GSO 12" RC Dissambly.

[RobH]

You were a braver man than I last night Doug!

I looked at the sky and thought better of it, and by the time I took the hound out for his bedtime walk it was just starting to rain!

I'm not quite sure I understand you properly......was it the adjustment of the primary collimation screws that cured tyhe astigmastism, or putting the secondary against the back plate?

From my experiences, I found that when I pulled the primary back as far as it would go, my mirror pinching got worse, so I'll be trying mine with the mirror pulled back far enough to adjust it, but no further.

Today's job is to build a basic alt/az cradle to put the scope on in order to aim it properly when using an artificial star as I can't see anything but the sky from inside the obsy!

[silentrunning]

You were a braver man than I last night Doug!

I looked at the sky and thought better of it, and by the time I took the hound out for his bedtime walk it was just starting to rain!

I'm not quite sure I understand you properly......was it the adjustment of the primary collimation screws that cured tyhe astigmastism, or putting the secondary against the back plate?

From my experiences, I found that when I pulled the primary back as far as it would go, my mirror pinching got worse, so I'll be trying mine with the mirror pulled back far enough to adjust it, but no further.

Today's job is to build a basic alt/az cradle to put the scope on in order to aim it properly when using an artificial star as I can't see anything but the sky from inside the obsy!

Hi Rob

I was beginning to gnaw my own leg off so I took what chance I could with the weather/seeing etc, a close call though.

It was an adjustment to the primary that finally got rid of the on-axis astigmatism. Moving the secondary back on to its back plate was just to get it back to a known starting point. Things may be different for your set up as my focuser moves with the mirror and no doubt this has some bearing on how the adjustments are realised.

Pulling the mirror all the way back initially gave me horrendous mirror pinch until I ensured the clips were loose enough not to touch the mirror even when it is pulled right back. This can be done whilst the rear assembly is off the scope.

How did installing the springs etc go?

Doug

[RobH]

No problem installing the springs Doug, I used a metal weld type stuff which took 24 hours to cure but ewon't fall off, and baffled the fans using cork tile also.

I moved the mirror clips out as far as they would go so they are nowhere near the mirror surface now.

I also adjusted my truss while using my glatter collimator to send a beam at the secondary. I tweaked the truss alignment until the laser was as close to the centre of the secondary as I could get it, tightened everything right up, and then drilled and tapped holes in the plates that hold the truss and put M4 grub screws iinto them, which will stop any movemnt of the truss when I move it.

Final adjustement of the focuser was done after thesecondary had been collimated, by using a tilt plate on the focuser. This only needed the slightest tweak though.

[kirkster501]

Nice work guys here. I love my RC scope, it is fabulous!

Blasted weather is naff! Not been out for a fortnight, and I am buying my first ever CCD next week. Seriously make me wonder sometimes...

[RobH]

Naff weather indeed

If I can get this scope working well, then I may well invest in one of the CF 8 inch scopes to cover the middle distance (so to speak) between the 12 and my 80mm, plus, it should work fine on my HEQ5 too.

I see you're in Nottingham Kirkster.....I was born and raised there, in Beeston,and only moved away in 2002.

[silentrunning]

Nice work guys here. I love my RC scope, it is fabulous!

Blasted weather is naff! Not been out for a fortnight, and I am buying my first ever CCD next week. Seriously make me wonder sometimes...

What CCD have you got in mind?

[RobH]

Don't get a more recent SX.....go for an Atik.

[kirkster501]

Naff weather indeed

If I can get this scope working well, then I may well invest in one of the CF 8 inch scopes to cover the middle distance (so to speak) between the 12 and my 80mm, plus, it should work fine on my HEQ5 too.

I see you're in Nottingham Kirkster.....I was born and raised there, in Beeston,and only moved away in 2002.

... I worked at Siemens and at Plessey's in the Rylands for many years

[kirkster501]

Don't get a more recent SX.....go for an Atik.

An Atik 460 is my plan Rob....

[RobH]

An Atik 460 is my plan Rob....

Excellent choice......a brilliant camera.

My Dad did his apprenticeship at Ericsonns, and spent his entire career there, and at Plessey as it later became. He was in charge of a lab and headed the international comittee that decided on our wire and cable colour codes.

[marcoverstraaten]

What an excellent thread this is for me! I pulled some hair out the last 2 months!

Now I totally rebuild an 10" a couple of years ago allready, forcing me to dive into the RC collimation, and I totally got used with it.

First option for me was to get rid of the faulty design of focusser & mirrorcell. So I did on the 10" & now on my 12".

I have the same issues with the 12" with perfect collimation.

This is the best a got so far:

I haven't come up with your idea, so this opens up new possibilities! Thanks!

[marcoverstraaten]

And as far as checking axis alignment with focusser:

I removed the secundary and taped a small mirror on top of the secondary mount cell. On the outside.

