8" RC focus and focal length issues

[Crispy5573]

Hi all, I bought a second hand 8" TS Optics RC (GSO) and I have tried everything I have found here and elswhere about these scopes then finally took it to RVO (my local astronomy store). They checked it out for me and said it seemed fine and the collimation was almost perfect so they tweaked it a little for me. 

The problem I have is it won't get to a sharp focus and when I do a plate solve the focal length comes up at 1785 ! These scopes are meant to be around 1600-1625 and everyone says the distance between the mirrors is critical.

I believe is you close the distance between the mirrors it will increase the focal length at a ratio of about 10 to1 (1mm closer gives 10mm more focal length) If something has happened to this scope before I bought it then somehow the mirrors are about 16mm too close together ! 

Just to make matters worse I bought a Feathertouch focuser and M90 connection ring, 2 M90 extension rings and a focuser tilt plate for it.

Anyone had a similar experience or able to offer advice please I'm ready for donating it to the local dump. 

[newbie alert]

As a guess as I've never used a RC, is your spacing distance bang on spec, as that will have a impact on both focus and focal length... It happened with my SCT.. 

Also I wouldn't get hung up on getting the FL perfect as I've not come across a bang on the money spec'd scope yet, from SCT to refractors, but they're using too long than too short..

[davies07]

Hi,

You are not alone to be struggling with an RC8.  There are many threads concerning collimating and adjusting the focal length of these scopes. They can be tricky to get right but produce really nice images when correct.  From the information you give I would think the collimation is correct but the mirrors are too close together and the scope is undercorrected (I think that’s the right description).

I owned one of these for for five years and it gave excellent service once adjusted. Interestingly, the focal length of mine was 1660mm.  At this focal length a Ronchi test showed no spherical aberration and the scope was ‘correct’. So your correct focal length might not be the quoted 1624mm. You’ll need to do a Ronchi test to find the correct mirror spacing.

Have a look at posts in this thread

https://stargazerslounge.com/messenger/331416/?tab=comments#comment-730724

I’ve posted some links to documents I’ve stored on Dropbox. You should be able to download these and read up on collimation and doing a Ronchi test.  Different people have their own favourite approach to doing collimation of these scopes but I think the hole in card technique works.  I’ve moved up to a 10” RC but am still using the hole in card method even though the mechanical design of the 10” is better than the 8”. The hole in card method was developed by Es Reid by the way.  
 

I’m  away from home at present so will not be able to give quick responses to questions until the weekend. Your scope can be checked and corrected.  If you don’t think you could do it yourself, you could always send it to Es to check (Cambridge). 

David

 

 

[Crispy5573]

41 minutes ago, newbie alert said:

As a guess as I've never used a RC, is your spacing distance bang on spec, as that will have a impact on both focus and focal length... It happened with my SCT.. 

Also I wouldn't get hung up on getting the FL perfect as I've not come across a bang on the money spec'd scope yet, from SCT to refractors, but they're using too long than too short..

Thanks for your reply newbie alert. I'm not too bothered about the focal length but from all I've read the spacing on an RC is critical. I don't know what it should be or how to measure it.

Edited December 27, 2022 by Crispy5573
Error

[Crispy5573]

16 minutes ago, davies07 said:

Hi,

You are not alone to be struggling with an RC8.  There are many threads concerning collimating and adjusting the focal length of these scopes. They can be tricky to get right but produce really nice images when correct.  From the information you give I would think the collimation is correct but the mirrors are too close together and the scope is undercorrected (I think that’s the right description).

I owned one of these for for five years and it gave excellent service once adjusted. Interestingly, the focal length of mine was 1660mm.  At this focal length a Ronchi test showed no spherical aberration and the scope was ‘correct’. So your correct focal length might not be the quoted 1624mm. You’ll need to do a Ronchi test to find the correct mirror spacing.

Have a look at posts in this thread

https://stargazerslounge.com/messenger/331416/?tab=comments#comment-730724

I’ve posted some links to documents I’ve stored on Dropbox. You should be able to download these and read up on collimation and doing a Ronchi test.  Different people have their own favourite approach to doing collimation of these scopes but I think the hole in card technique works.  I’ve moved up to a 10” RC but am still using the hole in card method even though the mechanical design of the 10” is better than the 8”. The hole in card method was developed by Es Reid by the way.  
 

