GSO RC8 collimation issue

[pluto]

Hi all, sorry for another RC collimation thread...

I've got an RC8 that I've had for a couple of months and I'm not having much success collimating it.
I got it second hand and, as it had undergone a Perth to Sydney road trip to get to me, it's in need of a good collimation.

First I got it looking good just adjusting the secondary with a cheshire. Then I star tested it and got the secondary collimation spot on. But there was still a collimation error so I figured the primary needed some attention.
Then I followed this:
http://deepspaceplace.com/gso8rccollimate.php
At first I though I was getting closer to collimation but, long story short, I wasn't.

Anyway I've just got a Tak collimation scope and was hoping things would go a bit smoother now but it looks like the secondary mirror isn't centre spotted (see image). Am I missing something?

So I'm sure both the primary and secondary mirrors are significantly off collimation but I have no idea how I can collimate the primary when I'm not sure if of the state of the secondary...?

Any help/advice would be very gratefully received!

(the other pic is looking through the Tak collimation scope after getting as close as I could with a cheshire, the orange centre is because I'm shining a torch at the light hole thing and it's a different colour than the white monitor the scope is pointed at)

[Paul Haese]

You will have to centre spot the secondary yourself. Early models of these scopes did not have centre spotting. You can take the secondary assembly out and get a centre spot from some where like cats eye collimation.

Good to hear you have a Tak collimation scope for this task. Many people try this task without the collimation scope and it fails dismally.

[pluto]

Thanks Paul, I'll give it a go!

Any reason to not just use a very precisely placed dot from a black marker?
I've seen mention of this on CN and it sounds like it works...?

[Paul Haese]

Yep a block dot would work. Just be really steady with the placement.

[pluto]

I'm thinking I'll 3D print up a little thing that fits over the secondary holder with a small hole that fits the tip of a sharpie or something. That should minimise the chance of me stuffing it up!

[glend]

Sorry to resurrect this thread but I am having similiar problems with my new RC8. It's brand new, and of course arrived out of collimation. I have a good laser collimator that I use on my newts and it is precisely collimated itself. I have a Cheshire as well. My RC8 does have a centre spotted secondary, and I have no problem using the laser to target the centre spot precisely and bounce back onto the display rectile of the laser; however, when I look at it through the Cheshire it appears to be out slightly. I followed the Astro-tech collimation instructions published online for the RC8. The primary appears perfect and I have not touched that. it's probably close but I have not been able to do a star test because of the darn cloud.
So that's the background.

Question 1: Do I really need to buy a collimation ring for the focuser? I read some threads that say it is not necessary to plumb the focuser using the ring unless I am using a camera with a full size sensor - mine is APS-C. Is that correct?

Question 2: Do I really need a Tak collimation scope? Sure it would be nice to have but people do seem to be able to get this thing collimated without it. Can it be done with my Cheshire and Laser, mine seems close and Astro-tech publishes instructions for those tools. What would I gain with the Tak?

Any other suggestions?

Thanks folks.:thanx:

[Paul Haese]

Yes and Yes. The ring will ensure you are in the centre of the optical line at your sensor and the Tak scope will get you collimated in about 5 minutes once you view Ken Crawfords video on his site. With practice you will tell the very fine collimation to really get these scopes singing, however the Tak scope gets you 99% there.

[glend]

Ok I have bought the collimation ring and should have it Monday. In the meantime I have gone through all of Peter's thread on this subject and tried the Hall of Mirrors technique, barlowed my laser with two different barlows, tried the Cheshire again etc. The Cheshure seems to be my check point. I did have some good results with barlowing my laser, got good images of the baffle rings and centred all of that and surprise the Cheshire agree - the first time two different techniques agreed.

I will hold off on the Tak as I can't afford it right now, and I would have to get an adaptor at $ as well. Bintel and/or Andrews should rent out the Tak collimator.

[glend]

Did a star test tonight - it was rubbish. Star defocused showed shifted centre spot to the right and trying to adjust the secondary to correct it just mucked it up more. Brought it in and re-did the collimation on the bench using the Cheshire and barlowed laser - look perfect again but that's how it looked before the star test. Will wait for the collimation ring, if that doesn't fix it i might just send it back.

