GSO 12" RC Dissambly.

[silentrunning]

For anyone interested I have been trying to collimate my 12" GSO RC which is proving interesting to say the least! I have taken the scope to pieces a number of times in order to attack mirror pinch etc and thought I would post these pictures as their is not a great deal out there relating to this beasty!

Its easy enough to remove the rear assembly and mirror cell from the telescope tube...

The main baffel uncrews from the central part of the cell leaving a large screw arrangment that presses lightly down on the mirror holding it in place..

The mirror cell is held to the rear assembly (that contains the fans) only by the collimation 'pull' screws. This is the back of the mirror cell once removed from the rear assembly.... Its a peculiarity of this scope that the focuser and camera etc all hang of the mirror cell rather than the rear assembly of the scope...

This picture shows one of the three areas that the collimation screws lock into, the hole in the side appears to be where the bitumen has been squeezed through

The mirror is held in the cell by adhesive strips on the back - seen in one of the pictures above, by bitumen, by the large central ring and by these retaining clips...

These clips were quite tight but I have now loosened them, and retightened with a thin paper sheet between the clip and the mirror. After tightening this is gently pulled out leaving a hair breadth of a gap between the clip and the mirror. So at that stage at least there is absolutely no pressure from the clips on the mirror. I actually inverted the mirror cell and placed it in various angles to see if there was any mirror movement that would place additional pressure on these clips and there was none.

I am beginning to think that the weight of the focuser, camera etc which pulls on the centre of the mirror cell gets transferred to the three supporting pillars of the push collimation screws seen next to the spring in the picture above and this distorts the mirror resultign in the triangular stars. This may be backed up by this image of the ring that was close to the zenith. The more central weight of the focuser etc may have amplified the effect. Interesting that the stars have become three perfect little stars!!!!

I now need to figure out a way of supporting the mirror cell within the rear assembly in a way that allows the collimation screws to continue to work and without stressing the mirror in a opposite way !

Doug

[silentrunning]

Compare this image of M5 which was close to the celestial equator, The triangular artifact is still there but not so obvious...

[ollypenrice]

Personally I think it is time these 'kit form' telescopes were brought to book. I've no axe to grind because I don't get involved with them but I'd be interested to know how you feel as a buyer? Are you happy to be doing this work? That really is a genuine and open question. Maybe you are. Maybe not. Just interested.

Olly

[laser_jock99]

Maybe they should come with a warning label "this scope may require time, intelligence & re-engineering to get working properly". If you have none of these skills be prepared to spend 4x as much....

Going back to the triangular star problem- are the only three points of support on the cell the adjuster screws?

[silentrunning]

The scope was second hand at a price way below new. I'm prepared to play around with this to try and get it to what I would call an acceptable standard but I would be going nuts if I had payed the new price for it.

Yes the mirror cell is only in contact with the rear assembly via the adjuster screws and all the weight of focuser etc hangs of the mirror cel!!

[perfrej]

It is the same with the 190MN. It has excellent optics and not so excellent mechanics. As for your "collimation-screws-hold-the-sideway-weight-of-the-primary-holder" problem, that is easily fixed. I had to do it on my 190MN which shows the same problem.

You take a rubber hose with suitable properties to act as padding between the cell and the tube and jam it in all the way around. I would also suggest replacing the springs around the three collimation screws with either O-rings (2 or 3) or a short piece of stiffer and thicker rubber hose. Collimation screws should be screwed all the way in to start with, and collimation should be perfomed by losening as little as possible on the screws so that they always are as short as possible.

The above procedure will eliminate the problem of the cell sliding back and forth in the tube laterally. You probably have three nylon screws that keep the mirror glass centered in the holder as well. These need to be handled the same way as the clamps - gently.

All in all, once you get them tuned they're probably good enough

/per

[RobH]

Hi Doug.

Thanks for taking the time to post these pics....a useful resource for aqnyone with a GSO Ritchey Chretien, as they are all built the same way.

As far as I know though, the only ones to show astigmatism due to stress on the primary mirror are the 12's.

Mine was exactly the same until the mirror cell was altered and the whole thing built into a carbon truss. I didn't do this myself but am tempted to take it apart to see what exactly was done.

Cheers

Rob

[silentrunning]

Thanks per,

I was just beginning to deduce that some astigmatism that seems intermittent was due to sideways movement of the mirror as a result of the mirror cell being inadequately supported by the three collimation screws allowing some lateral movement of the cell within the tube. I have just this morning been thinking about how to reduce this and length wise movement as well and had deduced the same aproach - keep the colli screws as tight as possible and use some kind of padding to prevent lateral movement. I will also consider replacing the spring with some stiffer rubber supports. To prevent pinching I am considering putting a large rubber O ring or a number of springs around the central stalk that will transfer the weigtht from the mirror cell to the rear of the tube. My only concern is trying to make all this work whilst allowing some movement for collimation. Stiff but not solid rubber I guess.

