79 replies to this topic

[pterodyne]

Yes, thank everyone who has contributed to this thread.  Likely it will help others in a similar situation.

 

My remote hands person (Ernest) is next going to loosen the retention screws on the primary and Ill record another focus sequence.  When we last checked on the collimation the retention screws were loose, even though they were tight the prior time so he tightened them, possibly too much.  This must be due to thermal variations between the two attempts.  By loosen, I just mean back them off, and then tighten slightly.

[Timo I]

Some additional thoughts on the tightness of those screws from my side. In my 10" RCT the locking screws were always very tight so that the collimation screws were tightly pressed against the primary mirror cell's base. No looseness there. What I try to say here is that kind of GSO RCT scope will keep its' collimation very well, when those collimation screws have been locked properly down. Please remember, that it's quite possible if those collimation screws for your primary mirror could be maybe even 1/4 turn away from the correct setting. It all depends on the previous owner's actions on that scope. How much adjustments he has done for the primary cell?

 

When I have told about the very-very narrow tolerences for the correct collimation, it is 100% true when you are near the perfect collimation point. But based on that video images, your scope's collimation is now way off from the correct collimation and that requires bigger collimation adjustments. Don't be too shy doing those adjustments as long as you have good control over what you are doing there

From my view it looks like your current collimation setting can be so much "off the grid", that it really demands that kind of 1/4 turn adjustment(s) or more (!) for your primary mirror to begin with. You just need to iterate there with those adjustment(s) how much collimation adjustment is actually needed there. So please make a well controlled adjustment for one of the most appropriate collimation screw there (=tighten or loosen that specific collimation screw with some amount, maybe 1/4 turn + lock it down) and then take another test image. Judge from the test image's unfocussed star shapes, if the stars are getting better "balanced" than before with that latest adjustment? If not, then you need to turn the direction around and back off with your adjustment. Or then you might need to continue turning the same collimation screw into the same direction (maybe 1/8... 1/16 turns more), because the adjustment was not enough on the first attempt. You will know that by the look of those stars!

 

It will be a process of taking test images and making (small) controlled adjustments for one collimation screw at a time. Paper and pencil might be useful there for recording what you have done there. (I have been awake doing those adjustments at 3 AM, when your sleepy mind starts to get too hazy to remember what you have done last and if that was a move to the worse or better ) But whenever you get there (=near the 100% correct collimation point), then you must minimize your adjustments you are making for those screws. You will notice then that the RCT hyperbolic mirrors start to "reject each other" (just like two similar magnetic poles), when their alignment starts approaching that nearly 100% accurate collimation point. The nearer you come to the absolutely perfect collimation point, the more difficult it will be making such small adjustments, which are just required image-wise. There you will notice how narrow the adjustment tolerance is there, but not before that. Good luck there!

[pterodyne]

Just bumping my own thread here.  Still freezing in Park County Colorado.  Hopefully Ill be doing an under the stars collimation via DSI method next month.

[chris3c273chris]

Interested to hear how it goes. Good luck.

[Mark326]

This is an excellent thread and very timely for my own current situation.    

 

Having noticed my star centers were slightly off across entire field of view, left side heavy. I decide the primary mirror needed a tweak, well long story short the RC scope is now horribly out of collimation. Weather has not cooperated in getting back out under the stars, reading that DSI guide though outlines the steps well. Confident this will get me back up and running.

 

Havent received the Glatter I ordered with holographic attachment yet. Would like some accurate way to do this in daylight on a test bench. Will see what comes first, a run at it with laser or a clear night.

[Mil.Dave]

Hi Timo, et al,

 

I recently upgraded from a rather old 8" SCT to a new TS (GSO) 10" RC-Truss telescope, and am in the process of undertaking collimation using the DSI method. One piece of data that I've not been able to find, however, is the correct spacing of the mirrors. I will be taking test images for plate solving to determine what the inter-mirror spacing is at the moment but obviously need to know what it should be. Going by the suppliers websites, they simply state the focal length of 2000 mm and a back focus distance of 235 or 250 mm depending on which site one looks at?

 

If you, or any other group member, are able to provide the actual distance required between the mirrors I'd be most grateful.

