fluoride coatings) w/ a 32x eyepiece... shows a veritable 'river' of wind running across the range, making the subtle shifts in the wind jump out at you, and making it easier to dope the wind for the next shot.
That is what I'm asking about w/ the C90 and C130 scopes and mirage. I accept that with the mirage 'running' picking out bullet holes is going to be more difficult... if the scope can 'see' the mirage and the changes in the wind, that helps a lot though.
Monte
What I was referring to w/ regards to the mirage... for HP it is not uncommon to intentionally pull the focus back 1/2-2/3 of the way to the target, and watch the mirage across the middle of the range, rather than at the target. A scope like my old fixed 22x Champion's Choice,hah!) CC45 hardly shows anything. My Kowa TSN-661,no fluoride coatings) shows mirage OK. A Leica Televid 77 APO,fluoride coatings) w/ a 32x eyepiece... shows a veritable 'river' of wind running across the range, making the subtle shifts in the wind jump out at you, and making it easier to dope the wind for the next shot.
That is what I'm asking about w/ the C90 and C130 scopes and mirage. I accept that with the mirage 'running' picking out bullet holes is going to be more difficult... if the scope can 'see' the mirage and the changes in the wind, that helps a lot though.
Monte
Fred Bohl
Gold $$ Contributor
Monte
Because the C90MAK and more so the C130MAK offer greater real resolution and narrower fields of view at higher powers than refractors,conventional spotting scopes), they do provide more detailed view of the mirage and wind that is there.
However, the focus offset method you use could be a problem. They are normally focused by moving the large main mirror and precise focusing is very touchy,in and out of focus with very little mirror/knob movement). Your Leica Televid 77 has that very smooth two stage,coarse & fine) focusing motion of the erector cell. I'm used to touchy focusing at high magnification but I'll admit that it is an acquired skill,easier on those Celestrons than Meade/Weaver MAKS).
Because the C90MAK and more so the C130MAK offer greater real resolution and narrower fields of view at higher powers than refractors,conventional spotting scopes), they do provide more detailed view of the mirage and wind that is there.
However, the focus offset method you use could be a problem. They are normally focused by moving the large main mirror and precise focusing is very touchy,in and out of focus with very little mirror/knob movement). Your Leica Televid 77 has that very smooth two stage,coarse & fine) focusing motion of the erector cell. I'm used to touchy focusing at high magnification but I'll admit that it is an acquired skill,easier on those Celestrons than Meade/Weaver MAKS).
Fred,
Would you be so kind as to address depth of field in this discussion also? For example, how the depth of field would compare between the various scopes, what increases/decreases it, etc. I had forgotten how important it can be until Monte brought up the issue of reading mirage.
BTW, I really appreciate the amount of effort that you have put into this thread. I've learned a lot reading through it as it has developed.
robert
Would you be so kind as to address depth of field in this discussion also? For example, how the depth of field would compare between the various scopes, what increases/decreases it, etc. I had forgotten how important it can be until Monte brought up the issue of reading mirage.
BTW, I really appreciate the amount of effort that you have put into this thread. I've learned a lot reading through it as it has developed.
robert
Armature testing report on Kowa 821 and Celestron C130MAK using televue click stop 8-24mm lens,19mm lens,24mm fixed lens and Celestron 32mm lens for test.Also testing the Kowa with 27mm LER.All testing at 425yd.Conditions warm 90 light wind,clear.The Kowa was very clear but no bullet holes were seethe power of the 27x was to low to distinguish holes clearly.The LER was a great feature good field of view and very easy target acquisition.At 425yd.the Kowa didn't stand up to the Celestron 130MAK, not even close.The 19mm-105x showed to much mirage and degradation of target for my personal liking.The 24-8 was fairly good on 24mm-83.3x but not as clear as I would like.The higher settings ended up having to much target degradation.The best clarity and bullet hole picture was with the 30mm-63x Celestron lens that came with the unit.Spot on Crystal clear picture.Touching 6mm holes were distinguishable.A very important note here on clarity is that the lens was used with out the 45 degree prism.Lens was used in the top of the Celestron 90 degree with the mirror button switched.Using the 45 adapter didn't give nearly as clear of an image.I'm sure Fred could explain why and probably has the solution to the poorer resolution using the 45.Perhaps TeleVue EverBrite 60degree prism would solve the problem.I'm sold on the C130MAK and with Freds expertise and patients and my further testing we will arrive at the ultimate lens or lenses for up to 600yd.The Celestron C130MAK at 300.00 is Great.Any new information I will report. Pat
Fred Bohl
Gold $$ Contributor
Robert Streich
First let me say your welcome. I do hope many have learned some practical optics from this thread. A very wise man once told me me that the best way to understand something was to explain it to someone else. I think I may be the one doing the most learning.
