Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

RC8 collimation: reducer vs native FL

  • Please log in to reply
11 replies to this topic

#1 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 11 Jun 2021

Posted 08 July 2021 - 10:43 AM

Hi everybody,

there are so many posts about RC collimation that one gets lost... I ask for your help understanding if I'm right direction or not.

 

I used CCD Inspector and the famous DSI collimation procedure to **** my GSO RC8 collimation, both at native FL and with AP 0.67x reducer (actually the TS version at about 82mm, so a little more than 0.67x).

 

What I found is that at native focal lenght collimation is OK, my conclusion is that I don't need to do anything more to improve it: DSI method shows balanced and round defocused stars all over the frame, CCDI shows good values. See pictures in next post.

 

Using the reducer, things becomes much more questionable:  DSI method shows unbalanced, non round stars across the frame; CCDI return uneven plots. See pictures in the further post.

So I conclude that the reducer lowers image quality and maybe has something wrong.

 

Then my points are:

  1. Curvature % shown by CCDI are not as low as hoped, in both configuration. But, at native focal lenght the difference between min and max FWHM looks thin enough for me. So I would not worry about it. Do you agree?
     
  2. Do you agree that my instrument collimation is actually good enough, or should I improve it?
     
  3. I'm a little surprised that the reducer worsens results so much. Actually, the impact on my final deep sky picutres is probably not that huge, but CCDI and DSI method suggests something is not ok. On the other hand I wouldn't do anything since my OTA at native FL looks ok, concluding the reducer is not good enough. Do you agree with my conclusion / is all of this normal in you experience?

Thanks everybody for your support.

D



#2 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 11 Jun 2021

Posted 08 July 2021 - 10:48 AM

RC8 at Native Focal Lenght 1625mm

Attached Thumbnails

  • 1. RC8 1600 in focus.jpg
  • 2. RC8 1600 CCD Inspector Curvaturejpg.jpg
  • 3. RC8 1600 CCD Inspector 3D.jpg
  • 4. RC8 1600 DSI defocus.jpg


#3 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 11 Jun 2021

Posted 08 July 2021 - 11:02 AM

RC8 at 1089mm with TS 0.67x reducer

Attached Thumbnails

  • 1. RC8 1089 in focus.jpg
  • 2. RC8 1089 CCD Inspector Curvature.jpg
  • 3. RC8 1089 CCD Inspector 3D.jpg
  • 4. RC8 1089 DSI defocus.jpg

Edited by davide.cattani, 08 July 2021 - 11:04 AM.


#4 TinySpeck

TinySpeck

    Messenger

  • ****-
  • Posts: 467
  • Joined: 08 Oct 2017
  • Loc: Seattle area

Posted 10 July 2021 - 12:16 PM

One problem is the GSO 8" RC is 1600 mm focal length, not 1625 etc as you read all over the web.  Have a look at this post for some background there.  So I would recommend correcting your focal length first.

 

I have had similar elongated star trouble with the AP CCDT67 (which I think is the same as you're using).  0.67x didn't work well, and I got better results changing the backfocus for about 0.75x reduction.  That still wasn't perfect, but it was much better.

 

It is indeed a mess out there as far as collimation procedures!  I've added to it myself, and I may have another one about to erupt.


Edited by TinySpeck, 10 July 2021 - 12:18 PM.

  • davide.cattani likes this

#5 Terry White

Terry White

    Ranger 4

  • *****
  • Posts: 332
  • Joined: 13 Sep 2017
  • Loc: Knoxville, Tennessee.

Posted 10 July 2021 - 02:26 PM

One problem is the GSO 8" RC is 1600 mm focal length, not 1625 etc as you read all over the web.  Have a look at this post for some background there.  So I would recommend correcting your focal length first.

 

I have had similar elongated star trouble with the AP CCDT67 (which I think is the same as you're using).  0.67x didn't work well, and I got better results changing the backfocus for about 0.75x reduction.  That still wasn't perfect, but it was much better.

 

It is indeed a mess out there as far as collimation procedures!  I've added to it myself, and I may have another one about to erupt.

