GSO 12" RC Dissambly.

[silentrunning]

I would just add that I adjusted the mirror retention clamps so that none of them touch the mirror at all. I used two sheets of thin paper between the clamps and the mirror to get a very small spacing. The main mirror is held securly by the central locking mechanism that uses a large rubber O ring so no chance of any z movement and the clamps remain in place more as a safety net. I also packed cork in between the mirror cell and the rear assembly so that the mirror cell is not just supported laterally by the 3 collimation screws and this also minimises x-y movement. I may also pack some cork between the sides of the mirror and the mirror cell to further lock the mirror without causing pinch.

Doug

[kirkster501]

Olly I guess this is where you differ from some us - your paying guests need to crack on with imaging and not scope tinkering. I have a bit more time for the tinkering side (it's in my blood) so am happy to fettle a bit if the optics are let down by the supplied mechanics.

Scope tinkering is obligatory in amateur astronomy! Olly is actually a professional if you think about it - paying guests and what not. If I were he RC and Newtonian scopes would be the last choice in the world to be buying.

[kirkster501]

Steve, if I recall in another thread, you were getting a Howie Glatter laser + plus the concentric circle holograph. Did you get it and if so what was your opinion of it?

Look at my blog in sig Mike. I got on very well indeed with it. However, you can achieve as good with the good old Cheshire. The HG concentric ring holograph just confirms what I do with the Cheshire. To anyone reading this, there really is nothing to be afraid about with these scopes. If you are familiar with the principles of collimation of a Newt/SCT etc with a Cheshire you are 90% there.

[silentrunning]

To anyone reading this, there really is nothing to be afraid about with these scopes. If you are familiar with the principles of collimation of a Newt/SCT etc with a Cheshire you are 90% there.

I would have to disagree with that. All the mods and the time spent on this scope have been critical to collimating it and I still have work to do. I have heard the 8" and the 10" do not have the issues associated with the 12" - primarily mirror distortion caused by a badly designed rear assembly. I have collimated Newtonians in a matter of minutes and this is a whole different ball game.

[r3i]

Hi Steve,

Thanks, I actually came across it this afternoon. Very useful resource I reckon for anyone collimating these kind of scopes, will have to add it onto the Links page on my web site.

[kirkster501]

I would have to disagree with that. All the mods and the time spent on this scope have been critical to collimating it and I still have work to do. I have heard the 8" and the 10" do not have the issues associated with the 12" - primarily mirror distortion caused by a badly designed rear assembly. I have collimated Newtonians in a matter of minutes and this is a whole different ball game.

Each to their own Doug. Maybe the 12" is different. Maybe other 8" versions are too. But as far as my own RC8 is concerned its been straightforward enough as soon as you get your head around them, which doe snot take long. One thing that does not help the GSO RC design is the fact that by default the focuser and priamry are joined as one unit.

[silentrunning]

One fundamental difference between the GSO 8" and the GSO 12" is that the 8" is supported by collimation screws that are close to the central region of the mirror (http://deepspaceplace.com/gso8rcpointing.php) whilst they are way out on the periphery of the 12" (above) which means that central mirror cell sag is inevitable in the 12" with its increased weight only adding to the problem. Without the additional supports I have added, this scope could not be adeqautly collimated and indeed others have gone to much greater efforts than I have to sort out these issues with the 12".

I have also found that the 12" is extremely sensitive to any level of decentred optics and/or tilt and 'perfect' collimation in a chesire can be way out on subsequent star tests. Add to this the complication of a ccd plane that is in the basic setup rigidly connected to the main mirror cell and almost certainly not square with the mirror and you have a combination of factors that for me at least has proved extremely trying. I also have 10" GSO revelation dob and a Meade LX200 10" SCT each of which I can collimate within a matter of minutes.

Appreciate all comments but I posted the initial report as an aid to anyone else struggling with the 12" and a warning to anyone considering buying this particular beast.

Doug

[RobH]

I completely agree Doug.

I've collimated many scopes without any problem, and have been fiddling with scopes for 45 years, but collimating the 12 inch RC is the hardest job I've ever had to tackle, allthough, strictly speaking the issue isn't just about collimation, but has as much to do with mechanical issues with the scope too.

I had a 10 inch GSO based RC on loan from Ian King, and could collimate that in a flash.

If I get a chance before the clouds roll in, I'll see how mine performs now I've got the truss assembly in line with the optical axis of the primary. I found this to be misaligned by a fraction, which was putting the axis of the secondary 5mm away from the axis of the primary.

I'm doing all my initial checks visually in order to keep the optical train as simple as possible. If it looks okay visually, then I'll add the camera and see how it behaves then.