I’m  away from home at present so will not be able to give quick responses to questions until the weekend. Your scope can be checked and corrected.  If you don’t think you could do it yourself, you could always send it to Es to check (Cambridge). 

David

 

 

Thanks David,

 

All I need to know now is how to get to the Drobox and possibly a phone number/email address for Es Reid. The link to the comments doesn't work for some reason.

Ron

[Cornelius Varley]

10 hours ago, davies07 said:

Have a look at posts in this thread

https://stargazerslounge.com/messenger/331416/?tab=comments#comment-730724

The link goes to your private messenger, no one can read those.

[teoria_del_big_bang]

Got the T shirt on that one and little advice I can give but following the thread in case I see something new.
I did exactly the same, bought a second hand IOptron RC6, although I think all makes are actually the same scopes, different colour and brandname.
Also because it was 2nd hand and not knowing for sure what the previous owner had done I also took it to RVO for proper colimation , I also added the tilt plate and a better focusser (Baader Diamond Steeltrack) which i think is a must as the stock focusser is nogood at all for AP. Also further cost in adding a full length dovetail on top and all the counter weight kits to be able to balance it without it being almost out of the clamp on the mount.
And to be honest I think I was 99% the way there and did get some images one night but it was obvious from the images the colimation was not perfect,
So eager to get imaging I tried adjusting the secondary, which I thought was the correct thing to do, but did not get it any better.
After asking about it on this site (I will see if I can find the thread) the replies I got indicated I should have adjusted the primary as it would more thanlikely have been collimated properly by RVO but that was without the heavy CCD and filterwheel added which, because of the fundemental issue of the focusser being connected to the primary, will move the mirror enough to take it out of colimation and needs final colimation in situe with all the imaging train attached, and then maybe needs some counter adjustment on the tilt plate after this primary has been tweaked..

That was my understanding anyway, forgive me if I understood wrongly.

Unfortunately because I moved the secondary and was not sure at all I had adjusted it back to where RVO set it I was amost back to square one 😞  and wished I had realised all I needed was some slight adjustment to the primary and tilt plate and was probably there.

Whilst I am not one to give up that happened at beggining of this year and the weather and cloud cover was so bad in my part of the country I never really had the time to try again, poor excuse I know 🙂 

Now one thing in your OP that interests me is your suggesion the mirro spacing maybe wrong due to the reported focal lenght. I had not thought of that as I always wondered if the mirror spacing was correct (not that that seemed to be my issue when taking the images) , is the reported FL that precise when plate-solving you can trust it ? I always thought it was a bit of an approximation but if it is pretty accurate then that would be good to know.
EDIT: Having read all the replies properly seems this maybe a red-herring so forget this thought 🙂 

I hope you make good progress as I still have mine and would like to carry on if 2023 is any clearere in Yorkshire as , like yourself, have invested much more than the initial cost of the scope, in both time and money, and would like to reap some rewards.

Steve

Edited December 27, 2022 by teoria_del_big_bang

[Crispy5573]

58 minutes ago, Cornelius Varley said:

The link goes to you private messenger, no one can read those.

Apparently not even the person it was sent to! I get the message "you do not have permission to view this"

[Crispy5573]

36 minutes ago, teoria_del_big_bang said:

Got the T shirt on that one and little advice I can give but following the thread in case I see something new.
I did exactly the same, bought a second hand IOptron RC6, although I think all makes are actually the same scopes, different colour and brandname.
Also because it was 2nd hand and not knowing for sure what the previous owner had done I also took it to RVO for proper colimation , I also added the tilt plate and a better focusser (Baader Diamond Steeltrack) which i think is a must as the stock focusser is nogood at all for AP. Also further cost in adding a full length dovetail on top and all the counter weight kits to be able to balance it without it being almost out of the clamp on the mount.
And to be honest I think I was 99% the way there and did get some images one night but it was obvious from the images the colimation was not perfect,
So eager to get imaging I tried adjusting the secondary, which I thought was the correct thing to do, but did not get it any better.
After asking about it on this site (I will see if I can find the thread) the replies I got indicated I should have adjusted the primary as it would more thanlikely have been collimated properly by RVO but that was without the heavy CCD and filterwheel added which, because of the fundemental issue of the focusser being connected to the primary, will move the mirror enough to take it out of colimation and needs final colimation in situe with all the imaging train attached, and then maybe needs some counter adjustment on the tilt plate after this primary has been tweaked..

