Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

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Martin_S
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Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Martin_S »

I'm not sure if this is the right section, so moderators feel free to delete or move to the appropriate section.

Here is a method I used for Collimating my GSO 6" Ritchey Chretien. I don't know if anyone else uses this method.


I would suggest that you familiarise yourself with the Instructions that came with your Ritchey Chretien optical tube
before using this method.

You will need the following to use this method.

1. Barrel type astronomy camera such as a Rising Tech 224 or ASI mini camera with a C mount connection.
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2. C type TV lens. An adjustable aperture is essential for getting the correct depth of field.
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3. A bright source of diffused light.

4. A PC with Sharpcap or similar imaging software

5. Allen keys

Now you are ready to Collimate your optical tube.

1. Assemble the camera and lens as shown in the picture.
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2. Insert the Camera and lens assembly into the focuser, use 75mm of spacers in front of the focuser.
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3. Point the optical tube towards a bright diffused light source. This will illuminate the inside of the opical tube.
20200529_220937resized.jpg
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4. Open your imaging software and adjust exposure settings,then set focus and aperture on the small camera lens
to get an image similar to this, adjusting the f ratio will give you the correct depth of field.
You will need to remove your camera a few times from the focuser during collimation to to adjust your image to optimum settings.
collimated secondary.jpg
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5. Start with your secondary mirror cell. There should be a small doughnut shaped ring
on the center of the secondary mirror, make sure that you can see this with your camera.
Once you are confident that this is visible, center the doughnut within the reflection as per your instructions.
The secondary mirror is now collimated.
collimation 29-5-20.jpg
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6. Now move on to the primary mirror. Adjust your image to show a thin ring on the outside edge of your image.
This ring should be concentric if not adjust your primary mirror as per your instructions.
Once you can see a thin even ring your primary mirror is collimated. The outer ring on my image is dark on one side due
to poor illumination as I used a small desk lamp and a sheet of paper.
[/attachment]
collimation notes.jpg
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7. Your optical tube is now bench collimated and ready for a star test and final adjustment.
My star test was perfect so no further adjustments were needed.


Martin


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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by marktownley »

Very informative! Will this work with an SCT too?


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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by rsfoto »

Hi Martin,

Very interesting and thank you. Will try to do it on my 8" RC from Astrotech.


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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Martin_S »

Interesting question Mark,It would work on a Classic Cassegrain as they have fixed optics. The SCT optical system is a lot more complex. I'd be more inclined to use an artificial star and a camera with a basic SCT as there are to many variables in the optical train.

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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Montana »

This should be in the Library section Martin :)

Mark, I don't think it is possible to align the primary mirror on my SCT I think it is fixed, I have only ever done the secondary mirror. Mine also has no dot reference point either, so again this method will not help. The only way collimation can be done is at night with an out of focus star, I use my 3x barlow + camera. Although my Televue powermate in WL on the Sun was pretty good alternative the problem then comes in the fact that the collimation screws are under the Baader solar film ;)

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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Merlin66 »

Well done Martin!
Looks like you have a great white light set-up.
Just a note. Al's collimating tool (http://www.iceinspace.com.au/forum/show ... ool&page=2) is a very good tool to assist in collimating.


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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by christian viladrich »

Excellent idea. Well done !


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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Martin_S »

Merlin66 wrote: Tue Jun 02, 2020 11:27 pm Well done Martin!
Looks like you have a great white light set-up.
Just a note. Al's collimating tool (http://www.iceinspace.com.au/forum/show ... ool&page=2) is a very good tool to assist in collimating.
Thanks Ken , I will give this a try.

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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Carbon60 »

Ingenious, Martin.

I use a Takahashi collimating scope and my eyeball, but of course I can’t be at both ends of the scope at the same time to look through the scope and adjust the secondary. It would be so much more convenient to use a computer screen to relay the image. This is an excellent solution.

Stu.


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Re: Benchtop Ritchey Chretien Collimating with an astronomy imaging camera.

Post by Astrograph »

Just a note for people trying to use this collimation method on similar scopes.

The GSO RC6 is the simplest version and has limited collimation adjustment. To collimate a Cassegrain format telescope the basic steps are actually as follows;

Step 1 - The first thing you must do is collimate the image train. On the 8" GSO RC and larger, there is a collimation flange that the focuser is mounted to. You must first align this to the centre spot on the secondary. Traditionally this is done using a Glatter or Farpoint Laser. Using Martins method I guess you would use a cross hair overlay and centre this on the spot. However that very much depends if the chipset is actually centred in the camera body which is not always the case.

Step 2 - Once the image train is centred now you align the secondary back on the laser or camera as Martin describes.

Step 3 - The next step is to adjust the primary. You can do it as explained by Martin but traditionally it is done on a star so the shadow of the secondary is concentric. This is where Al's collimation aid is useful.

A classic cassegrain and RC are done in exactly the same way but for perfect collimation you need the focuser on a seperate tilt plate. The early GSO RC's (and the RC6) have a problem in that adjusting the primary also moves the focuser so you end up decollimating the image train when you adjust the primary. Therefore you have to try and do it back to front.

An SCT is different. It has no spot on the secondary to start with. The normal tool for this is a Hotech Advanced collimator. This is quite difficult to use as it must be perfectly centred to the scope and the scope and it must be on a floor which does not move when you step on it. Hotech have two online videos about collimating an SCT

With a Meade or Celestron 'C' you can only adjust the secondary. If you add a tilt adjuster on the back you can then adjust the image train. For the Edge range you can also centre the corrector plate.

Really with an SCT, point it at a star and collimate it. There is a limit to how good it can get as the primary mirror flops all over the place anyway and focusing using the primary will shift collimation significantly.


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