By depth of field I assume that you mean it in the photographic sense. The distance from the first point of acceptable focus through the point of sharp focus and beyond to the last point of acceptable focus.
In that sense telescope designs the have a long focal length relative their objective diameter,high f/#) have a longer depth of field than those that have lower ratios of focal length to objective diameter,lower f/#). Unlike a typical camera lens system where the f/# is controlled by the aperture opening of an adjustable diaphragm, telescopes of a particular f/# can change depth of field by changing eyepieces.
Eyepieces have an internal aperture limit inherent in the basic design and so have their own f/#. By using eyepieces of wider apparent field of view design you can reduce the telescope system depth of field. This will in turn increase the field of view and reduce the effective magnification.
To summarize, longer focal lengths and higher magnifications increase depth of field. And the field of view is decreased.
Before trying to make use of these relationships keep in mind that for long range target observation, the spotting scope as well as the target rifle scope will probably be used at magnifications high enough that the system will be diffraction limited so that even at so called sharp focus the image will be slightly blurred and therefore the ideas of acceptable focus and depth of focus become somewhat blurred themselves.
First let me say your welcome. I do hope many have learned some practical optics from this thread. A very wise man once told me me that the best way to understand something was to explain it to someone else. I think I may be the one doing the most learning.
By depth of field I assume that you mean it in the photographic sense. The distance from the first point of acceptable focus through the point of sharp focus and beyond to the last point of acceptable focus.
In that sense telescope designs the have a long focal length relative their objective diameter,high f/#) have a longer depth of field than those that have lower ratios of focal length to objective diameter,lower f/#). Unlike a typical camera lens system where the f/# is controlled by the aperture opening of an adjustable diaphragm, telescopes of a particular f/# can change depth of field by changing eyepieces.
Eyepieces have an internal aperture limit inherent in the basic design and so have their own f/#. By using eyepieces of wider apparent field of view design you can reduce the telescope system depth of field. This will in turn increase the field of view and reduce the effective magnification.
To summarize, longer focal lengths and higher magnifications increase depth of field. And the field of view is decreased.
Before trying to make use of these relationships keep in mind that for long range target observation, the spotting scope as well as the target rifle scope will probably be used at magnifications high enough that the system will be diffraction limited so that even at so called sharp focus the image will be slightly blurred and therefore the ideas of acceptable focus and depth of focus become somewhat blurred themselves.
Fred Bohl
Gold $$ Contributor
Pat
Sorry for the delay in responding. I have been busy on some testing of my own for the last two days. I have enlisted the assistance of a fellow shooter and armature astronomer in a project to deal with the mirage and air quality problems. We will be reporting on that with an article or at least another thread at a later date,as a teaser - very good initial results). I also wanted to put together a hopefully helpful response that will help all parties. Here goes.
The Maksutov-Cassegrain spotting scopes we have been discussing - Celestron C90MAK & C130MAK also the Meade/Weaver ETX90 & ETX125 - are of a very similar design and share common image formation and delivery scheme. The optical tube assembly forms an image using a double spherical surface corrector plate and two spherical mirrors,no refracting lenses) and delivers an image that is inverted,up is down) and reversed,left is right).
At the rear is a flip-mirror,first surface mirror) that when down,horizontal) allows the image to pass through to the rear port,still inverted and reversed), but when up,45º) reflects the path out the top port and inverts the image,makes it right side up) but leaves it reversed. Eyepieces are designed not to invert or revert the image. Therefore with the eyepiece in the top port and flip-mirror up, the telescope,optical tube plus eyepiece) will deliver an erect,up is up) but reversed,left is right) image. And with the eyepiece in the rear port and flip-mirror down, the telescope,optical tube plus eyepiece) will deliver an inverted,up is down) and reversed,left is right) image.
