11 replies to this topic

[mehdymo]

Hi every body,

 

It seems like the distance between mirrors in an RC telescope is important to have a good cillimation as the mirrors are hyperbolic. There are a few methods to have the correct distance as follows:

- Plate Solving

- Putting the sensor at the given back focus an change the secondary position until the stars become focused

- Set the distances manually (if you know them) and collimate based on these distances

 

The third option may not be very accurate, but it would be a good start point to try the other two. I am wondering if people can provide the correct distances for different RC sizes and brands (mine is Orion RC 6") to make a good reference for everyone here.

 

I have noticed there is a small difference in the manuals (check the attached files), thus we can gather more information and make a better record here. Please note that:

- The distance for the back focus is better to be mentioned from the rear cell to the sensor as we may have different focusers.

- The distance from secondary to primary can be mentioned based on the secondary distance from the holder. It is consist of two separate distances.

                         1- The first one is the distance between the bottom of the holder to the top of secondary cell which is set by the central screw in the holder

                         2- The variable distance that can be set by the locking ring on the secondary body

   Instead of 1 and 2, we can measure the the distance between the top of the holder to the bottom of the secondary cell which needs a little more work.

 

Please let me know if anything else can be added or if there is anything wrong, so I can go ahead and correct it asap.

Thanks in advance for your help. The first picture is from Orion manual ( I combined them in Paint) and the second one is from Ioptron manual.

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Edited by mehdymo, 16 April 2020 - 06:55 PM.

[TOMDEY]

Each individual telescope is different. What the manufacturers provide is the design nominal spacing for your make and model, not necessarily best for your particular as-built OTA SN. Because of that, the gold standard is to start with any one of your ballpark settings up there, but to then further tweak about that nominal, to drive the spherical aberration to zero. (this is Zernike 1st order spherical). And that would ideally involve an interferometer and autocollimating flat. But (thankfully!), in practice, all you need do is scrutinize a star at high mag and adjust the PM/SM despace until the spherical aberration goes away. That's really all there is to it!

 

You will find that your scope has best imagery at that setting. At that point, you tag your actual best backfocus, and forever stick with that.

 

[We go through that exercise on all of our aerospace "Ritchey Front Ends". This technique is also identical to what microscopists do when finessing for variable specimen cover slip thicknesses; they adjust the spacing twixt eyepiece and objective until the spherical aberration goes away. It takes only a minute or so and then the instrument is ideal for the rest of the session. Cheap amateur microscopes don't provide that adjustment.]   Tom

Edited by TOMDEY, 16 April 2020 - 08:34 PM.

[mehdymo]

Thanks TOM, the reason that I asked for the distances is that many of us remove the secondary during collimation and when we put it back, the distance is changed. I touched the locking ring as well. Thus, before spending many nights to figure out how much the screw should be loosen and how much the ring should be, I wanted to have a rough estimation. Then I can start doing what you mentioned. I have seen many people have asked for this in different threads and thought that it would be useful to gather these information here. Tonight, I will use method 2 and then 1 to see if I can get it right and then follow what you said and perform the collimation. If it is successful, I will provide the first set of data for Orion RC 6" here.

[mehdymo]

Last night was a success. I used method two and fixed the sensor at the back focus of ~240mm. It took me a long time to find Arcturus in the field as it was defocused and it is hard to find any star at this focal length without a finder scope. However, when I found it, it again took a long time to find a good combination of the distances on the secondary to focus the star and secure the secondary in place. Finally, I took an image and used method one and uploaded an image to Astrometry.net to plate solve it and validate the second method and found that it is very accurate. Please see the attached picture. I will post the accurate distances in the next post.

 

 

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[mehdymo]

Here is my Orion RC 6" secondary structure/distances which works very well for me. Please note that it may be a little different for your scope, but I guess it is very close for other scopes and should be a good start point.

 

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[andysea]

I seem to remember that one can dial in the correct distance between the mirrors by using an eyepiece with a Ronchi screen. I need to research this however.

You are correct about the mirror spacing. I think that is why my RC focuses by moving the secondary. As long as the camera is at the correct back focus then the mirrors come in focus at the correct distance.

[mehdymo]

Thanks for your information. I have seen your posts about RC collimation and they are very informative. I used this method to correct the spacing and then used the Hotech Advanced Laser Collimation tool for collimation. It worked very well. Now my cheap RC image quality is better than my expensive Edge HD.

I seem to remember that one can dial in the correct distance between the mirrors by using an eyepiece with a Ronchi screen. I need to research this however.

You are correct about the mirror spacing. I think that is why my RC focuses by moving the secondary. As long as the camera is at the correct back focus then the mirrors come in focus at the correct distance.

[astrobananeck]

Here is my Orion RC 6" secondary structure/distances which works very well for me. Please note that it may be a little different for your scope, but I guess it is very close for other scopes and should be a good start point.

Did you adjust your FL using the center screw on the spider (the one with a phillips head) or did you rather loosen the knurled collar ("d5" on your image) to then rotate the secondary housing ("L2")? I saw another CN member mention the latter as the preferred method as it mostly retains collimation if I understand correctly.

[cuzimthedad]

Moving to Cats & Casses where all things RC are discussed...

[astrobananeck]

So I contacted Teleskop Service (TS) support regarding this and they answered that, yes, indeed, FL should be adjusted to match the factory specification. They also said the best procedure was using the knurled collar and secondary mirror housing, rather than the center Phillips head screw although that latter "method" is also acceptable to them.

[MitchAlsup]

As explained in "Reflecting Telescope Optics" R. N. Wilson Book 2::

 

The collimation tolerances of the RC telescope are sufficiently stiff that any bending of the truss/tube is likely to harm collimation (this is because the optical axis of the secondary and camera have to remain on-axis with the primary.

 

Without any bending of the truss/tube, THEN the focal distance between secondary and foal plane will not be moving enough to be outside of the range where a focuser can get the job done.

[astrobananeck]

So I managed to bring back my FL from 1406 mm (after initial collimation) to 1379 mm. Ultimately I couldn't use just the secondary mirror housing (it was all the way up already) so I did use the center Phillips screw. It only took a very slight adjustment so I don't think the secondary mirror stability is any different from its previous position. I then recollimated the secondary only (there was no on-axis coma so I didn't need to retouch the primary), refocused and took a few pictures that I measured with CCD Inspector. All looking good as you can see on the picture below (I used M44 to test) with just a slight residual tilt that I might try to remove by retouching the secondary collimation ever so slightly next time. For now, I'm content:

 

 

 

As explained in "Reflecting Telescope Optics" R. N. Wilson Book 2::

 

The collimation tolerances of the RC telescope are sufficiently stiff that any bending of the truss/tube is likely to harm collimation (this is because the optical axis of the secondary and camera have to remain on-axis with the primary.

 

Without any bending of the truss/tube, THEN the focal distance between secondary and foal plane will not be moving enough to be outside of the range where a focuser can get the job done.

In the process, my focus point (measured by the ZWO EAF steps) went from 6480 to 209 which is quite a significant shift (expected). FWHM seems to be down by about 0.3" but there were clouds and guiding was not perfect so I'll try again when we have a clear night here.