 

Kind regards,

Dave

[pterodyne]

When I had the 10" (steel tube version), it always plate solved between 1998 and 2002 depending on temperature.  I would not adjust the mirror spacing unless you have reason to believe it's off.  In my case, I was plate solving at 2815 or so on a scope that should be 2850.  This is way off.  And because my V-curves had a definite bottom (however POOR) it was inferred that the distance between the two mirrors was off.  The scope was reaching a "best" focus so my backfocus spacing was correct.  Conversely, had I simply added more back focus using the rings, the scope would not have had a bottom to the V-curve and therefore not focusing.

 

If someone sees a flaw in that logic, please tell me!  Either way the aberrations I'm seeing aren't spherical so I don't think spacing is my primary issue, although it was AN issue.

[Mil.Dave]

Hi Pterodyne,

 

Thanks for your measurements and supporting information. I'll keep your advice my mind.

[Timo I]

Regarding the distance between mirrors of a RCT scope.

This web blog has the most accurate data on the distance between mirrors, which I have been able to find out from the Internet:

http://interferomete...retien-gso.html

Teleskop Austria's blog tells this:

 

Like all Cassegrain Systems, in the Ritchey-Chrétien ist the spherical aberration is dependent on the distance primary to secondary, there is only one optimal distance. At the same time the backfocus (distance focal plane to tube) also changes with that distance. If the secondary is brought nearer to the primary, for each 1mm the backfocus increases 10mm.  Sometimes more backfocus is needed, so how changes the spherical aberration respectively? To get an idea about that I measured some primary-secondary distances, and just read the terms for spherical aberration alone to avoid influence from test impurities. Here are the resulting Strehl values, distances given are from secondary holder to spider (means I changed the secondary only, inverse to mirror distances):
6mm - 99%
8mm - 99%
9,5mm - 98%
12mm - 93%
14mm - 79%
With decreasing distance secondary to primary, the undercorrection increases. Interesting, there ist no linear behaviour, first it goes slightly off, then rapid. If you ever need to adjust the distance, I advice to use a Ronchi grating, as zone errors can be deceiptive if you plan to use the star test. They are less careful in polishing under the secondary! Btw: At delivery the scope came with correct distance, just the collimation was fairly off, so it performed only around the diffraction limit. I also remember that I rotated the secondary to get rid af some astigmatism when collimation was otherwise perfect.

 

And I have confirmed that to be true with own previous 10" RCT scope too, but with a more experimental way (with the usage of CCD Inspector software).

 

Here's status for "somewhat correctly" collimated 10" RCT, when mirrors are too far apart from each other:

https://astrokuva.ga...90_measured.jpg

Star fields for a similar (but not the same) images (sorry):

https://astrokuva.ga...CCDImage191.jpg

https://astrokuva.ga...ure_problem.jpg

Not that perfect star shapes there... eh, but over then (14-10-2015) my collimation accuracy was not that great either?

 

Then here's image change with appox. 1...1,5 mm (0,03937 to 0.0590551 inches) closer distance between those mirrors:

https://astrokuva.ga...easurements.jpg

Field had "flattened" there...

 

NOTE: Here's CCDI:s definition for "curvature" in its' 3D Curvature Plot Viewer: (its' not actual field curvature we're talking here, but different aberration amounts)

CCDInspector extracts thousands of stars from each image and computes their FWHM.

Then, a polynomial function is fitted to the distribution of FWHM values. This is then plotted on a 3-D surface, with varying colors, as follows:

Black:    lowest FWHM
Blue:      slightly defocused
Green:    more defocused
Red:       highest defocus

 

And here's some warning for the above test too.

That curvature measurement in CCDI changes also with the collimation accuracy of those RCT mirrors!

I know that CCDI's 3-D curvature plot flattens out, when collimation accuracy for RCT mirrors gets better and better.

 

Here's just some comparison with that: (15-10-2015)

https://astrokuva.ga..._comparison.jpg

https://astrokuva.ga...CCDImage285.jpg

(star field is from the middle image there)

 

After some time (29-03-2016) I took once again the distance of those mirrors into a closer inspection with more "managed" results.

I wrote over then this story into our Finnish astronomy forum:

https://www.avaruus....41722#msg141722

(You can click on the images to get full size images and watch out how star field changes a bit in the image corners.)

 

There I presented how the distance change between mirrors was affecting to the CCDI's 3-D curvature plot, when about one total turn for those secondary mirror collimation screws was done (and then secondary mirror was moved closer to the primary mirror by that one-screw-turn amount). The collimation accuracy was kept about the same, while doing that mirror distance shift. I loosened central locking screw a bit, then turned all three collimation screws 90 degrees each. Took collimation images and collimated the view. Then again loosened the central screw and turned collimation screws 90 degrees, collimated again. etc. etc. until full turn was done. If you scroll that message thread lower you can read about the 04-04-2016 update for further (minor) distance adjustment done for the mirror distance. You can watch there, how the star images got better and collimation accurary was getting nearly perfect. That's a long-time process there with my RCT collimation, but just like they say in that Teleskop Austria's blog there is only one optimal distance for this...  

[David07]

Hi guys,

 

In my experience, the collimation of the GSO RCs, that is the design where the focuser extension tube is attached to the primary mirror support, is extraordinarily difficult. Any method based on using a laser or Cheshire inserted into the focuser is immediately compromised because of the lack of a fixed reference in the telescope design. There are unknown alignment errors between the primary mirror and it’s holder and the mirror holder and the focuser. Moreover, moving the primary mirror, when collimating the scope, also changes the pointing direction of the focuser. I spent 18 months fiddling about with my RC8 until I read the Austria Teleskop blog and that gave me the idea for the collimation method I now use. 

 

I began by taking the scope to an optician friend with an optical bench and artificial star.  The first discovery was that after I had spent weeks of adjusting the secondary to primary separation to get the specified focal length of 1624mm, a Ronchi grating test quickly revealed that the scope was overcorrected: the mirrors were too far apart. We adjusted the mirror separation until we got a properly corrected image. The focal length was 1660mm, as it still is. So problem number one, the actual focal length might not be as advertised. 

 

To collimate the scope:

 

With the scope on a bench, I removed the focuser and extension tubes. I then measured, with a calliper, the inside diameter of the central hole in the primary mirror holder. I cut a piece a styrene about 2mm thick to exactly fit the hole and drilled a 1mm hole in its centre. Now gently fit the styrene disc into the central hole of the primary mirror holder.

 

Next, mark the position of the central screw holding the secondary mirror. A little dab of white acrylic paint, for example. Also, mark the side of the secondary boss and mirror holder. This will enable you to replace the secondary exactly in the correct place and not alter the scope focal length. 

 

Carefully unscrew the secondary mirror central screw, holding the secondary mirror. Count the number of turns to release the screw and note the number. Don’t touch the three collimation screws. 

 

Place a light behind the telescope and looking from the front of the scope, sight the hole in the centre of the styrene disc though the centre of the hole that the secondary securing screw came from. Now adjust the primary mirror so that the reflections of the secondary support vanes coincide with the support vanes themselves. You should see a tiny spot of light in the centre of the secondary screw hole and the reflections of the support vanes will be hidden behind the vanes themselves. 

 

The primary mirror is now aligned with the secondary holder. 

 

Replace the secondary mirror. Count the number of turns and align the paint marks. 

 

Next, reach into the scope tube and unscrew the baffle tube. Let it rest on the inside of the tube. 

 

Now bring the lamp to the front of the tube and set it to shine onto the styrene disc. 

 

Go go to the back of the scope and look through the small hole on the centre of the styrene disc. You should see the central alignment ring on the secondary. Adjust the secondary until the reflection of the hole in the centre of the styrene disc is centred in the secondary alignment ring. 

 

The secondary is now pointing at the centre of the primary mirror.

 

Replace the baffle tube. Replace the focuser etc. Sell your laser collimator. 

 

Check the collimation on a star in the centre of the field of view. It should be really close. 

 

On a night of good seeing I get HFD readings of 1.6 to 1.7 arc seconds at focus. Previously I couldn’t get below 2.5 arc seconds. I don’t use a tilt plate and I’m pleased with the images I get.

 

Hope this helps,

 

David

[akulapanam]

Hi guys,

In my experience, the collimation of the GSO RCs, that is the design where the focuser extension tube is attached to the primary mirror support, is extraordinarily difficult. Any method based on using a laser or Cheshire inserted into the focuser is immediately compromised because of the lack of a fixed reference in the telescope design. There are unknown alignment errors between the primary mirror and it’s holder and the mirror holder and the focuser. Moreover, moving the primary mirror, when collimating the scope, also changes the pointing direction of the focuser. I spent 18 months fiddling about with my RC8 until I read the Austria Teleskop blog and that gave me the idea for the collimation method I now use.

I began by taking the scope to an optician friend with an optical bench and artificial star. The first discovery was that after I had spent weeks of adjusting the secondary to primary separation to get the specified focal length of 1624mm, a Ronchi grating test quickly revealed that the scope was overcorrected: the mirrors were too far apart. We adjusted the mirror separation until we got a properly corrected image. The focal length was 1660mm, as it still is. So problem number one, the actual focal length might not be as advertised.

To collimate the scope:

With the scope on a bench, I removed the focuser and extension tubes. I then measured, with a calliper, the inside diameter of the central hole in the primary mirror holder. I cut a piece a styrene about 2mm thick to exactly fit the hole and drilled a 1mm hole in its centre. Now gently fit the styrene disc into the central hole of the primary mirror holder.

Next, mark the position of the central screw holding the secondary mirror. A little dab of white acrylic paint, for example. Also, mark the side of the secondary boss and mirror holder. This will enable you to replace the secondary exactly in the correct place and not alter the scope focal length.

Carefully unscrew the secondary mirror central screw, holding the secondary mirror. Count the number of turns to release the screw and note the number. Don’t touch the three collimation screws.

Place a light behind the telescope and looking from the front of the scope, sight the hole in the centre of the styrene disc though the centre of the hole that the secondary securing screw came from. Now adjust the primary mirror so that the reflections of the secondary support vanes coincide with the support vanes themselves. You should see a tiny spot of light in the centre of the secondary screw hole and the reflections of the support vanes will be hidden behind the vanes themselves.

The primary mirror is now aligned with the secondary holder.

Replace the secondary mirror. Count the number of turns and align the paint marks.

Next, reach into the scope tube and unscrew the baffle tube. Let it rest on the inside of the tube.

Now bring the lamp to the front of the tube and set it to shine onto the styrene disc.

Go go to the back of the scope and look through the small hole on the centre of the styrene disc. You should see the central alignment ring on the secondary. Adjust the secondary until the reflection of the hole in the centre of the styrene disc is centred in the secondary alignment ring.

The secondary is now pointing at the centre of the primary mirror.

Replace the baffle tube. Replace the focuser etc. Sell your laser collimator.

Check the collimation on a star in the centre of the field of view. It should be really close.

On a night of good seeing I get HFD readings of 1.6 to 1.7 arc seconds at focus. Previously I couldn’t get below 2.5 arc seconds. I don’t use a tilt plate and I’m pleased with the images I get.

Hope this helps,

David

Agree with the focuser connected to mirror on non 2nd gen + RC being an issue and the potential mirror misspacing BUT that is a pretty extreme method to collimate an RC.

The steps really are as simple as:
-Laser in focuser to verify focuser mechanically aligned with secondary
-Tak scope to adjust secondary tilt
-collimate on a star to adjust primary

[David07]

Agree with the focuser connected to mirror on non 2nd gen + RC being an issue and the potential mirror misspacing BUT that is a pretty extreme method to collimate an RC.

The steps really are as simple as:
-Laser in focuser to verify focuser mechanically aligned with secondary
-Tak scope to adjust secondary tilt
-collimate on a star to adjust primary

Yes, I would agree that these steps will achieve collimation of an RC where the focuser is attached to the back plate of the scope. But in the cheaper GSO scopes, where the focuser is attached to the primary mirror support, the method fails at step 3 because adjusting the primary mirror when collimating on a star will also move the focuser and undo its alignment with the secondary mirror.

 

The best you can achieve with this method is what I term a 'squinted collimation'. I've tried to show it in the diagram, below:

 

 

The focuser extension tube is attached to the primary mirror support and has a small pointing error. You can adjust the secondary mirror to centre the laser spot, but the laser beam is now at some angle to the tube axis. Later, when you adjust the primary mirror on a star, the best you can achieve is to have the mirrors parallel - so the 'hall of mirrors' effect will look OK, but the optical axes of the primary and secondary mirrors will be parallel, at best, not colinear. So the scope is not collimated - and never can be with this setup. 

 

My method has the benefit that the focuser and its pointing error plays no part in the collimation procedure. You are simply dealing with the primary and secondary mirrors. I'll post a few photographs showing how I did it.

[David07]

Here are a few photos showing the collimation procedure I described.

 

 

This is the plastic (styrene) disc I made to collimate my GSO RC8. It is 57.5mm in diameter.

 

 

Here is the plastic disc inserted into the back of the scope. The hole in the centre is just about in the centre of the primary mirror and provides a reference point.

 

 

Here are the markings on the secondary mirror mount to ensure I put it back in the right place. I made the markings before discovering the focal length issue so they are not in the right place. I take a photo of the setup to ensure I get it back to the right place.

 

 

More in the next post.

 

 

 

 

  

 

 

[David07]

Here is the view from the front of the scope.

 

 

The bright dot near the centre of the secondary mirror mounting hole is the lamp shining through the hole in the plastic disc. You can just about make out the reflections of the secondary support vanes in the primary mirror. The vanes and their reflection need to line up while the spot of light is dead centre in the secondary mirror hole. Once done, the primary mirror is pointing square-on to the secondary support vanes.

 

I've removed the baffle tube and here is the view through the peephole in the plastic disc, looking toward the secondary mirror. Note the lamp is now shining onto the plastic disc from the front - you can just see it, top right.

 

 

You can just see the reflection of the hole in the plastic disc. Adjust the secondary to centre reflection in the secondary mirror adjustment ring. Job done.

 

Now check it on the sky.

 

Adjusting the two mirrors are independent operations, there is no iteration needed.

 

A note on putting the baffle tube back: be careful not to touch the mirror with the tube. Locate the tube onto the thread on the primary mirror and unscrew it until you hear two clicks as the threads engage. Now carefully screw it in and not too tightly. 

Edited by David07, 18 March 2019 - 01:12 PM.

[David07]

By the way, I mentioned that I spent some time measuring the focal length of my RC8. I did this by taking an image of an open cluster and then comparing it with a star chart and field of view markings produced by the Cartes du Ciel (Skychart) software. I overlaid the image and chart in Photoshop to get a match. It all took a lot of time and I then discovered that PixInsight offers an image analysis facility under Script/Image Analysis/Image Solver which saves a lot of work.

 

This is the graph of scope focal length vs the number of turns of the secondary holder screw that I developed for my RC8.

 

 

In its initial state, the secondary screw had 8 turns on it, corresponding to a focal length of 1628mm but was over-corrected at this focal length. It is now sitting at 1663mm focal length where I get a fully corrected image. The secondary holding screw moves the mirror 1mm per turn. So the ratio of mirror movement to change of focal length is nearly 20:1. 

 

Note: the actual number of secondary screw turns required for a particular focal length depends on where the primary mirror is. I keep my primary mirror close to its support, so I have just a couple of turns on the primary collimation screw from the fully screwed in position. This is to keep as little screw thread as possible exposed and carrying the primary mirror. The reason is that on the cheaper GSO scopes, these three little screws are also carrying the focuser tube, focuser and imaging system. The whole setup will flex if you load too much onto the focuser and have the primary mirror hanging out on the three collimation screws.

Edited by David07, 18 March 2019 - 01:08 PM.

[Timo I]

Yes, I would agree that these steps will achieve collimation of an RC where the focuser is attached to the back plate of the scope. But in the cheaper GSO scopes, where the focuser is attached to the primary mirror support, the method fails at step 3 because adjusting the primary mirror when collimating on a star will also move the focuser and undo its alignment with the secondary mirror.

The best you can achieve with this method is what I term a 'squinted collimation'. I've tried to show it in the diagram, below:

 

Words of wisdom there!

