TS-Optics RC8 - Collimation : next step ?

Equipment Ritchey-Chrétien Collimation
JérémieDawn LowryJohn Hayesandrea tasselli
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Jérémie avatar
Jérémie avatar
Hi All,

I received 2 weeks ago a TS Optics 8'' Ritchey Chretien, and tried to collimate it (out of the box, the stars were just horrible...). I plan to use it first with an AstroPhysics 0,67x telecompressor (reducer) to get more light and less guiding troubles.

Therefore, I will use it only with a 25mm M90 extension tube (not the 50mm), a M90 tilting adapter flange and focuser (in that order, to be able to access collimation screw of the primary). And therefore, this is with this "short" optical train that I have tried to collimate it (without the focal reducer during the collimation process of course).

Until now my procedure has been :

- Remove the "top" part of the central baffle - without opening the telescope, just by unscrewing it, passing my hand between the spider - in order to be able to see the reflection of the doughnut of the secondary mirror on the primary.

- Using my Takahashi collimation scope, I have carefully adjusted the primary mirror so that thin circle of light around the bunch of reflections is concentric to them and even. I can barely see the circle of light because of my short optical train. I have also tried with a 2'' 80mm extender tube between the focuser and the collimation scope, but results are similar in the end.

- Then I performed a first adjustment of the secondary so that the doughnut appears to be concentric with the collimation scope center black hole / spot (where we look at).

- Then I fitted my Howie Glatter 2'' laser (just the spot) in the eyepiece holder of the telescope to adjust the M90 tilting adapter until the laser hit the doughnut (it was already within it, but touching it on one side). So minor tweaks. I have used my DSLR and a 105mm macro lense to use my liveview to be as precise as possible. I also checked the Howie Glatter was calibrated by turning it on itself within the eyepiece holder, and the spot did not move and remained almost exactly in place.

- Then I adjusted the secondary mirror until the laser beam hitting the doughnut was coming back to itself (again using the DSLR and playing peekaboo with the laser to see alternatively the emission spot and the sum of the two).

- Then I mounted the holographic attachement to the Howie Glatter to get the circular pattern projected : some say you need to re-adjust the secondary until you have concentric circles on the primary (by focusing more or less until one of the circle is on the edge of the primary, then adjusting the seconday to have the full circle on the edge); others say you only check the projection on the wall, and adjust the primary until your circles are plain and not half there. I have chosen this option, doing minor tweaks only to the primary.

- Then reiterated the process a few times : tilt ring to hit the doughnut, check that the spot is coming back to itself (alignement focuser / secondary) and then again adjusting the primary with the concentric circles.

- I also checked if the infinite reflections between the 2 mirrors were good : and they were, straight to infinity, looking from just above the secondary (no right or left deviation) and the same when looking just on the left of the secondary (no up or down deviations).

- Screw back the top part of the central baffle, trying not to touch the secondary (thought it shouldn't move that easily...)

THEN : the truth. The test under the stars.

As you can see below, it went wrong.... My stars are in really bad shape (compared to what I was expecting from the books).

I thought I could finalize from there. I understood I would see the shadow of the secondary mirror more or less centered in the star, and adjust the primary just a tad to make everything perfect and symmetrical.

But playing with the primary had some effects, but not enough to make thing well. I did not want to screw my initial collimation and stopped after making more than a 1/2 turn of screw (I did that once, and when looking through the Taka collimation scope, I saw the primary was completely off).

Maybe I should have ?

Then I tried to play a bit with the secondary : almost nothing changed (with 1/4 turns of screws... that's big). Then thought about the tilt ring (more weight ?), but the same, nothing improved really.

From where I left, given the strange shape of the star, what should have been my next step ? Primary ? Secondary ?

Thansk for your time, again...

Nota Bene : below there is only 1 serie of shot on a bright star (500ms exposure) with various focuser position. I just duplicated it and make one of the series bright to better see the shape of the star / stain...

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Andrea Alessandrelli avatar
Andrea Alessandrelli avatar
Hello Jeremie, 

my 8'' RC is very sensitive to cool down issue and is capable to give me horrible stars (0.7 eccentricity!) when not cooled properly and wonderful stars (0.2/0.3 eccentricity) when cooled. So it's good to remove the cap and let it cool down at ambient temperature for at least a couple of hours before using it. 