That was my understanding anyway, forgive me if I understood wrongly.

Unfortunately because I moved the secondary and was not sure at all I had adjusted it back to where RVO set it I was amost back to square one 😞  and wished I had realised all I needed was some slight adjustment to the primary and tilt plate and was probably there.

Whilst I am not one to give up that happened at beggining of this year and the weather and cloud cover was so bad in my part of the country I never really had the time to try again, poor excuse I know 🙂 

Now one thing in your OP that interests me is your suggesion the mirro spacing maybe wrong due to the reported focal lenght. I had not thought of that as I always wondered if the mirror spacing was correct (not that that seemed to be my issue when taking the images) , is the reported FL that precise when plate-solving you can trust it ? I always thought it was a bit of an approximation but if it is pretty accurate then that would be good to know.
EDIT: Having read all the replies properly seems this maybe a red-herring so forget this thought 🙂 

I hope you make good progress as I still have mine and would like to carry on if 2023 is any clearere in Yorkshire as , like yourself, have invested much more than the initial cost of the scope, in both time and money, and would like to reap some rewards.

Steve

Thanks Steve I'll let you know if I find a solution.

Ron

[davies07]

https://www.dropbox.com/s/dbhncjtzfursg6b/Collimating GSO Ritchey with a plastic disc V2.pdf?dl=0

Try this link for the document on collimation. 

https://www.dropbox.com/s/i68gibodxszoga6/Testing with Ronchi eyepiece.pdf?dl=0

 

and this one for the document on checking the focal length.

I will PM you Es’s details.

 

D

 

[inFINNity Deck]

The original discussion David refers to is here:

I joined that discussion, learned a lot from David (I still use his disc) and others, tweaked the RC-collimation method slightly and described it here (open it in Chrome to get it translated): https://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

In the meanwhile I have collimated four RCs, both tube and truss ones and of 8" and 10" aperture. Some have been back several times, not because collimation was off, but to make further adjustments of their focal lengths or mechanics (after which collimation was required). All perform as should. One of the scopes was altered to have a larger back-focus distance, so that may have been the case with your scope as well.

Main importance is to have the focuser disconnected from the primary mirror, but achieving that may require quite a bit of machining. Another point is to adjust the mirrors by only using two of the three screws as in that way you avoid that the mirror-distance changes in the course of several collimations. Best is to paste a small sticker (piece of tape) over the screw that serves as reference.

If you want to get the focal length right, it is of key importance to know the design value for your scope. The Ronchi-test can be of assistance, but is not very sensitive, so may result in a focal length being off by quite a few millimetres (but then, if the Ronchi-test does not detect aberration, then it will not affect imaging).

Nicolàs

[Crispy5573]

1 hour ago, inFINNity Deck said:

The original discussion David refers to is here:

I joined that discussion, learned a lot from David (I still use his disc) and others, tweaked the RC-collimation method slightly and described it here (open it in Chrome to get it translated): https://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

In the meanwhile I have collimated four RCs, both tube and truss ones and of 8" and 10" aperture. Some have been back several times, not because collimation was off, but to make further adjustments of their focal lengths or mechanics (after which collimation was required). All perform as should. One of the scopes was altered to have a larger back-focus distance, so that may have been the case with your scope as well.

Main importance is to have the focuser disconnected from the primary mirror, but achieving that may require quite a bit of machining. Another point is to adjust the mirrors by only using two of the three screws as in that way you avoid that the mirror-distance changes in the course of several collimations. Best is to paste a small sticker (piece of tape) over the screw that serves as reference.

If you want to get the focal length right, it is of key importance to know the design value for your scope. The Ronchi-test can be of assistance, but is not very sensitive, so may result in a focal length being off by quite a few millimetres (but then, if the Ronchi-test does not detect aberration, then it will not affect imaging).

Nicolàs

Thanks Nicolas, 

I've had quite a bit of advice and several different techniques have been shown to me. I'm planning on using bits of different methods ( some good advice in your thread thanks) to suit the equipment I already have. If this fails I intend to take it to someone who knows how to correct this properly. 

Ron