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Collimating a GSO RC6 - Issues!


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I have been trying to collimate my RC6 using stars and a variation of the DSI method and using a REEGO Pro tool (similar to the TS LED Collimator).

When I look into the collimation tool it looks absolutely bang on, even then I would expect to have to do some minor tweaks on a star..

But something is very wrong, and not just small tweaks I think.

 

Camera is a AA 1600m and a TS CCD47 reducer at approximately 85mm of backfocus.

 

field.png.67cb9266ab25d2a989388ed6a95c8395.png

This is the edges of my field, when the large star in the middle is in focus (used bhatinov mask), NOT GOOD.

 

740063193_focusin.png.fee408fa34e6ffec1141fb6807c9c3d4.png

This is a star defocused INSIDE of focus, looks OK I think (except some specks on the mirror).

1504160100_focusout.png.7348e1f39c16d472bf58d555bbe56341.png

This is the same star defocused OUTSIDE of focus, this is much more worse..

 

Does anyone know if the fact that the star shape is altered so much is due to some sort of image train shift / sagging of the focuser?

 

Any input is much appreciated, this thing is really driving me nuts!!

 

Edited by jjosefsen
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  • jjosefsen changed the title to Collimating a GSO RC6 - Issues!
4 minutes ago, vlaiv said:

First step would be to loose focal reducer.

It could be some silly misalignment in reducer lens or reducer not being square with focuser or whatever.

You want to take that variable out of the equation.

I had the same issue with an Astro-Tech Field Flattener, but I guess the same point remains - the FF could be faulty.

Should be clear tomorrow, I will try again without anything else in there.

I am wondering if maybe it could also be that the focuser is just not square to the primary mirror, and I would need a tilt plate adapter.

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10 minutes ago, jjosefsen said:

I am wondering if maybe it could also be that the focuser is just not square to the primary mirror, and I would need a tilt plate adapter.

I'm not entirely sure if angled focuser will present itself like above - one side of focus ok and other not concentric.

What could happen is focuser tube sag.

Good image is with focuser tube moved inside focuser - and bad image is with focuser tube drawn outside, right?

Maybe it is focuser tube that is tilting with respect to focuser when you rack it too far out. Do you have any sort of 2" extension like this one:

https://www.firstlightoptics.com/adapters/astro-essentials-2-35mm-focus-extension-tube.html

that you can use to get out focus image - without racking focuser that much out?

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24 minutes ago, vlaiv said:

I'm not entirely sure if angled focuser will present itself like above - one side of focus ok and other not concentric.

What could happen is focuser tube sag.

Good image is with focuser tube moved inside focuser - and bad image is with focuser tube drawn outside, right?

Maybe it is focuser tube that is tilting with respect to focuser when you rack it too far out. Do you have any sort of 2" extension like this one:

https://www.firstlightoptics.com/adapters/astro-essentials-2-35mm-focus-extension-tube.html

that you can use to get out focus image - without racking focuser that much out?

Yes good is inside, bad is outside.. So sagging makes sense as a possible culprit.

Checking the outside focus with extension tubes is doable I think, will give that a go.

Thanks for the feedback.

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8 minutes ago, Adreneline said:

My GSO 6" RC suffered vey badly from droop in the focus unit. I replaced it with a Baader Diamond Steeltrack and it made a huge difference.

Adrian

It could be yeah... I am just hessitant to throw more money at this scope than I already have.. ;)

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1 hour ago, jjosefsen said:

I have been trying to collimate my RC6 using stars and a variation of the DSI method and using a REEGO Pro tool (similar to the TS LED Collimator).

When I look into the collimation tool it looks absolutely bang on, even then I would expect to have to do some minor tweaks on a star..

But something is very wrong, and not just small tweaks I think.

 

Camera is a AA 1600m and a TS CCD47 reducer at approximately 85mm of backfocus.

 

field.png.67cb9266ab25d2a989388ed6a95c8395.png

This is the edges of my field, when the large star in the middle is in focus (used bhatinov mask), NOT GOOD.

 

740063193_focusin.png.fee408fa34e6ffec1141fb6807c9c3d4.png

This is a star defocused INSIDE of focus, looks OK I think (except some specks on the mirror).

1504160100_focusout.png.7348e1f39c16d472bf58d555bbe56341.png

This is the same star defocused OUTSIDE of focus, this is much more worse..

 

Does anyone know if the fact that the star shape is altered so much is due to some sort of image train shift / sagging of the focuser?

 

Any input is much appreciated, this thing is really driving me nuts!!

 

I'm reading this as at some stage I want to buy a RC but I'm almost scared to!

With my sct I get a  different shift in the doughnut either side of focus but that's the primary shifting.. Rc have them fixed don't they?

 

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So I tried something else just now..

Used a laser to make sure the focuser was pointing straight at the center of the secondary mirror, and rotating the laser to make sure it still pointing straight at the secondary dot.

When I then checked the collimation with my REEGO, and even jsut looking into the focuser it is obvious... But circle that you are supposed to see around the primary mirror, that has to be concentric or equally tick all the way around is completely misaligned.

Does this mean that I have a misaligned focuser? And would the tilt aadapter work in this scenario?

Edited by jjosefsen
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4 minutes ago, jjosefsen said:

Does this mean that I have a misaligned focuser? And would the tilt aadapter work in this scenario?

If you fix your primary and dot from laser is not in center after that - then yes, it is down to focuser (I'd try several focuser positions to see if it makes a difference in where laser ends up - even without fixing primary).

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21 hours ago, Adreneline said:

My GSO 6" RC suffered vey badly from droop in the focus unit. I replaced it with a Baader Diamond Steeltrack and it made a huge difference.

Adrian

Hi,

I was looking at a BDS focuser,but they don't make one with a M90 flange as such..

But for thir refractor version of the BDS it is possible to buy an adapter for Skywatcher Refractors with M90 connection to scope, is this what you did?

Adapter: https://www.baader-planetarium.com/en/rt-adapter-m90x1.5-diamond-steeltrack-(skywatcher).html

Focuser: https://www.baader-planetarium.com/en/2"-bds-rt-baader-diamond-steeltrack.html

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Hi.

That looks like what I ordered:

https://www.firstlightoptics.com/baader-diamond-steeltrack-focusers/baader-steeltrack-diamond-rt-adapter-m90x1-for-refractors-gso-rc-scopes.html

Like you, I could have done without the extra expense but trying to compensate for the droop was like trying to solve the wrong problem and constantly chasing my tail. I'd fix one problem and create another.

One word of warning! With the Baader fitted I've found it very easy to find focus with my ASI294MC and my Canon M6 MkII with the supplied spacers. I don't use the RC for narrow band.

However, on the odd occasion I want to use an eye piece to allow friends/family to do a bit of stargazing, just using a SW 28mm eye piece, there is no way I can achieve focus with the Baader as the drawer tube is too long. I have to refit the stock focuser. It's a minor thing but just thought I'd mention it.

HTH

Adrian

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3 minutes ago, Adreneline said:

Hi.

That looks like what I ordered:

https://www.firstlightoptics.com/baader-diamond-steeltrack-focusers/baader-steeltrack-diamond-rt-adapter-m90x1-for-refractors-gso-rc-scopes.html

Like you, I could have done without the extra expense but trying to compensate for the droop was like trying to solve the wrong problem and constantly chasing my tail. I'd fix one problem and create another.

One word of warning! With the Baader fitted I've found it very easy to find focus with my ASI294MC and my Canon M6 MkII with the supplied spacers. I don't use the RC for narrow band.

However, on the odd occasion I want to use an eye piece to allow friends/family to do a bit of stargazing, just using a SW 28mm eye piece, there is no way I can achieve focus with the Baader as the drawer tube is too long. I have to refit the stock focuser. It's a minor thing but just thought I'd mention it.

HTH

Adrian

Thank you, and thanks for the headsup.

If I may I have one more question.. :)

Does it have built in collimation? It doesn't look like it, but wanted to be sure. :)

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2 minutes ago, jjosefsen said:

Does it have built in collimation? It doesn't look like it, but wanted to be sure

I'm not sure I totally understand the question, however, it is possible to compensate for out of parallel / not orthogonal because the Baader has a built in tilt adjustment. I am pleased - very pleased! - I have not had to use this feature.

Apologies if I have misunderstood your question.