For most astronomical observations, the top port position for the eyepiece is desirable for objects well above the horizon. In some cases it will be desirable or necessary,such as with a eyepiece focuser devise and/or to more comfortably observe objects closer to the horizon) to use the rear port and this is done by using what is commonly called a diagonal,or star diagonal). These are adapters fitted between the eyepiece and an eyepiece holder attached to the rear port that uses a first surface mirror aligned within the housing to reflect the image through a 90º,or 60º in the TeleVue 60º Everbrite Diagonal) path and invert it to deliver an erect,up is up) but reversed,left is right) image thus matching the image orientation of the top port eyepiece. Note that these mirror based diagonals are fairly easy to make inexpensively with precise alignment, high transmission efficiency and near zero added image distortion.
When used for applications such as bird watching or target spotting it is very desirable to have an erect,up is up) and normal,left is left) image orientation. For in-line designs double prism configurations,several forms) are used to produce the erect,up is up) and normal,left is left) image orientation. The four Maksutov-Cassegrain spotting scopes name above come with a 45º erect image eyepiece holder to mount on the rear port. These and angled design spotting scopes internally use a Schmidt prism to provide the image correction to an erect,up is up) and normal,left is left) image orientation. Note that the Schmidt prisms,and all prisms) used to invert and revert an image while deviating it through an angle of 45º are complex devises that are difficult to make inexpensively and those supplied will show transmission losses and added image distortion.
To maximize image quality it would be best to use the rear port and learn to deal with an inverted,up is down) and reversed,left is right) image. This would require an adapter and eyepiece holder for the rear port. These work for all four above mentioned scopes: Meade - Rear Cell Adapter for ETX Telescopes [ME-07036],$25 at OPT); and Meade - Eyepiece Holder for Schmidt-Cassegrain or Maksutov-Cassegrain Telescopes [ME-07182],$25 at OPT). To get an erect,up is up) and normal,left is left) image orientation you could use a better erecting devise such as the Williams Optics 1.25†45º Erecting Prism [WD-EP125],$68 at Williams Optics) with the adapter and eyepiece holder on the rear port.
As long as we are spending like drunken sailors, the next accessory to have is the Vixen - Piggyback 1/4"-20 DX Fine Adjustment Unit [VX-3562],$90 at OPT). Install one of these little gems between your tripod and scope and the fine slow pan and till control will make you kick yourself for not using one all along. Also get a set of vibration isolation pads for your tripod feet such as the Meade - #895 Vibration Isolation Pads [ME-07368],$50 at OPT).
Now for the big buck item. To deal with the touchy focusing issue we can use the mirror movement focus provided as the coarse focus then add an eyepiece focus unit for fine focus adjustment. The best type is Crayford type which is extremely smooth and backlash free. The suggested one is the JMI NGF-miniC 1.25 in Crayford Focuser,$150 at Jim's Mobile Incorporated) or for the ultimate motorized version the JMI NGF-miniCM 1.25 in Crayford Focuser,$290 at Jim's Mobile Incorporated). The motorized version is not as extravagant as you might think. Since it is operated with a hand control on a light cable to the focuser, fine focusing by remote avoids almost all the shake imparted to the scope during manual focusing reducing the time to sharp focus. Note that if you go with a focuser right away you can apply the $25 to the focuser that you do not need for the eyepiece holder since the focuser tube is an eyepiece holder.
This may seem to be getting very expensive but if you start with a C130MAK and add the NGF-miniCM focuser, Williams WD-EP125 Prism, Rear Cell Adapter, Vixen Fine Adjustment Unit, and two Vixen Eyepieces you are still under $1000.
Sorry for the delay in responding. I have been busy on some testing of my own for the last two days. I have enlisted the assistance of a fellow shooter and armature astronomer in a project to deal with the mirage and air quality problems. We will be reporting on that with an article or at least another thread at a later date,as a teaser - very good initial results). I also wanted to put together a hopefully helpful response that will help all parties. Here goes.
The Maksutov-Cassegrain spotting scopes we have been discussing - Celestron C90MAK & C130MAK also the Meade/Weaver ETX90 & ETX125 - are of a very similar design and share common image formation and delivery scheme. The optical tube assembly forms an image using a double spherical surface corrector plate and two spherical mirrors,no refracting lenses) and delivers an image that is inverted,up is down) and reversed,left is right).