[silentrunning]

Hi Rob

The biggest single improvement to the mirror pinch issue - which superficially is all I can see wrong with your test images above - was to completely free the mirror from the retaining clips. On the standard setup tightening the collimation screws pulls the cell down into the rear assembly and the retaining clips screws make contact with the rear assembly. The tighter the colli screws are done up the more pressure is put on the clips which in my case bends the clips down onto the mirror. Once I ensured that they were clear of the mirror - especially when back in the rear assembly with the screws tightened - my mirror pinch issues vanished and the out of focus stars were completely circular for the first time.

Now I need to move onto image tilt issues - watch this space!

Doug

[peter shah]

I cant say I've ever had a scope that I've not had to mess and fiddle with......some say its all part of the fun...that's fine if you have plenty of clear nights........

[RobH]

Bingo !!!

The scope's not perfect yet, but way better.

There's a tiny hint of pinching still, but I'm going to add springs like you did Doug, and shift the retaining clips a little further out.

The truss managed to shift a tad while putting it onto the mount, so the laser centre spot was just on the edge of the centre donut, so this still need a bit of work with a file.

I think I might add a set of 8 bottle screws to the rear assembly to form a lower truss and make it much more rigid.

These aren't issues that any standard GSO scope owners will have as your tubes are solid, but my scope, being an open truss construction, is effectively 2 parts, the primary mirror assembly and the truss/secondary held together with 4 losmandy plates, and this is the weak point.

The 8 and 10 inch versions of this scope haven't shown any of these problems as they're much lighter and the mechanical stress is of a much lower order than with the 12.

[silentrunning]

That's great news Rob.

I would suggest loosening the clips of completely if that is still possible with your setup. Leave them in place off course and tightened up but off the mirror. My fresnel rings were completely circular after doing this and the springs and cork stopped the elliptical stars appearing as the scope moved to different decs.

Doug

[RobH]

Here are a few pics of my scope in bits, and the modified rear cell, which the focuser attaches to, rather than the rear of primary.

You can also see the amount the secondary is offcentre by. The laser is dead inline with the centreline of the focuser assembly.

[silentrunning]

Hi Rob

Is that with the secondary collimation screws removed and the secondary flush up against the back plate or at the point where you reached decent star shapes?

Doug

[Gedan]

This has been a very educational thread but I have to say it scares the bejeezus out of me!!

[RobH]

Hi Rob

Is that with the secondary collimation screws removed and the secondary flush up against the back plate or at the point where you reached decent star shapes?

Doug

That's with the secondary up against the back plate Doug, before I loosend the truss from the 4 plates holding it and heaved it into line a bit more.

This has been a very educational thread but I have to say it scares the bejeezus out of me!!

And me too before I started to take things apart, but it's proving to be very enjoyable and educational too, and at the end, we'll hopefully not only have better performing scopes, but a lot more knowledge about them too

[silentrunning]

Hi Rob

I have found Jareds descripiton (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5804518/Main/5544974) of collimating the 10" very interesting and it seems to make real sense given what I have learnt of the 12" having hopefully fixed the mirror pinch and movement issues. Might be of use to you as well as we move on from these problems.

One thing pointed out in various places is that the secondary centre marking may not be true to either the secondariy's mechanical or optical axis! I have yet to confirm this as I have yet to achieve perfect collimation but I have generally avoided using it just in case. I plan to use a combination of jareds approach and checking for on-axis coma and off axis astigmatism when we get a clear sky.

Doug

[RobH]

Hi Doug.

I did conmfirm that the mechanical centre of my secondary corresponds to the centre mark, but plan on using the coma/astigmatism method for fine tuning, although with a small chip camera this isn't easy as the astigmatism doesn't show up clearly.

Might have to stick my DSLR on there for that, or do it visually.

I have an artificial star enroute too but need to find somewhere that's got a clear 50 metres over grass and that's secure to set up for testing.

[kirkster501]

The centre mark on the secondary of my RC8 precisely matches the optical center. I really rate Jared's Hall Of Mirrors technique. It is astonishingly accurate; 1/100 or so of a turn, the tiniest tweak - just enough to break the natural tension on the screw - adjusts the alignment. I get it almost exactly correct with the Cheshire and fine tune it with hall of mirrors. There is no need for a star test in my experience - it is bang on accurate.

[silentrunning]

Jareds technique makes sense - align optically without the focuser arrangement being involved until the very end. Unfortunately another difference between the 12" and the smaller scopes is that with the 12" the secondary obstructions is too large to see all but the first spider vane reflections but nevertheless a very good starting point.

Whichever approach you use nothing comes close to a star test. Imperceptible asymmetries can spread the energy distribution of a point source so that fainter stars may no longer rise above the noise level for any given exposure settings whilst those that do still appear perfect.

[silentrunning]

I think I have perfected my daytime (no star) collimation method for this scope. Assuming whatever mods have been made to remove mirror pinch/ sag etc...

1. Tighten all primary collimation screws as tight as possible and then tighten the locking grub screws.

2. Loosen or completely remove the secondary collimation screws.

3. Tighten the secondary's' central screw so the secondary is flush against the back plate. This will put the secondary as close to the mechanical axis as possible given manufacturing tolerances.

4. Use Jareds method to align the spider vanes, the spider vane reflections, the centre spot and your eye. However do this initially with the primary adjusters, not the secondary. This will ensure that the primary is as close to parallel with the secondary as is possible before any secondary adjustments are made.

5. Loosen the secondarys central screw and tighten the secondarys collimation screws until the spider vanes etc are all aligned again.

6. Use a laser to tilt the focuser so that the reflected ray is as central as possible on the lasers plate - I do this by holding the laser flush up against the back of the chesire and firing the laser through the chesire sighting hole .

It may be necessary to iterate through steps 4 and 5 but remarkably I got almost perfect visual collimation (chesire) after step 4 without even replacing the secondarys' collimating screws!. Of Course I followed the process to the end so that when I finally get to do a star test I can make those - hopefully - , final tweaks.

Doug

[RobH]

Hi Doug.

My springs arrived today so it's time to take the scope apart again.

How did you attach them to your mirror? It looks like tape.

I'm taking a slightly different approach as my scope design is different.

My only fixed point is the focuser, which is attached to the back of the scope, not the mirror cell. I'm checking that this in line with the mechanical axis by rotating the whole assembly, not the rotating part of the focuser.

If my laser dot stays in the same place, then this is correct.

I'm going to use this and careful measurement to ensure that my secondary is exactly aligned with the same axis.

Once this is done, I'm going to drill some small holes through the plates that hold the truss and into the lower truss ring and the ring that holds the mirror cell..

These will be tapped and I'll put a grub screw into each, probably M4. This should prevent any future slippage.

While I've got the mirror cell open, I'm also going to put cork pads on the back of the mirror where the 3 fans are.

This will stop them blowing directly oto the mirror rear and causing cold spots, which will affect collimation too. The airflow should still circulate around the mirror and help to keep dew off.

[silentrunning]

Hi Rob

I just used double sided tape to attach the springs to the mirror cell spokes. You need to ensure they are central enough not to touch the elevated parts of the rear assembly so that they make nice flat contact with it.

The above method for collimating should overcome any problems with components not being completely aligned with the mechanical axis of the scope. It should result in perfect alignment of the optical axis of the mirror and secondary and the final step with the focuser tilt plate should get the image plane aligned and perpendicular to this. Of course perfect alignment with the mechanical axis would be preferable. My one does seem to be very close if not spot on in this respect.

I am hoping that only secondary adjustments will be necessary if and when we get a clear night.

Good luck with the mods. I take it no trip to IK is now necessary?

Doug

[RobH]

Hi Doug.

I've delayed my visit to Ian as there seemed little point of getting the scope aligned there and then bringing it home just to take it apart again!

After I've done all of the mods etc, if it still isn't performing correctly, then it'll go to Ians, and if it still has an issue after that, then Astrosib or Officina Stellare will be getting an order.

I'd much rather get it right though, certainly from a financial point of view!

I have an atrificial star ordered too, and enough room in the lane at the back of the house to get the required distance for proper testing. Hopefully it'll arrive before too long.

[silentrunning]

Well I thought last year was bad but this has been the worst early summer weather I can ever remember for our hobby!

However, I managed to get out last night in another force 4 wind with clouds hurling across the sky underneath a full moon lit haze. Appalling conditions but as long as I could see a star I could at least get a rough idea as too where the above procedure had left my scope collimation wise.

An out of focus Arcturus looked promising, completely circular pattern but as I pulled the focus in it became apparent that a slight on-axis astigmatism was present the axis of the elliptical star would switch through 90 degrees passing through focus. All attempts to fix this with secondary adjustments failed and in fact these adjustments had no effect on the on-axis astigmatism at all. This suggested to me that the primary and secondary axis were not exactly aligned so I would need to tweak the primary adjusters. As I now had very little idea were the secondary was I loosened the colli screws and tightened the central bolt to place the secondary flush with its back plate. I know from previous attempts that this is at least visually a good starting point for alignment (see above) and indeed the star looked as it did at the start.

Fortunately the first primary adjuster I tried I loosened by about 1/16 of a turn and the star shape improved enormously. Some finer adjustments and I had a circular star on-axis for the first time! No coma, no astigmatism, no mirror pinch. Praise the lord!

I quickly pushed the star to all four corners of the field of view to see what off-axis aberrations where present but I really needed a fainter star and better conditions for this test. It looked good though and then clouds stopped play. Just got everything in doors before it started raining. Its still raining now.

If this is it then I will be tempted to remove the secondary collimation screws altogether and leave it tight up against the back plate!! That a be a first.

Doug