Thanks Rob, I would be fascinated to see what mods have been done to your mirror cell.

Doug

[laser_jock99]

On my F4 imaging Newtonians I did away with the useless 'mirror locking' bolts and replaced them with extra springs- thus giving the mirror cell six points of contact instead of three. Detail of the 'six spring' mod here:

I'm not sure if the RC has 'mirror locks' though or any means of apply extra support points in the Z axis?

Two ways of stopping X-Y shift of the mirror cell are detailed here:

Again whether these methods are applicable to the RC I don't know?

I'm glad you did pay full price for the scope- as Olly say's it's nice to have a useable scope from the off and not an engineering challenge!

[laser_jock99]

I'm glad you did pay full price for the scope- as Olly say's it's nice to have a useable scope from the off and not an engineering challenge!

Should have read- glad you DIDN'T pay full price!

[silentrunning]

Thanks laser do you have the second link - they both point to the same thread.

Doug

[kirkster501]

I am of the opinion that with most scopes involving a mirror then some DIY'ing is required to get it "right" for oneself.

I have made modifications to both my GSO RC8 scope and my 12" Dob.

[RobH]

I'm not sure if the RC has 'mirror locks' though or any means of apply extra support points in the Z axis?

Two ways of stopping X-Y shift of the mirror cell are detailed here: http://stargazerslou...ng#entry1918274

RC's don't have mirror locks. You only need these on SCT's, as focusing is done by moving the primary mirror, so it needs locking afterwards. You still tend to get a bit of mirror slop going through the meridian though on some of them.

On my 12 inch RC, even before the mod was done, I didn't get any X-Y shift at all, just astigmatism caused by the focuser and imaging gear all hanging off the back of the primary.

[kirkster501]

I am very pleased with my RC8 - its a lovely piece of kit and very well made. This thread is very useful in case I need to take it apart to clean the mirror some day!

The issue with the GSO brand of RC scope is the fact that the mirror is coupled to the focuser with no direct means of disconnecting the two to allow for independent adjustment and collimation. You need the focuser decoupling adjustment plate from Altair-Astro or Teleskop-service to provide that decoupling - another £80 or so - to get it perfect.

[silentrunning]

Beware that the RC8 is totally different in design to the 12" - as far as I have seen on the web. . Not much here will be of use to you if you decide to venture in.

I have the focuser decoupling ring from Teleskop which compensates for the equivalent of chip tilt with the focuser not being completely orthogonal to the mirror but it does not to solve the weight hanging of the back of the mirror cell as it screws into the cell in the same way as the focuser - in that sense it only adds its own weight to the problem and it is probably not needed in chips below 22mm in size anyway.

I fear with the stock mirror cell that some x-y movement is inevitable with just three bolts holding the mirror cell in place. At the moment I am set on fixing the mirror pinch astigmatism associated with the weight being applied to the center of the mirror. Without having a special mirror cell built I intend to try some springs and a large rubber O ring to support the main mirror.

Doug

[r3i]

Beware that the RC8 is totally different in design to the 12" - as far as I have seen on the web. . Not much here will be of use to you if you decide to venture in.

The best guide that I found for taking the RC8 to bits is here: http://deepspaceplace.com/gso8rcpointing.php

The TS Focus Collimator is definitely a must have for these scopes if sticking with the stock focuser - it makes collimating the full optical train doable and relatively straightforward.

[laser_jock99]

Thanks laser do you have the second link - they both point to the same thread.

Doug

Here you go http://stargazerslounge.com/topic/185419-mirror-stability-modifications-collimation-retention/page__hl__+six%20+spring#entry1927113 as said, not sure if it's applicable to your mirror cell.

[RobH]

An update on this.......I've never managed to get my star shapes perfect, even after the mirror cell redesign, but put it down to a collimation issue.

However, I've been seriously thorough with my collimation procedures, measuring, and using 3 different types of collimation tool (Glatter, Hotech and Tak collimation scope) plus a cheshire.

Last night was clear for 4 hours and I spent the entire time testing, adjusting and retesting, but still my stars weren't round.

In the end, I went back to basics and used a high power eyepiece (should have done this to begin with!), and bingo.....pinched optics. I assume it's the primary.

The scope is now off the mount and about to be completely disassembled.

Here are 2 test images, inside and outside focus....no wonder I couldn't get my stars round.

[laser_jock99]

You would think manufactureres would bench test scopes on an artificial star before they ship?

[ollypenrice]

An update on this.......I've never managed to get my star shapes perfect, even after the mirror cell redesign, but put it down to a collimation issue.

However, I've been seriously thorough with my collimation procedures, measuring, and using 3 different types of collimation tool (Glatter, Hotech and Tak collimation scope) plus a cheshire.