At the rear is a flip-mirror,first surface mirror) that when down,horizontal) allows the image to pass through to the rear port,still inverted and reversed), but when up,45º) reflects the path out the top port and inverts the image,makes it right side up) but leaves it reversed. Eyepieces are designed not to invert or revert the image. Therefore with the eyepiece in the top port and flip-mirror up, the telescope,optical tube plus eyepiece) will deliver an erect,up is up) but reversed,left is right) image. And with the eyepiece in the rear port and flip-mirror down, the telescope,optical tube plus eyepiece) will deliver an inverted,up is down) and reversed,left is right) image.
For most astronomical observations, the top port position for the eyepiece is desirable for objects well above the horizon. In some cases it will be desirable or necessary,such as with a eyepiece focuser devise and/or to more comfortably observe objects closer to the horizon) to use the rear port and this is done by using what is commonly called a diagonal,or star diagonal). These are adapters fitted between the eyepiece and an eyepiece holder attached to the rear port that uses a first surface mirror aligned within the housing to reflect the image through a 90º,or 60º in the TeleVue 60º Everbrite Diagonal) path and invert it to deliver an erect,up is up) but reversed,left is right) image thus matching the image orientation of the top port eyepiece. Note that these mirror based diagonals are fairly easy to make inexpensively with precise alignment, high transmission efficiency and near zero added image distortion.
When used for applications such as bird watching or target spotting it is very desirable to have an erect,up is up) and normal,left is left) image orientation. For in-line designs double prism configurations,several forms) are used to produce the erect,up is up) and normal,left is left) image orientation. The four Maksutov-Cassegrain spotting scopes name above come with a 45º erect image eyepiece holder to mount on the rear port. These and angled design spotting scopes internally use a Schmidt prism to provide the image correction to an erect,up is up) and normal,left is left) image orientation. Note that the Schmidt prisms,and all prisms) used to invert and revert an image while deviating it through an angle of 45º are complex devises that are difficult to make inexpensively and those supplied will show transmission losses and added image distortion.
To maximize image quality it would be best to use the rear port and learn to deal with an inverted,up is down) and reversed,left is right) image. This would require an adapter and eyepiece holder for the rear port. These work for all four above mentioned scopes: Meade - Rear Cell Adapter for ETX Telescopes [ME-07036],$25 at OPT); and Meade - Eyepiece Holder for Schmidt-Cassegrain or Maksutov-Cassegrain Telescopes [ME-07182],$25 at OPT). To get an erect,up is up) and normal,left is left) image orientation you could use a better erecting devise such as the Williams Optics 1.25†45º Erecting Prism [WD-EP125],$68 at Williams Optics) with the adapter and eyepiece holder on the rear port.
As long as we are spending like drunken sailors, the next accessory to have is the Vixen - Piggyback 1/4"-20 DX Fine Adjustment Unit [VX-3562],$90 at OPT). Install one of these little gems between your tripod and scope and the fine slow pan and till control will make you kick yourself for not using one all along. Also get a set of vibration isolation pads for your tripod feet such as the Meade - #895 Vibration Isolation Pads [ME-07368],$50 at OPT).
Now for the big buck item. To deal with the touchy focusing issue we can use the mirror movement focus provided as the coarse focus then add an eyepiece focus unit for fine focus adjustment. The best type is Crayford type which is extremely smooth and backlash free. The suggested one is the JMI NGF-miniC 1.25 in Crayford Focuser,$150 at Jim's Mobile Incorporated) or for the ultimate motorized version the JMI NGF-miniCM 1.25 in Crayford Focuser,$290 at Jim's Mobile Incorporated). The motorized version is not as extravagant as you might think. Since it is operated with a hand control on a light cable to the focuser, fine focusing by remote avoids almost all the shake imparted to the scope during manual focusing reducing the time to sharp focus. Note that if you go with a focuser right away you can apply the $25 to the focuser that you do not need for the eyepiece holder since the focuser tube is an eyepiece holder.
This may seem to be getting very expensive but if you start with a C130MAK and add the NGF-miniCM focuser, Williams WD-EP125 Prism, Rear Cell Adapter, Vixen Fine Adjustment Unit, and two Vixen Eyepieces you are still under $1000.
Fred your the best.Fantastic information.The Williams Prism should be a must,sounds good.Vixen fine adj. sounds like the ticket.Focus $ I will suffer for now.Money will go to another lens.Using the celestron 32mm for now.Watching Ebay for lenses.Thanks for your great input it is greatly appreciated.You can see 8 pages of post.Keep us posted on your findings.I will further report.Pat
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