Whatever adjustment you want do to the primary mirror during a star collimation, it should be done late at night when the scope is well cooled. 

I don't have an howie glatter and takahashi but for me this method works fine: 

Collimate the primary on a star , look for the illumination of the defocused star and adjust the primary until the illumination seems even. After that look for the baffle rings and make them look concentric still with primary mirror adjustments. Any adjustment you do, recenter the star. Then take care of the secondary mirror if necessary. 

Then the day after check with a cheshire and you will see how the collimation can be refined, usually at this point is a matter of micro adjustments. After that you should be good for a while, the GSO/TS RC is not a scope that loses collimation easily. 

I hope it helps! 

Andrea
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Jérémie avatar
Jérémie avatar
Thanks for those tips Andrea
John Hayes avatar
John Hayes avatar
I can see right away from your star images that you have two problems.   First, I see significant spherical aberration, which implies a spacing error--either between the primary and secondary or in the sensor placement relative to the design focal plane.  Second, I see a lot of coma, which implies secondary misalignment.  The very first thing in aligning ANY Cassegrain type system (include a RC) is to set the sensor at the precise correct back working distance specified by the design.  THEN set the mirror spacing to minimize spherical.  I don't know how that is done with your system.  In my CDK system, we do that using a Ronchi ruling.  

After the mirror spacing is set, then align the secondary to minimize coma.  We typically start by centering the secondary shadow when the system is out of focus--but that's only for course alignment.  The actual alignment is done to eliminate coma when the system is focused.  You'll need a high powered eyepiece to closely examine the Airy pattern to produce a round, evenly illuminated pattern.  

Unless you get lucky, simple geometric alignment tricks might get you close but they are unlikely to produce a precisely aligned system.

Good luck with it!

John
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andrea tasselli avatar
andrea tasselli avatar
I think you have a cooling down issue to deal first (it is seen by the frayed outer edge of the dim side of the Fresnel rings). As other have said wait a couple hours with the scope vent open. Check that the outside temperature is stable and has been for at least1/2 hours. Coarse collimation with defocused star to get in the right ball-park. Then adjust you secondary with in-focus high power EP (giving at least 300x). Every time you adjust, bring the star back in the centre and repeat. Forget all those Howie laser things, they are not getting you where you want to be.
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Jérémie avatar
Jérémie avatar
John Hayes:
I don't know how that is done with your system


Thanks @John Hayes for your feedback.

The TSO RC8 is a « cheap » / simple design, as the focuser is directly connected to the plate that holds the primary, and the secondary is hold by a spider you can’t adjust. In theory, they expect that the secondary is centered in the tube and at the right distance from the primary for the design to work correctly... but many users of the RC8 buy an additional tilt ring to allow focuser adjustment when the primary is moved, iteratively.

So in short : no way to play on the distance between mirrors (except maybe the central screw of the secondary : but the manufacturer tells to never touch it)

That leaves with : 3 pairs of screws at the back of the primary mirror (push - pull system) and 3 screws on the secondary to play with (and by the way, I am wondering if that doesn’t impact the distance between mirrors if you don’t alway compensate 1 turn on one screw by 2 half of « un-turn » on the other ones). I must make it work with that (well, plus 3 pairs of push pull screws as well on the tilt ring I added).

I know it is lacking of adjustments systems, but I have seen people getting nice results from it...
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Jérémie avatar
Jérémie avatar
andrea tasselli:
I think you have a cooling down issue to deal first (it is seen by the frayed outer edge of the dim side of the Fresnel rings). As other have said wait a couple hours with the scope vent open. Check that the outside temperature is stable and has been for at least1/2 hours. Coarse collimation with defocused star to get in the right ball-park. Then adjust you secondary with in-focus high power EP (giving at least 300x). Every time you adjust, bring the star back in the centre and repeat. Forget all those Howie laser things, they are not getting you where you want to be.