Last night was clear for 4 hours and I spent the entire time testing, adjusting and retesting, but still my stars weren't round.

In the end, I went back to basics and used a high power eyepiece (should have done this to begin with!), and bingo.....pinched optics. I assume it's the primary.

The scope is now off the mount and about to be completely disassembled.

Here are 2 test images, inside and outside focus....no wonder I couldn't get my stars round.

Classic pinched optics and no mistake so at least you know where you are. Reading this thread again makes me feel that, as I have always suspected, these scopes are not for me. If you are up for them, then fine.

I cannot say that I relish dealing with Orion Optics (because I don't think that one should have to beg to obtain a manual for a new telescope, for instance) but Yves' ODK14 does work well. There was one bungle regarding baffling but Ralf, our friendly optical genius, saw it as soon as he looked through the eyepieceless scope and made a baffle to sort it. That done, it holds collimation well - very well - and gives good results.

But here's a question; is there an imaging reflector that I would buy myself with my own cash for use with guests? (That means it has to work, now, tonight, no messing.) ANd the answer is probably No!

Olly

[kirkster501]

As long as there are no mechanical issues such as pinched optics, misaligned focuser, etc and the scope is otherwise sound, all you need to collimate an RC scope is a Cheshire EP and a nice, bright day with the scope outside. You can get it spot on accurate with just the Cheshire and a steady hand turning the screws 1/50 of a turn.

EDIT- bad punctuation.

[r3i]

As long as there are no mechanical issues such as pinched optics, misaligned focuser, etc and the scope is otherwise sound, all you need to collimate an RC scope is a Cheshire EP and a nice, bright day with the scope outside. You can get it spot on accurate with just the Cheshire and a steady hand turning the screws 1/50 of a turn.

EDIT- bad punctuation.

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?

[laser_jock99]

But here's a question; is there an imaging reflector that I would buy myself with my own cash for use with guests? (That means it has to work, now, tonight, no messing.) ANd the answer is probably No!

Olly

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.

[RobH]

Spent a good 12 hours taking the scope apart, checking everything, reassembling and testing yesterday (and very enjoyable it was ).

Once reassembled, the glatter collimator was still putting the spot 5mm from the centre of the secondary.

I measured absolutley everything, so I know the centre of the secondary is in the correct place, the seconday cell is centred in the truss, and the truss is true.

The laser is concentric in the focuser etc, so the only point where there was any play was the attachment between the truss and the main body of the scope.

I loosened everything up and shimmed one angle of this. That put the laser spot dead centre on the secondary.

The scope is now back on the mount, and I'm about to get a ball bearing on a stick 40 metres away which, if the weather allows, I'll use the reflection of the sun coming off this as a point source, or otherwise fire a laser at it.

This should allow me to see if I've at least cured any pinching, although I couldn't find anything that may be causing this.

[silentrunning]

Good news I think Ive cracked it - the collimation issue that is not the main mirror!

In order to get rid of any mirror pinch I applied six springs to the back of the main mirror to support its middle and take up the weight of the focuser, a school boy solution but I really didn't want to try and build a multipoint point support mirror cell...

Once back in the rear assembly I tightened the main collimation screws as tight as I dared. Once back on the telescope with the focuser removed I made some tiny adjustments to the primary collimation screws to align the main baffle with the telescope . I did this by looking to the extreme edge of the rear aperture and equalising what i could see of the clips that hold the spider to the main tube. This would at least be a starting point for the main mirror posiiton. I then loosened all the secondary collimation screws and tightened the main secodary assemblty screw until its housing was flush against the back of its support. Looking through a chesire I was suprised how close to collimation the scope was after these crude steps. I left it like this with no pressure on the secondary screws as I wanted to be sure that these wern't causing mirror pinch either.

Last night through twilight and with a force 4 wind blowing I was able to check collimation using some stars at various decs. The out of focus stars were the closest to cicrcular I have so far achieved and I was able to remove some minor on-axis coma but lossening the secondary and making traditional adjustments to the secondary collimation screws.

I managed just 4 unguided 20s images of M13 through twilight. About 100 more where ruined by the wind. I have stacked the 4 here - no darks or flats, just loads of stretching....

Just 80 seconds through twilight!

As part of this whole struggle I also downloaded a program called ATMOS that allowed me to model the GSO 12" RC and produce spot diagrams and point spread functions at different areas of the fov. This was a god send. I had imagined that whenever a star appeared as multiple stars mirror pinch was probably to blame but generating the PSF for a star at the edge of a decentered field I managed to pretty much exactly match some of the double star effects I was seeing. So mirror pinch was not to blame for these effects which are inherent in the RC way out on the edge of the field. I still nead to centre and flatten the field but at least I'm working on the right issue now!

Doug