FredBohl

[StraightPipes]

Can anyone tell me if FredBohl still visits this site? Or, if Fred happens across this post I have a few questions on "spotting scopes".
I have read all of his posts but need an answer to an "updated" question.

 

[Fred Bohl]

I'm alive and still reading the posts but was on a trip most of April and am still getting caught up with chores.

Ask away and I'll try to be prompt with my reply.

 

[StraightPipes]

Great news, hope your trip was a great time also.

I have been playing with a Celestron spotter for a while now, but it is time to upgrade to a larger "MAK". I have found a source for the C-130. Have read all that you have posted here, so this wont take long. I believe that the C-130 and its 2000mm focal lenght is well suited for resolving bullet holes. Simply, all that we need this spotter to do is see bullet holes. Resolution and all of the other problems will be secondary and can be tolerated if we dont expect "too" much from this scope. As in all scopes that suffer when trying to do what we do here.
Fred, is the C-130 still the recommended scope, or is there something better sense this was last discussed. It can still be purchased for a good price. That is, if the vendors web site is up to date.
You also stated that you were working on a "mirage" fix. Did you ever get anywhere with this?

 

[Fred Bohl]

Since the C-130 Mak Spotting Scope has been discontinued by Celestron I have to be hesitant in recommending it. However, if you can still find a new or "near new" one at a reasonable price, I still love mine for bullet hole spotting so I would buy it if I had to replace mine.

The experiments in imaging and image processing to help deal with mirage are working well enough to be very helpful in teaching me to "read the wind and find the real target position" but not well enough to turn into a practical product/system for general range use.

 

[StraightPipes]

Hello Fred, thanks for getting back on this. Yes, it was discontinued but I believe they still warrant the C130. In researching this scope, I have found a "fix" or modification that greatly improves the image. It involves the removal of 1 or 2 optical windows located in the rear of the C130. The star gazers have been making this mod for quite awhile now. What are your thoughts on this?
Second; will the 45* erector hinder image quality as much as they claim? There are "inline erector eyepieces" that are supposed to work better. Some are 5-6" long without the eyepiece, but I think you can find some quite a bit shorter. Or, is there another component that will erect the image?

 

[Fred Bohl]

StraightPipes,

First on the "fix" -- those flat glass lenses refered to seal the main optical tube from dirty air and water vapor and for that reason alone should be retained -- just keep them clean.

As to your other question, let us first review a little so that we and others reading this thread are all on the same page.

Paraphrased from older posts:

The Maksutov-Cassegrain Celestron C130MAK 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 C130MAK comes 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. The ScopeTronix Visual Back STVB ($30 at scopetronic.com) or the combination of Meade - Rear Cell Adapter for ETX Telescopes [ME-07036] ($25 at OPT) plus Meade - Eyepiece Holder for Schmidt-Cassegrain or Maksutov-Cassegrain Telescopes [ME-07182] ($25 at OPT) will work fine. I only view through the rear cell using one of these and either a lens or digital imager. 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 accessories to have are the Celestron - Heavy-Duty Alt-Azimuth Tripod [CE-93607]($90 at OPT) and 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.

 

[StraightPipes]

Hi Fred, so what you are suggesting to improve image quality is to swap out the supplied 45* erector with the Williams model? Also to use the rear port? I understand this direction. Also, the in-line erector eyepieces "dont" do as well of a job producing a quality image?
I will re-read your previous post, more than once. If moisture and dust are the concerns for "not" removing the windows that I mention, will you agree regardless that image quality will be better when they are removed?
I am listening to you, Im not telling you. I have a bad habit of pushing the "performance" envelope. I take advice like "more is better".

 

[Fred Bohl]

StraightPipes,

In your first paragraph: yes, yes and yes - we are in agreement.

Removing the flat windows will provide a slight but detectable image quality improvement. However the improvement will quickly be defeated by the accumulation of moisture and dirt on the critical mirrors inside the optical tube. Also note that it is extremely difficult to clean those mirrors without damaging their surfaces and altering collimation (alignment). IMHO the improvement in image quality will not improve target analysis sufficiently to justify the risk of mirror surface damage.

 

[StraightPipes]

Thank you Fred. I have thought about playing around inside this scope and degrading collimation in the process. If I do not purchase a "lemon" I will take your advice and stay out of the internals. In my research, I see that lemons(bad images) can and do happen. I guess collimation is off due to shipping or rough handling. Only then will I rip into the internals.

Thank you

 

[StraightPipes]

Fred, I sent you a PM.