Thanks @andrea tasselli  and @Andrea Alessandrelli : indeed, I usually put it outside 1 hour before imaging. Maybe less that night.
I will be more careful next time. 
Then on your procedures, it seems easy for you, and without laser and stuff - maybe a cheschire for Andrea A, so that gives hope. I will try with my artificial star as to be more comfortable to make my tests. Doing outside in the cold and the dark is really frustrating...
Ron avatar
Ron avatar
Jeremie,

I have used a Cheshire, Howie Glatter laser, and various imaging techniques to collimate my TPO RC 8" scope.  I had a similar result as you with the Glatter system and the Cheshire is just difficult for me.

The method I now use, and then check with the Glatter laser, is found at:
https://deepspaceplace.com/gso8rccollimate.php
I've iterated 2-3 times and get very good results.  At this point, the Glatter laser shows it's perfect.  Why I can't just use the laser is beyond me, though.

Good luck,
Ron
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andrea tasselli avatar
andrea tasselli avatar
Jérémie:
John Hayes:
I don't know how that is done with your system


Thanks @John Hayes for your feedback.

The TSO RC8 is a « cheap » / simple design, as the focuser is directly connected to the plate that holds the primary, and the secondary is hold by a spider you can’t adjust. In theory, they expect that the secondary is centered in the tube and at the right distance from the primary for the design to work correctly... but many users of the RC8 buy an additional tilt ring to allow focuser adjustment when the primary is moved, iteratively.

So in short : no way to play on the distance between mirrors (except maybe the central screw of the secondary : but the manufacturer tells to never touch it)

That leaves with : 3 pairs of screws at the back of the primary mirror (push - pull system) and 3 screws on the secondary to play with (and by the way, I am wondering if that doesn’t impact the distance between mirrors if you don’t alway compensate 1 turn on one screw by 2 half of « un-turn » on the other ones). I must make it work with that (well, plus 3 pairs of push pull screws as well on the tilt ring I added).

I know it is lacking of adjustments systems, but I have seen people getting nice results from it...

The spherical aberration induced by de-space error isn't that large compared to what half a turn more or less will give you. I'd assume your collimation screw be M5 or M6, in which case one whole turn would only offset at most 0.8 mm to 1 mm and for a typical RC @ f/8 and that would result in around 1/15th to 1/20th of wave PV if such movement were to result in a longitudinal shift  of the same magnitude, which it does not. I would result in a tilt of the primary/secondary, by and large. So do not fret about it.

Coma sensitivity to tilt (irrelevant which one is tilting with respect to) is far larger. 0.1 degree would approximately correspond to 2/5th of wavefront error PV. If we assume that the collimating screw is an M5 (with 0.8mm pitch) acting at the outer radius of the primary mirror one full turn would result in a around 2 FULL wave of error PV, which is rather large.
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Andrea Alessandrelli avatar
Andrea Alessandrelli avatar
Jérémie:
maybe a cheschire for Andrea A, so that gives hope. I will try with my artificial star as to be more comfortable to make my tests. Doing outside in the cold and the dark is really frustrating...

A full Cheshire collimation always gave me unwanted results. But once you have collimated on a star it becomes a powerful instrument for fine tuning. At that point it will be a matter of finding micro asymmetries and adjust them. 

Also another thing that I've forgot to mention: In a system like the RC is very important to avoid the clamping and use thread instead. Try to clamp a cheshire when you are collimated and you'll see how it goes  So if you have the stock focuser, that would be the first thing that needs to be replaced.
Mark Eby avatar
Mark Eby avatar
Hello Jeremie,

I used to have an Astro-Tech 8" RC, and I eventually learned how to collimate it quite well. Take a look at my website for instructions:
    http://www.skydude.me/

Hope it helps,
Mark
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Jérémie avatar
Jérémie avatar
Thanks @Mark Eby , I will check your procedure on your website.
Jérémie avatar
Jérémie avatar
Found an interesting post on Cloudy Nights about distance between mirrors. It seems these RC8 from GSO (and probably rebranded) don’t come at the designed focal length. They say you shouldn’t touch the central screw, but it may be necessary. I will also check my focal length by plate solving and see what it gives...