Gerrit, I read your thread. Yes, there's a lot of contradictory information posted by vendors websites on RCs. Without resorting to specific vendor bashing, let me just say that I have found that the descriptions on some of the store-brand RC's to be highly misleading, so I take what they say with a grain of salt. I have tried to get some of this vendor disinformation corrected, to no avail. My only comments are that the entrance pupil, the smallest diameter in your OTA, will define the active diameter of your mirror. This is smaller than the mirror OD to get the mirror clamps on the mirror OD out of the field of view. A Ronchi eyepiece or Ronchi camera adapter can measure any spherical aberration, due to an incorrect focal length setting, and allow you to move the primary in or out to achieve zero spherical aberration at the correct RC focal length. Once that is done the DSI guide is by far the best method for collimation, IMHO.


Edited by Terry White, 10 July 2021 - 02:27 PM.

  • davide.cattani likes this

#6 David07

David07

    Sputnik

  • -----
  • Posts: 29
  • Joined: 19 Mar 2018
  • Loc: Cambridge, UK

Posted 17 July 2021 - 03:22 PM

I don't think you can be prescriptive about the correct focal length of an RC8, you get what the glass produces. I agree with Terry, you need to test the spherical aberration with a Ronchi grating to get an assessment of the scope. My RC8 arrived giving a focal length of 1604mm but was assessed by a professional optician as being grossly overcorrected - mirrors too far apart. After some work he arrived at a fully corrected condition with parallel Ronchi lines inside and outside focus. The focal length is 1660mm and that is how it is today. My back focus is consequently quite long.

 

I would accept that this must be something of an exceptional sample. I've tested other RC8s with a Ronchi grating and they come out in the 1620 to 1630mm range. I've not seen any that test as short a FL as 1600mm. 

 

On the CCDT67 reducer, I agree and a 0.67x reduction is pushing it too much. I use 0.7x and that seems to work.

 

One other aspect of the RC8 is to understand that the weight of your imaging train hangs on the three collimation pull screws. These screws need to be snug or else your collimation will change as you move the scope around the sky or change the load hanging off the back.


Edited by David07, 17 July 2021 - 03:53 PM.

  • TinySpeck and davide.cattani like this

#7 TinySpeck

TinySpeck

    Messenger

  • ****-
  • Posts: 467
  • Joined: 08 Oct 2017
  • Loc: Seattle area

Posted 20 July 2021 - 08:07 PM

I don't think you can be prescriptive about the correct focal length of an RC8, you get what the glass produces. I agree with Terry, you need to test the spherical aberration with a Ronchi grating to get an assessment of the scope. My RC8 arrived giving a focal length of 1604mm but was assessed by a professional optician as being grossly overcorrected - mirrors too far apart. After some work he arrived at a fully corrected condition with parallel Ronchi lines inside and outside focus. The focal length is 1660mm and that is how it is today. My back focus is consequently quite long.

 

I would accept that this must be something of an exceptional sample. I've tested other RC8s with a Ronchi grating and they come out in the 1620 to 1630mm range. I've not seen any that test as short a FL as 1600mm. 

 

Your photos above say "native focal length 1625mm" and "1089mm with TS 0.67x reducer".  Did you determine an actual 1660 mm focal length after all of the above?  How has that affected your CCDI results and the elongated stars?

 

I'm a Ronchi noob, I hardly know anything about it.  What I see is the Ronchi test being used on individual mirrors, where parallel lines inside and outside the radius of curvature indicate a perfect spherical reflector.  That wouldn't apply here.  I'd love to know more about Ronchi testing to determine RC focal length if you know of some links.  @Terry White, feel free to chime in too!


Edited by TinySpeck, 20 July 2021 - 08:10 PM.

  • xthestreams and davide.cattani like this

#8 Terry White

Terry White

    Ranger 4

  • *****
  • Posts: 332
  • Joined: 13 Sep 2017
  • Loc: Knoxville, Tennessee.

Posted 21 July 2021 - 06:58 AM

What I see is the Ronchi test being used on individual mirrors, where parallel lines inside and outside the radius of curvature indicate a perfect spherical reflector.  That wouldn't apply here.  I'd love to know more about Ronchi testing to determine RC focal length if you know of some links.  @Terry White, feel free to chime in too!

No, the Ronchi test works fine for finding the RC's correct mirror spacing. For example, this has been covered on CN here. The Ronchi patterns are shown here.