Adrian

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3 hours ago, Adreneline said:

I'm not sure I totally understand the question, however, it is possible to compensate for out of parallel / not orthogonal because the Baader has a built in tilt adjustment. I am pleased - very pleased! - I have not had to use this feature.

Apologies if I have misunderstood your question.

Adrian

No that was exactly what I was trying to ask :) So thanks. :)

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So I had some real progress last night..

Having aligned the focuser to the secondary yielded the best stars I have had to date on this thing..

So the optical axis between the primary and secondary mirror is quite a bit out of alignment right now, but apparently having alignment between the focuser and secondary is "more important". That is a bit of simplification obviously, but seems correct in my instance.

I have ordered a tilt plate adapter for M90 connection, this will allow me to align the primary to the secondary again, and then use the tilt plate to align the focuser.

Hopefully this will sort things out.

 

With the reducer in the imaging train, my focus point is quite close to the scope, I don't have to extend the focuser much, so I think I can get away with the stock focuser for now. But upgrading that will definitely be on the todo list if I can get this thing humming to my liking. :)

I also fine tuned the distance between the mirrors further, and it was a massive improvement to focus and led to much tighter stars across the entire field.

Edited by jjosefsen
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Yes, sorry, Steve. Here goes:

I think the OP has two problems: his RC6 is out of collimation and there seems to be significant tilt between the pointing of the primary mirror and the optical train. It will not be possible to properly collimate the scope with any device that is inserted into the focuser. Any attempt to correct the tilt due to the misalignment will result in a scope with what I term "squinted collimation": the mirrors will be parallel (at best) but the optical axes of the mirrors will not be aligned. The way to proceed is to remove the optical train completely and collimate the primary and secondary mirrors on their own on the bench as described here:

https://www.dropbox.com/s/avpu2vn6s3ynsz5/Collimating GSO Ritchey Chretien with a plastic disc V2.pdf?dl=0

These notes deal with the RC8 but there is an article on the specific collimation of the RC6 in this issue of my club newsletter:

https://www.dropbox.com/s/ni0n7yvsfrwis40/September 2020 Newsletter.pdf?dl=0

Once close collimation of the primary and secondary mirrors has been achieved on the bench, it remains to test the scope on a star test to eliminate any residual coma. This should be done on a star in the centre of the field of view. The scope should be just a smidge out of focus 'out' not a huge doughnut. Here is an image from a star test on my RC8.

image.png.927b40b7a0424265cea6966b0cef15a3.png

The focuser is around 100 counts (0.25 mm) out of focus out. The blue lines are part of the centre mark on the N.I.N.A. image viewer. Note the central Poison spot. I adjust the primary mirror to get the Poison spot central. The required adjustments to the primary should be very slight indeed.

The secondary should not be adjusted at this stage. Check the symmetry of how the stars look in the corners. They should look the same in each corner. 

The methods I've described are due to Es Reid, by the way. The method I've described is based on the techniques he uses in his commercial work. 

As to the misalignment of the optical train with the primary mirror: I think this is a common problem with the GSO designed RC6 and RC8. On my RC8, the primary mirror is secured to the primary support structure with a ring clamp and an O-ring between the clamp and the glass.

I had a huge misalignment on my RC8 and I found it to be due to a perished O-ring, so I replaced it. To inspect the O-ring does need you to dismantle the primary mirror from its holder but its not a difficult job if done with care. While you have the primary mirror in your hand, you could paint the edge with black acrylic paint (Warhammer Chaos Black worked well) to hide the rough edge of the glass and the marks of the clamp that held the mirror while it was silvered. Doing this will really tidy up the halo around your star images.

Do not over-tighten the ring clamp. You'll stress the mirror.

Finally, you can use the tilt focuser to remove any remaining tilt between the focuser and the secondary. I do have a bit of residual tilt on my scope but I don't correct for it. My images seem fine without it.

By the way, if I put a laser on my scope it says it is out of collimation!! 

Sorry for my reticence in posting on the open forum, but I've said all this (mostly) before but forums seem to have short memories. The information is here if people would search for it.

 

David

 

 

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On 26/11/2021 at 11:37, davies07 said:

Yes, sorry, Steve. Here goes:

I think the OP has two problems: his RC6 is out of collimation and there seems to be significant tilt between the pointing of the primary mirror and the optical train. It will not be possible to properly collimate the scope with any device that is inserted into the focuser. Any attempt to correct the tilt due to the misalignment will result in a scope with what I term "squinted collimation": the mirrors will be parallel (at best) but the optical axes of the mirrors will not be aligned. The way to proceed is to remove the optical train completely and collimate the primary and secondary mirrors on their own on the bench as described here:

https://www.dropbox.com/s/avpu2vn6s3ynsz5/Collimating GSO Ritchey Chretien with a plastic disc V2.pdf?dl=0

These notes deal with the RC8 but there is an article on the specific collimation of the RC6 in this issue of my club newsletter:

https://www.dropbox.com/s/ni0n7yvsfrwis40/September 2020 Newsletter.pdf?dl=0

Once close collimation of the primary and secondary mirrors has been achieved on the bench, it remains to test the scope on a star test to eliminate any residual coma. This should be done on a star in the centre of the field of view. The scope should be just a smidge out of focus 'out' not a huge doughnut. Here is an image from a star test on my RC8.

image.png.927b40b7a0424265cea6966b0cef15a3.png

The focuser is around 100 counts (0.25 mm) out of focus out. The blue lines are part of the centre mark on the N.I.N.A. image viewer. Note the central Poison spot. I adjust the primary mirror to get the Poison spot central. The required adjustments to the primary should be very slight indeed.

The secondary should not be adjusted at this stage. Check the symmetry of how the stars look in the corners. They should look the same in each corner. 

The methods I've described are due to Es Reid, by the way. The method I've described is based on the techniques he uses in his commercial work. 

As to the misalignment of the optical train with the primary mirror: I think this is a common problem with the GSO designed RC6 and RC8. On my RC8, the primary mirror is secured to the primary support structure with a ring clamp and an O-ring between the clamp and the glass.

I had a huge misalignment on my RC8 and I found it to be due to a perished O-ring, so I replaced it. To inspect the O-ring does need you to dismantle the primary mirror from its holder but its not a difficult job if done with care. While you have the primary mirror in your hand, you could paint the edge with black acrylic paint (Warhammer Chaos Black worked well) to hide the rough edge of the glass and the marks of the clamp that held the mirror while it was silvered. Doing this will really tidy up the halo around your star images.

Do not over-tighten the ring clamp. You'll stress the mirror.

Finally, you can use the tilt focuser to remove any remaining tilt between the focuser and the secondary. I do have a bit of residual tilt on my scope but I don't correct for it. My images seem fine without it.

By the way, if I put a laser on my scope it says it is out of collimation!! 

Sorry for my reticence in posting on the open forum, but I've said all this (mostly) before but forums seem to have short memories. The information is here if people would search for it.

 

David

 

 

Thank you so much for this, very well written information, it is a very interesting read.

I am currently visiting family and I only have my phone, so will take another look at this when I get home to my PC.

I really hope I can get this thing working well, as I plan to upgrade it to an 8" version at some point, maybe when I finally get around to building an observatory.

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On 26/11/2021 at 11:44, FLO said:

@jjosefsen  Was your RC6 purchased from us?  

If yes then we'll happily put it on the optical bench and check it for you. 

No, this is a second hand scope I found locally.

Unfortunately my favourite astro shop (you) is suddenly very far away due to taxes and stuff. 🥺

Edited by jjosefsen
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23 minutes ago, jjosefsen said:

I really hope I can get this thing working well, as I plan to upgrade it to an 8" version at some point, maybe when I finally get around to building an observatory.

I followed the really good advice from David when I was trying to collimate my RC8. I have developed my own method (albeit very similar to that described by David but using the REEGO (TSRCKOLLI). Follow the guidance carefully and you will get there. It is worth the effort as the RC's are excellent for AP.

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46 minutes ago, Clarkey said:

I followed the really good advice from David when I was trying to collimate my RC8. I have developed my own method (albeit very similar to that described by David but using the REEGO (TSRCKOLLI). Follow the guidance carefully and you will get there. It is worth the effort as the RC's are excellent for AP.

I have the REEGO and their new reflector module MoR.

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