https://www.cloudynights.com/topic/721482-my-take-on-gso-ritchey-chrétien-collimation/
SemiPro avatar
SemiPro avatar
Jérémie:
Found an interesting post on Cloudy Nights about distance between mirrors. It seems these RC8 from GSO (and probably rebranded) don’t come at the designed focal length. They say you shouldn’t touch the central screw, but it may be necessary. I will also check my focal length by plate solving and see what it gives...

https://www.cloudynights.com/topic/721482-my-take-on-gso-ritchey-chrétien-collimation/

Silently following this thread to see how others collimate their RC, however I am not sure how much I would trust a measurement from a piece of cardboard jammed into the tube to measure the primary. I can tell you from experience (from the work I do) that when it comes to measuring or making templates with cardboard and expecting accuracy your mileage may vary.
Torben van Hees avatar
Torben van Hees avatar
I can second the procedure documented by @Mark Eby. It is exactly what I use (basically following DSI but adjusting the secondary on in-focus stars). To get close I use the Tak scope (really quick, just to be in the ballpark), then tune on the stars. Looking at your image, thermal problems are a main culprit. You also decollimated the primary which is easy to do and hard to fix. My advice is not to try and get it perfect unless but stop at 90%. Primary and secondary adjustment are iterative procedures which you switch between. The main problem for me is that secondary adjustment is not smooth with the screws. Plan a whole night for collimation.

Using lasers is a dead end for this scope, they make things worse. The only laser that works for me is the Hotech Advanced SCT, a 600€ investment. And even that needs fine tuning with a star.
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Dawn Lowry avatar
Dawn Lowry avatar
ill jump in here.  I have a picture to include.  I feel I am a little off in collimation. ~. and maybe tilt but think collimation. (it has not been collimated for 9 months. ~. Dean at Starizona cleaned and collimated it for me summer 2020~. shipped it back and then helped me finalize the tweak on it).   ~So you know~I roll my scope in and out on a dolly from garage to driveway (30 feet and over a small little bump only).

  Hopefully this week...My process is as follows. I'll find a deep focused bright star.  I'll place my hand in the bottom of the front of the scope and make sure that my camera is oriented in the same way that will show my hand in the bottom of the scope. 

Using sharpcap.... will defocus the star and place the star in the Center of my screen.  I will adjust the primary on whichever axis is closest to the deviation~  So for example if it is deviated towards 4 o'clock and my screws are located at 10 ~2 and 6 ~~~I will use the 10 o'clock pair of screws to pull the star center back towards the center.   And I will keep doing that until I get the star perfectly in the middle.  

The part I don't understand is adjusting the....  Secondary on a focused star can you explain that?  Dean did not bring that part up..

Also. I have a very hard time. seeing if my laser is bouncing back. I have the howie glatter.  I do remove the baffle. I set up the camera and seperate computer monitor to see it clearly. ( guess I dont know what I am looking for). I do have a moonlite on it.