After that I should tell you that my 10" GSO RCT scope had been modified quite early on with this kind of mod:

https://astrokuva.ga...user_tuneup.jpg

https://astrokuva.ga...er_tuneup_2.jpg

As a consequence please be reminded, that I have had my own fair share of these collimation problems due to this 'squinted collimation' David describes there. (That kind of home made ATM work in the back of such extremely difficult RCT optics is 100% sure to give its own set of mechanical errors in the alignment for RCT mirrors. It took very long from myself to find out that kind of info over then and I think I never got 100% over with it in my 10" RCT scope so there was some slight mechanical error, which remained in that scope when I sold that.)

So thanks David, that kind of info is much appreciated here in this thread!

 

GSO engineers should have originally designed their RCT scope without that fundamental flaw in that "focuser fixed with main mirror cell" design in those RCT scopes! That kind of major design error has caused myself to despise almost anything Made in China now.

Edited by Timo I, 18 March 2019 - 10:50 AM.

[David07]

Words of wisdom there!

After that I should tell you that my 10" GSO RCT scope had been modified quite early on with this kind of mod:

https://astrokuva.ga...user_tuneup.jpg

https://astrokuva.ga...er_tuneup_2.jpg

As a consequence please be reminded, that I have had my own fair share of these collimation problems due to this 'squinted collimation' David describes there. (That kind of home made ATM work in the back of such extremely difficult RCT optics is 100% sure to give its own set of mechanical errors in the alignment for RCT mirrors. It took very long from myself to find out that kind of info over then and I think I never got 100% over with it in my 10" RCT scope so there was some slight mechanical error, which remained in that scope when I sold that.)

So thanks David, that kind of info is much appreciated here in this thread!

 

GSO engineers should have originally designed their RCT scope without that fundamental flaw in that "focuser fixed with main mirror cell" design in those RCT scopes! That kind of major design error has caused myself to despise almost anything Made in China now.

That is a brilliant mod. Wow! I wondered if there was an easy way to modify the back end to fix the focuser to the back plate. I think you have it.

[jkobservatory.net]

If anyone would like to have a go at Collimation on my 14GSO via teamviewer hook me up. I'll pay £50 for time



Sent from my SM-G975F using Tapatalk

[David07]

Jk,

 

i have sent you a PM with a suggested way ahead. 

 

David

[rockstarbill]

OP -- what was the end result of this work?

[pterodyne]

Sadly, my offgrid setup is completely down.  Im just now in Colorado (from Baja California Sur) to be able to work on it.  Im having to replace my 500Ah battery, going LiFePo4 this time (lithium batteries) so hopefully it will last for 10 years! 

 

As to the scope itself, the last thing I did was an under the stars collimation back in August.  It was looking promising, but then the battery issue made it so I can't use the observatory at all and I had to go back to Baja.  Im back for good now as we moved back to Colorado.  I have the new battery ready to go, so Im hoping to have a report on the status soon.

 

Thanks.

 

Bryan

[SavannahCarl]

Hi David07,

 

I was referred by someone at the astrophotography beginners pages on Facebook. I have a GSO 6" RC and have just completed your collimation method. I think I got everything right but I'm not sure which part the was the light baffle. I unscrewed the secondary mirror cell from the 13mm ring that held it, then screwed the mirror assembly directly to the secondary 'plate'. I was able to see and center the dot, so I guess what I did was ok. Anyway, if this works, I will be ecstatic. I won't be able to check before the end of the month, though. Dark Sky shows clouds and rain through the end of June!

[Jaspalchadha]

I spent several hours on collimating the 14 GSO RC and got a result that I am happy with. I spent several months resetting the scope close to factory reset.



Sent from my iPhone using Tapatalk

Edited by Jaspalchadha, 05 June 2020 - 05:36 AM.

[Ranger Tim]

Just when I thought I had seen enough of RC collimation threads for the rest of my life I found this one and became fascinated! Please keep posting pterodyne, I want to know how this develops. David and Timo deserve accolades for the help provided. I realize now I have stumbled through my own attempts with a Cro Magnon-like zeal, and only through divine providence have I kept any sort of accuracy with my RC.

[Timo I]

WOW! This thread has now been resurrected from death, it seems...(= these RCT collimation problems never die totally and some lucky users (like Jaspalchadha) survive from their long time misery due to these writings )

PS. Thanks Tim, I'm just glad to be able to help someone!