[glend]

Well after far too many hours spend peering into a Cheshire tube I have found a solution which is incredibly simple and allows quick collimation in the daytime on the bench (without having to buy a Tak Collimation Scope and Adaptor) - have to give credit to a user on Cloudy Nights who came up with this - ..WadeH237

Here is his thread:

http://www.cloudynights.com/topic/506196-collimating-an-astro-tech-at6rc/?hl=%2Brc+%2Bcollimation#entry66895 38

The solution is to remove the internal baffle tube for the collimation, which then allows you to see everything clearly: baffle rings, vanes, secondary holder, centre spot, etc.

This might horrify some but its pretty easy.

1. Put a masking tape arrow head on the scope tube and the front spider ring (at one of the screws) for a reference point when reinstalling it.

2. Undo the six screws holding the front ring to the tube (2 on the tube itself and 2 each on each of the two dovetails). The front ring slips off easily. Set it aside - you will clearly see the centre spot now on the secondary. You could also clean the secondary if needed.

3. Look inside the tube and you will see the black baffle tube in the centre. On the RC08 it is actually a two piece assembly, I suppose the bottom section was used for the RC06 and they put an extension on for the RC08. Reach down to the bottom of the baffle tube and grab it and turn it anit-clockwise, it should come loose easily and it is on a threaded collar above the mirror surface - so you should not need to worry about the mirror - although if you are put a mircofibre cloth over it. Unscrew the baffle tube, keeping an upward pull on it as you turn it so that when it comes loose it doesn't hit the mirror. The threaded collar is about 12mm long. Remove the baffle tube.

4. Now re-install the secondary spider/front ring using the exact same placement as when removed. Reinstall all the screws.

5. Go to the back of the scope, if you had removed the focuser before you did the baffle removal have a look inside the tube and you will see all the internal rings, vanes, secondary holder, secondary, centre spot, etc. Your now ready to collimate both secondary and primary (if required).

6. Reinstall the focuser if you had it removed.

7. Insert your Cheshire into the focuser and have a look at the current state of collimation. Now how you do the next bit is up to you, normal practice is to only adjust the secondary using the front secondary adjustment screws, get the circle over the black dot and then check the alignment of the secondary holder, vanes, inter and outer circles, etc. All of this is so much easier to do with the baffle tube out of the way.
Take out the Chesire and insert you laser into the focuser and check to see if it is right on the centre spot - it should be close if your Chesire circle was on the black dot. If the laser (and I assue your using a properly collimated laser), dot falls outside the centre spot your going to probalby need to get the GSO focuser collimation ring for your model. This ring gives you the ability to tweak the focuser tube aim and centre the laser beam precisely. I had to get one for my RC08, there was no other way of getting the beam into the middle of the centre spot.

8. It is possible to adjust the primary with this technique (only if it is really necessary) and I caution against that unless you somehow mucked up the factory settings when trying to collimate it before doing this. If you need to do it then do it in baby steps, a little tweak and then redo the secondary etc. Be aware that the GSO Collimation Ring covers the primary adjustment screws when the Ring is installed - what a brilliant piece of engineering that was. Baader makes one for the Baader focuser that does not cover the primary colliamtion screws, it's shown in the three part utube series on Rc Collimation.

9. So once your laser is precisely on the centre spot and returned to the centre of the laser bezel, and all the internals are where they should be you can reinstall the baffle tube. Just work in reverse, remove the secondary ring once again and screw the baffle tube onto it's threaded post/collar in the centre of the primary mirror.
This is also a good time to clean the mirror if you need to - after the baffle is back on, blow off any foreign material with a puffer, etc. You can also clean the secondary before you reinstall the ring.

10. Reinstall the secondary ring and tighten up all the screws. Do final check of the collimation, it is possible that you may have to tweak the secondary or the focuser adjustment slightly to get it precise again.

11. Do a Star Test. The collimation should be very close if not great. I'm very happy with mine, and not further adjustment is required at this time. I will start doing some images and work at refining the setup from them.

Good luck.

[glend]

I have a new probem with the RC, on the bench the collimation is perfect, however, on the mount its out significantly. It was collimated visually on the bench, checked with a star test visually, and looked close, however, when the camera star test is done it was way out. I adjusted the secondary only to make the camera star test perfect and did some test shots - and things looked good. Now back on the bench this morning it's way out again when checked visually. So what is the problem? Is it flex due to the DSLR ( used just my plain uncooled Canon, so not as heavy as the cooled camera).
The scope has the focuser collimation ring and that was used to centre the laser with full imaging ring/focuser setup during bench collimation.

I'm at a loss to know what the issues is now - any ideas?

[Paul Haese]

Have you centre spotted the secondary?

Which focusor are you using? With any luck it is not the one supplied by GSO. Those focusors are not great.

Check to see if all you collimation bolts front and rear are snug and not loose.

I doubt you will get collimation working with a Cheshire. Use the Tak scope only. It is most accurate.

The small screws on the primary are the locking nuts. Make sure those are snug but not over tightened.

In the Tak scope you will see a small black dot in the centre. Inside that you have to be able to see ever decreasing very thin white rings. Those rings are the inside baffles of the Tak scope. If those rings are slightly off centre you are not collimated. All the rings from those tiny ones to the very largest one must be concentric. The little tiny Tak rings are adjusted by a rear focusor collimation ring. If you don't have a rear focusor collimation ring you cannot do this adjustment.

Only a star test will get your scope fine tuned. There will be some very slight miscollimation after using the Tak scope but it should not be huge. If there is a huge difference between the two go hunting for something that is loose. Check the centre baffle ring to see that is just touching the primary and not completely loose. The 8 inch scope should hold collimation well.

My suspicion is that something might be loose or your bench collimation is not correct.

[glend]

Thanks for your response Paul. I have answered where I can below:

Have you centre spotted the secondary? It came from the factory centre spotted, and measuring it seems to confirm it's in the right spot.

Which focusor are you using? I am using the GSO rail focuser that come on the scope from the factory.

Check to see if all you collimation bolts front and rear are snug and not loose. Everything seemed tight, I know the back bolts on the collimation ring, and the primary were, I think the secondary was tight enough but I will check again..

I doubt you will get collimation working with a Cheshire. Use the Tak scope only. It is most accurate. Yes I know your feeling about the TAK scope but plenty of people collimate these things without one, and I have read everything on the internet about collimating them. I think the most accurate method is Wade's (on CNs) technique of removing the internal baffle tube which opens up the entire internals of the scope to the focuser view, that coupled with a Howie Glatter laser with the concentric ring attachment, and a Cheshire, is all many people need to get perfect collimation. I did remove the internal baffle, and I have disasembles a few scopes so I know how to put one back together properly, and its all back exactly as it was, no slack, no loose mirror, etc. The mirror is retained by a large O ring on the internal baffle mount, it is possible to remove the baffle tube without removing the baffle tube mount.

The small screws on the primary are the locking nuts. Make sure those are snug but not over tightened. Primary seems fine to me and all the vanes and concentric rings are lined up and evenly spaced (I measured them with a ruler when they were projected on my wall).

In the Tak scope you will see a small black dot in the centre. Inside that you have to be able to see ever decreasing very thin white rings. Those rings are the inside baffles of the Tak scope. If those rings are slightly off centre you are not collimated. All the rings from those tiny ones to the very largest one must be concentric. The little tiny Tak rings are adjusted by a rear focusor collimation ring. If you don't have a rear focusor collimation ring you cannot do this adjustment. I have a focuser collimation ring and it is lined up correctly at camera spacing distance. One problem with the GSO focuser collimation ring is that when it is installed you cannot reach or adjust the primary bolts, you need to remove it to get to them. I noticed in one of the utube videos a guy had a Baader focuser and a collimation ring with a bigger setback that allowed him to access the primary bolts - I'd like to have that setup.

Only a star test will get your scope fine tuned. There will be some very slight miscollimation after using the Tak scope but it should not be huge. If there is a huge difference between the two go hunting for something that is loose. Check the centre baffle ring to see that is just touching the primary and not completely loose. The 8 inch scope should hold collimation well. The visual star test looked ok, but the camera star test was bad, hence I do believe that something is loose, of sagging, but I can't find it - I believe it has to be in the assembly that screws onto the back of the scope (from the collimation ring right to the end of the focuser draw tube); as when the camera weight is applied it sags out of collimaiton.

My suspicion is that something might be loose or your bench collimation is not correct. Well I believe the bench collimation was spot on, and I went through many iterations to get it that way. I agree something is loose.

I don't have anymore time to spend on it this week as I am going to the dark site tomorrow and wanted it ready - it can stay home now, I'm actually sick of fiddling with it for awhile. I will give it another go when I return.:thanx:

[Paul Haese]

Then I think you most likely you have the problem with the focusor. Those focusors move around very badly. It does not take much flex to see different results. I found that even at different angles the collimation would shift quite a bit and yet it appeared to be reasonably firm.

I was not clear about the baffle area. I did mean the retaining ring. Sorry for the confusion.

[glend]

Thanks Paul. I have gone back to square one and started again on the focuser alignment and tightened the collimation ring right down and then adjusted from there - this seems to have solved the immeadiate problem; so it looks like one of the screws in the collimation ring was not as tight as it should have been. I can hold collimation with my uncooled DSLR but the cooled camera seems to be too heavy for the focuser assembly - still getting some sag. When using Liveview on the cooled camera, after centering the star I can move it on the screen with a simple touch on the bottom of the camera - not sure if this is causing a slight mount movement or if its the focuser. Perhaps it's better to not touch it at all.

Initial 5 minute 30 sec test shot on the Sculptor looked reasonable, but showed up slight elongation in the star field - which maybe guide/mount related. More tuning required.

I installed a Kendrick secondary heater and now I am thinking of removing it as it hangs over the side of the secondary shroud slightly and is thus in the light path. I tried to trim it before I installed it but it just does not fit right. The thick wiring is also a problem and even when taped to the top of the spider vane it is noticeable in a star test. I have the copper adhesive strips that came with the heater but have not installed them. It (the wiring and overhang of the heater) doesn't seem to be affecting the test image however, so is it worth worrying about? :question:

[Paul Haese]

Most likely the assembly of the focusor is causing the movement you see on live view. Until you can be certain that the elongation in the stars is caused by the guiding it will be hard to say with any certainty, but if you have good shaped stars after 30 seconds, it is most likely guiding causing the elongation.

With the secondary heater I taped the wiring one on top of the other and that is on top of one of the vanes of the secondary assembly. It causes a slight thickening on one of the diffraction spikes. I simply used electrical tape and it has held well for nearly 2 years now.

[glend]

Well I think I am getting pretty close now, and with the camera as well. I have tried to make the camera position collimation the priority. Visually the star test looks a little off but when the camera is on the focuser it looks pretty well centred. I don't how it will go in different attitudes of the mount. I have attached a couple of photos taken through the DSLR. What do you think?

Note that I have the Kendrick Secondary Heater installed and used the copper tape to cross the vane to keep any thicking of the vane to a minimum.

[Paul Haese]

Has to go down and to the right Glen. The star image has a second diffraction spike so collimation is reasonably close but with these scopes it might as well be a mile. Keep plugging away. If you are using the focusor that comes with the scope, it will move in different parts of the sky. First step is to eliminate that movement in my opinion.

[glend]

I should of mentioned the target was Rigil Kentarus - so a double, and I thought the secondspike was due to the minor star in the pair. The cloud has rolled in so I have to wait now. What is a good reasonably priced replacement focuser for these small RCs? And thanks Paul.

[Paul Haese]

You can't go wrong with a Moonlite or Feather Touch. The prices are variable but those focusors will prevent any flex and for imaging this is a real must.

[glend]

I have priced the replacement focusers, the Starlight and Moonlight are both going to cost around $500 USD plus shipping . If i bought direct from Moonlight in the US I could get it delivered for $600 AUD. The Baader Steel Track and adaptor is a lower cost option at $450 AUD landed but it seems to be exactly like the GSO rail focuser that I have on the RC08.
I use the exact same GSO rail focuser technology (newt version) on my 10" imaging newt and have never had a problem with focuser flex/sag on that scope with my heavy cooled DSLR (and it's a non-rotating scope so sometimes the camera is hanging at very severe angles).
I am having a real problem justifying spending half again the cost of what the RC08 cost me to buy a replacement focuser. Altair Astro advise on their website re the RC08 that the standard GSO linear rail focuser:

... will handle a DSLR or CCD camera, and a light weight filter wheel perfectly well, and is adequate in most imaging situations. Note this focuser has the Linear Bearing upgrade, and will outperform most Taiwanese dual speed focusers, both in rigidity and pulling power. It is perfectly adequate for a DSLR/CCD camera and light weight filter wheel.

I will take what steps I can to improve the stock focuser first.
So I have stripped down the stock focuser and tightened up the rail bearing screws (which seemed fine anyway) and firmed up the pinion shaft that presses against the rail - its all now tighter than my 10" newt focuser. I can not force any sideplay in the focuser rail nor up/down sag/flex that I can detect.
Reading old threads about this subject suggests that the bevel and stop ring placement on the focuser flange mount might be a source of bad fitting so I checked that as well and made sure the stop ring was in the best position to be pushed against by the locking bezel. I can't really do more to QA the stock focuser.
A new round of star tests will now be done at various mount attitudes looking for indications of sag/flex.

[Ausrock]

Glen,

What model F'touch will suit your scope?

Chris

[glend]

Chris,

FTF2015BCR-A20-271 is the product number I believe. Starlight store says $500 USD plus shipping if purchased direct.

[Ausrock]

I have a 2015BCR on my C9.25. If it would fit your RC I think I could spare it long enough for you to try out and see if it solved your problems.

Chris

[glend]

Thanks for the offer Chris. I would still need the GSO RC adaptor flange. I have a Moonlight on my iStar refractor and could use that as well to test but same problem, I need the adaptor flange.

[Ausrock]

Aaahhh........now I understand the "A20 271".......that had me buggered as to what it meant earlier :).

[glend]

I will see if I can get the Moonlight GSO RC adaptor flange, that should be a low cost test option, then I can move my Moonlight focuser onto the RC. The draw tube on the refractor focuser is too long for the RC but it should not matter for testing, and can always be changed if needed for permanent duty.

In the meantime I will test with the tightened up GSO focuser.

[glend]

I caved in and bought a TAK Collimation Scope and 2" adaptors today from Teleskop-Express (best price I could find that had stock and they will ship UPS tomorrow). I figured it was either that or sell the scope - guess I have to keep it now.;)

[Paul Haese]

Glad you finally caved. These Tak scopes are great. Click on the link below for instructions on Tak scope use.

I cannot find the video by Ken Crawford anymore. That was really easy to follow.

http://www.rcopticalsystems.com/support/images/rcos_alignment_collimation.pdf

If you get stuck, let me know, but I did the collimation on my scope in 5 minutes the very first time I used a Tak collimation scope.

Something to be really wary of though. When you get all the big rings concentric, looks for some really tiny thin rings in the centre dot. Those rings will be white and very small and are illuminated with the torch light on the tak scope illuminator (white disk). You can only see those if the torch is squarely centred on the white disk. Those rings show the inside of the tak scope. You get them centred by using camera collimation ring. It is critical that you get those really well centred.

Procedure wise you should get the secondary centred first, then the Primary. If either are too far out you will need to do an iterative process to get that sorted. Then do the tiny rings in the centre. The secondary position might change a bit after that, so adjust the secondary again and then finally tweak the tak rings.

Then do a star test. I have collimated a few guys scopes for our society (I am the instruments officer for the Astronomical Society of SA) in recent months and got the collimation so good that a star test reveals an ever so slight tweak if at all. The more particular you are with the tak scope the better the results you get.

[LewisM]

'bout time - stubborn bugger :) :)

[Paul Haese]

Found the video

http://www.rcopticalsystems.com/support/images/RCOS_Collimation_V3.wmv

[glend]

Thanks Paul. I had been using Stephen Kirk's utube three part RC08 collimation instructions and feel its very close now. The TAK Scope is another tool for better precision and may save me some time in the future if I stay with RCs.

[glend]

Still waiting for the TAK Scope to arrive, but messing around today with the stock focuser alignment, trying to work out why one side of the collimation ring has so much tilt to get the laser centred on the secondary spot, I stumbled on the reason : One of the extension tubes (the one that I had the focuser mounted on) had paint on the anodised bevel surface where the focuser engages to the tube. The paint was the material used to coat the the inside of the extension tube and had run on one area of the bevelled surface creating a non-symetrical bevel. This probably would not have mattered if it was not the extension that I used to attach the focuser. The fix, I hope, was to sand the paint off the bevelled face. This allowed the tilt to be near equalised.
Anyone buying a GSO RC with these extension tubes should inspect the bevelled surface if using the stock focuser.
Have to wonder what the next surprise will be.:question:

[glend]

All collimation problems now resolved with a perfect star test with camera load and no apparent focuser sag at various attitudes - and still waiting for the TAK scope to arrive :lol:

Once the TAK arrives I will double check everything but I swear it will be a cold day in hell before I ever mess with that primary setup again.