Edited by Terry White, 21 July 2021 - 07:03 AM.

  • TinySpeck and davide.cattani like this

#9 TinySpeck

TinySpeck

    Messenger

  • ****-
  • Posts: 467
  • Joined: 08 Oct 2017
  • Loc: Seattle area

Posted 21 July 2021 - 10:42 AM

No, the Ronchi test works fine for finding the RC's correct mirror spacing. For example, this has been covered on CN here. The Ronchi patterns are shown here.

Thank you, Terry, I'll study this.


  • davide.cattani likes this

#10 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 11 Jun 2021

Posted 21 July 2021 - 11:08 AM

Thanks everybody for your hints.

 

Platesolving says my native focal length is 1.618mm, so it should be in the "normal" range but not necessarily the correct value. 

 

I red the posts you suggested, I'm honest they're a little discouraging... By a first read, Ronchi grid would be an effective way to assess the distance, even though I guess a) a lot of effects will superimpose alltogher making it very difficult to interprete results, b) by varying secondary mirror distance (is it done by the central screw or the 3 collimations screws? I red both...) will not only need to refocus, but also to recollimate...

 

I'm very scared, my fear is to worsten the situation without being able to come back.

Maybe I'll buy the Ronchi tool, try to assess the situation and than think about next steps.

 

In the meanwhile I'lltry to image at full focal length (f/8...), which looks good enough by judging from the post #2 above.



#11 Terry White

Terry White

    Ranger 4

  • *****
  • Posts: 332
  • Joined: 13 Sep 2017
  • Loc: Knoxville, Tennessee.

Posted 21 July 2021 - 11:58 AM

Thanks everybody for your hints.

 

Platesolving says my native focal length is 1.618mm, so it should be in the "normal" range but not necessarily the correct value. 

 

I red the posts you suggested, I'm honest they're a little discouraging... By a first read, Ronchi grid would be an effective way to assess the distance, even though I guess a) a lot of effects will superimpose alltogher making it very difficult to interprete results, b) by varying secondary mirror distance (is it done by the central screw or the 3 collimations screws? I red both...) will not only need to refocus, but also to recollimate...

 

I'm very scared, my fear is to worsten the situation without being able to come back.

Maybe I'll buy the Ronchi tool, try to assess the situation and than think about next steps.

 

In the meanwhile I'lltry to image at full focal length (f/8...), which looks good enough by judging from the post #2 above.

David, you're right to be cautious when turning the screws on your RC's primary and secondary. For example, many RC owners have made their collimation worse by fiddling with the secondary, based on collimation tools that rely on geometric indicators such as secondary center spots and light baffles. This is all based on the fallacy that the RC's geometric and optical axes are the same. They're not, in general. Trying to fully understand your RC's collimation before you put a focal reducer on is a good idea too. There's a lot of contradictory information on RC collimating here on CN, IMHO. For example, I recommend that people leave the secondary screws (especially the center screw) alone when trying to adjust the mirror spacing. There are many sound reasons why adjusting the secondary center screw is a very bad idea when you're trying to make small and repeatable adjustments.


  • MikeECha and xthestreams like this

#12 xthestreams

xthestreams

    Ranger 4

  • -----
  • Posts: 308
  • Joined: 18 Feb 2020
  • Loc: Melbourne, Australia

Posted 21 July 2021 - 05:41 PM

A couple of things to add.

1. The spacing, if indeed it does need to be adjusted, is controlled by unlocking the large knurled ring around the base of the secondary, right where the secondary housing meets the spider vanes.

 

as Terry mentions, don’t mess with the Center screw.

 

2. Your collimation, at least to my eye is good but still a ways to go, hard to tell 100% but the aberrations do not look symmetric around the axis. It’s normal for the “central” spot to be slightly off axis, off axis, but they should be symmetric, per DSI guide. 
 

from what you’ve shared they do not appear to be, indicating more to be done tuning the secondary, I imagine/guess that the reducer is effectively magnifying that making it more obvious

 

I can’t speak to the impact of over correction, my understanding is that it increases astigmatism.

 

agree a Ronchi is not a silly investment. These are great and underrated instruments, mainly due to the awful/non existent documentation from GSO




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.







Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics