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RC mirror distances

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#1 mehdymo

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Posted 16 April 2020 - 06:54 PM

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.

Attached Thumbnails

  • From Orion Manual.jpg
  • From Ioptron Manual.jpg

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

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#2 TOMDEY

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Posted 16 April 2020 - 08:33 PM

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.

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#3 mehdymo

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Posted 16 April 2020 - 09:32 PM

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.



#4 mehdymo

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Posted 17 April 2020 - 02:52 PM

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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#5 mehdymo

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Posted 17 April 2020 - 03:39 PM

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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  • photo_2020-04-17_16-13-05 - Copy.jpg

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#6 andysea

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Posted 27 April 2020 - 12:48 AM

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.


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#7 mehdymo

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Posted 27 April 2020 - 03:27 AM

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.


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#8 astrobananeck

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Posted 10 April 2023 - 04:32 PM

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.



#9 cuzimthedad

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Posted 12 April 2023 - 03:26 PM

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


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#10 astrobananeck

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Posted 13 April 2023 - 07:27 AM

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.



#11 MitchAlsup

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Posted 13 April 2023 - 06:00 PM

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.



#12 astrobananeck

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Posted 13 April 2023 - 06:58 PM

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:

 

2023 04 14 013351
 

 

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.



#13 mehdymo

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Posted 26 February 2024 - 09:10 PM

I am glad it helped.


Edited by mehdymo, 26 February 2024 - 09:16 PM.


#14 Matthew Paul

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Posted 27 February 2024 - 12:53 PM

I'd love to see optical null tests for spherical aberration before and after people adjust their optic spacing to meet a magical ideal focal length. The spacing has nothing to do with collimation and everything to do with the overall spherical correction of the telescope. Each set of optics has a slight variation from an ideal focal length and conic constant. It's just the way that things are, there are manufacturing variations and tolerances.

 

The spacing can be adjusted slightly to offer the best spherical correction, the focal length differences are inconsequential and of no importance whatsoever. 

I ask, what is the point of having a "perfect" focal length if you spherical correction becomes worse? If the spacing is being adjusted based on a test for spherical aberration, then sure it makes sense, but to hit some ideal focal length and throw everything else out the window makes no sense to me. 

What is the reasoning for adjusting the spacing to meet a generic focal length given by retailers and manufacturers? What is the perceived benefit? 

Edited for spelling error


Edited by Matthew Paul, 27 February 2024 - 02:38 PM.

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#15 mehdymo

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Posted 04 March 2024 - 02:36 PM

It is not about changing the original spacing provided by the manufacturer. It is mostly for used scopes where the scope is not collimated anymore and people do all kind of adjustment to make it work. That would be a good starting point to achieve reasonable results.



#16 freestar8n

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Posted 04 March 2024 - 05:53 PM

Keep in mind that the focal length reported by a plate solve depends critically on the accuracy of the pixel spacing - which may be rounded only to two digits.  In that case there is no way the focal length is known to four digits - and 1406 could actually be 1452 or 1380.  

 

A plate solve gives the arc-sec per pixel very accurately - but to convert it to focal length you need to know the pixel spacing exactly - and it is often rounded.  The plate solve also may be rounded internally.

 

Rather than trust an app to tell you the focal length it might be better to do a plate solve yourself and get at least 4 decimals accuracy.  Then calculate the pixel spacing knowing the width of the sensor in mm and divide by the pixel count - and keep it to 4 or 5 decimals.  Then use those numbers to find the focal length based on fl = pixsize / asp * 206.265

 

If you do the math and it agrees with the app then you have confidence it isn't doing internal rounding and you can trust it for that camera.  But if you change apps or cameras you would need to check it again.

 

Either way it's perfectly fine if the optimal focal length in terms of aberration is different from the design - but the difference may not be as great as measured if the measured focal length is inaccurate due to rounding.

 

Frank


Edited by freestar8n, 04 March 2024 - 05:53 PM.


#17 fewayne

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Posted 05 March 2024 - 04:08 PM

YMMV, but I used software to measure and minimize spherical aberration, and then just accepted that as the "true" FL of my RC. Plate solves to 1625 (FWIW, given the inaccuracies Frank points out).


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