This is last nights cone image.  see the fringing and then the slight deviate to the right....

~~~do you collimation....issues?    (in the corners i do have oblong stars now too and never did before ( using the CCD67)  so I am blaming a slight collimation issue  ( slightly to the right..._  agree?)

I was contemplating going to the apex L. BUT I like the 1225mm focal length. and f6 is fine.  Thank you  in advance 
Dawn
Jérémie avatar
Jérémie avatar
Found this elegant method that avoids the iterative part of many of the procedures I have seen :
http://www.365telescopes.com/how-to-collimate-an-rc-the-williamson-method/

Most procedures with lasers starts by aligning the focuser to secondary (targeting the doughnut, then superposing the reflection of the laser beam on itself). Then adjusting the primary using holographic attachment or cheschire.

This one uses the property that when removing the secondary, you can superpose the primary reflection of a point-light source to itself to materialize the primary optical axis, then use the tilt ring to which the focuser is attached - and the laser - to superpose the laser to the light source. Then you have aligned the focuser to primary optical axis. 

Then you put back the secondary, and douhgnut align with the laser but this time adjusting the primary (and not the tilt). The focuser will move along, but now you just adjust the secondary so that the laser beam reflection is superposed to the laser emission.

Of course, I will probably need to adjust the distance between mirrors at some point to reduce spherical aberration as @John Hayes  mentionned. Will do that by measuring the focal length by astrometry, and adjust to spec later... 

Now, sky should be clear tonight, ready for a test...
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Torben van Hees avatar
Torben van Hees avatar
I saw that method, but did not want to totally remove the secondary. You are a braver man than me.
Dawn Lowry avatar
Dawn Lowry avatar
here are addl pictures of the corners ~~~. again I am blaming some collimation I hope...
John Hayes avatar
John Hayes avatar
Of course, I will probably need to adjust the distance between mirrors at some point to reduce spherical aberration as @John Hayes  mentionned. Will do that by measuring the focal length by astrometry, and adjust to spec later... 

Now, sky should be clear tonight, ready for a test...

I don't know where this idea came from but trying to set RC mirror spacing by measuring the focal length is not a good procedure.  Here's  why.  First (and probably least important,) astrometric solutions to determine EFL have a certain amount of noise so you won't get the same result each time you make the measurement.  Second, the EFL is very sensitive to small manufacture variations in the base curvature of the components so the EFL of each telescope will vary from the base design parameters.  If you go through an error tolerance, you'll see that the sensitivities are such that when you combine manufacturing variations with measurement noise, trying to set the EFL so that you have the correct mirror spacing can result in significant errors.  A lot of folks with RCs who try this approach are left scratching their heads over why their systems produce so much SA, which contributes to a lot of grumbling about how hard RCs are to align.  They are hard to align, but if you use the wrong procedures, it gets even harder!

The measurable distance that manufacturers typically use to properly figure the mirrors is the back working distance specified by the design.  That's the distance from a fixed mechanical fiducial to the focal plane.  Some folks call this spacing the "back focal distance". To understand why this number is so important, you have to understand how the optics are tested for figuring.  The aspheric figure of RC primary mirrors is typically made by testing the primary against an optical or holographic null system.  The aspheric secondary is then figured against the primary mirror by spacing the mirrors to produce a null at the correct BWD.  That means that setting the mirror spacing so that the focal plane falls precisely at the specified BWD guarantees proper optical correction--regardless of what EFL it produces.

John
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Jérémie avatar
Jérémie avatar
Very interesting @John Hayes . I won’t try to adjust that by measurement of FL then, but probably adjust them to minimize the spherical aberration, if that’s still visible after all my tweaks this afternoon.
Jérémie avatar
Jérémie avatar
Torben van Hees:
I saw that method, but did not want to totally remove the secondary. You are a braver man than me.

I was not very eager to do that, but the 3 screws were still at the same place, so when putting it back, I expect it to be +/- 0.5mm where it was before, if not almost exactly at the same place. And this method makes sense to me, though you will never reach perfect collimation of everything with these small RC : there is play in the focuser, enough that with heavy load compared to the laser, it will move anyway, not to mention that when you move primary, the focuser changes as well, etc...
I expect all the errors will be hidden by my bad guiding or bad seeing anyway :-)
But at least, I want to avoid comatic shapes of my stars...
Dawn Lowry avatar
Dawn Lowry avatar
so it seems....  http://www.skydude.me/at8rccoll.html. this applies alot to my situation... 

the adjustment of the primary. then the corners for the secondary with the INFOCUSED 1 min star....  still would welcome help like jeremie. or if i am hijacking the thread if that what this is called i can start a new one.  I would never mean to do that
Dawn
Dawn Lowry avatar
Dawn Lowry avatar
please ignore my posting .  I will start a new thred.  I  dont know how to delete them.  I apologize
Dawn
andrea tasselli avatar
andrea tasselli avatar
Dawn Lowry:
here are addl pictures of the corners ~~~. again I am blaming some collimation I hope...

I don't think is a collimation issue. Just field correction not 100%, a bit of residual astigmatism that's all. The flaring see in the other image is probably due to your camera/reducer just slightly tilted/off centre. If you want to see if your scope is collimated you need to produce in- and out-of focus of a star at the centre of the field/sensor. Start moving the focuser from inside the focus point all the way to in-focus and then out. Take pictures as you move the camera from inside to in-focus and then to out-of-focus. Check whether the Fresnel rings are concentric and evenly illuminated.
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