So now I can check the alignment of the system with a laser. (goes through the center screw hole of the secundary and back bouncing on the placed mirror)

Note: my focusser is seperated from the rear cell so this helps me adjusting the focusser.

[silentrunning]

That is exactly how my stars looked at the start of this process. I haven't solved it yet by I'm getting closer. The weather and short nights has got in the way of any progress for the last two weeks.

How did you make separate the focuser from the mirror cell? is that a standard 50mm extension screwed into a custom made adapter on the back of the rear assembly? Any chance of some more images with the various extensions and rings removed so I can see the back of the mirror cell through the rear aperture? That would give me a good feel for what you have done. You're flat mirror arrangement for checking focuser/secondary alignment crossed my mind and I may still give it a go for good measure.

Thanks for posting, looks like you have a fantastic observatory.

Doug

[marcoverstraaten]

Doug, it's a ring I made on my lathe. Mounted with bolts on the back, on the other end threads cut the size of the extension rings provided with the scope. So I can still use those. Moonlite has extension parts from there to match them to their focussers.

Now, I got the scope apart again. As you can see, the triangular star shapes are a bit tilted. I ligned up my camera to match dec/ra movement, so this tilt looks a bit odd. You'd expect to have 2 "points" of the triangular star to be horizontal in the frame to match the position of the mirror clamps. The mirror support frame has six "vanes" (can't make up another word for that, sorry), but only three of those have a small piece of cork. An that's the only support the main mirror has !!!! The position of these 3 pieces of cork are rotated in relation to the line up of the collimation screws. Overtighting the center baffle or side clips should cause these triangular stars. I hope you can visualize what I mean.......

A month earlier, super high end telescope & optics technician Hans Dekker tested my optics. They were fine after cutting the kit ( 3 positions on the side. before: astigmatism!! ). But the test was done horizontally. So, back home: triangular stars again...... AAAAARRRRGGGGHHH!

Yesterday I removed the cork supporting the mirror and put some tape on more (measured) places to give the mirror more support. I know this is not the correct way to support a mirror, but at least I should see some change in star shape. To be tested!

It is and was my goal to just use the optics and make a nice truss-design like I did wih the 10". So I can allways redesign a mirrorcell if necessary.

[marcoverstraaten]

Btw, I loosened all parts holding the mirror on its place after this. So clamps and center baffle are loose for the next test.

[silentrunning]

That is perfectly clear and recognizable as I have been analyzing the star shapes in relation to the clamps, cork, collimation screws and vanes in exactly the same way. Remarkably I am right at the same point of having near circular stars at low declination but once I head off toward the zenith the stars become triangular and actually split into two or three points of light!

Here are some stars from M5 at about 30 degrees alt...

and here is M13 at about 75 degrees alt...

Both imaged through massive amounts of twilight. The stars appear triangular or as three small stars across most of the field but may distort into just two stars in one or two areas in the out field.

I think the accepted wisdom would say this is mirror pinch or distortion with additional aberrations but by using ATMOS I have managed to model the star shapes I am seeing across the field with just very slight mirror decentering - 0.5mm and a tilt of 0.1mm at the edge which may be occuring when the scope is vertical as I collimate horizontally. It may well be a combination of all these factors!

I also may have had the collimation screws to tight for the above images as I did achieve much better star shapes when these where loosened - though I cant figure out why unless the mirror is pivoting on a spot additional to the collimation screw supports. Unfortunalty clouds prevented me from trying this when the images above where taken.

Coincidentally my next move was to to loosen everything and see what gives!

Doug

[silentrunning]

Oh and by the way the smallest change (0.05mm) in focus can change the shape of the stars enormously!

[silentrunning]

Here is a ATMOS diffraction PSF for a slightly decentred and tilted main mirror of the GSO 12 RC. The spot diagrams on the right show how the pattern changes away from centre and away from optimal focus. One thing that leads me to suspect that it is decentre and/or tilt over mirror pinch is that I haven't seen a centre star amongst the three at focus which I believe should be present for mirror pinch. Check out the patterns here.. http://www.telescope-optics.net/diffraction_pattern_and_aberrations.htm

Doug

[silentrunning]

And a close up to comapre with the above model...

[RobH]

I mamnaged to get my stars perfectly round, then pareked the scope at the end of the session, and the next day they were back to being slightly distorted again, so something had moved.

The scope is now packed away and ready to be shipped to France, where it's haveing a totally new multi point mirror cell built and fitted, and the whole truss assembly etc is being checked so that the mechanical axis and optical axis are the same.

I've had enough of faffing about.....I want to get some decent images with the scope, not spend every night trying to get good collimation again and again!

[silentrunning]

Don't blame you Rob, something is definitely still moving in my set up as well and it may be hard to put right given the standard rear assembly/mirror cell arrangment. Do let us know how the latest set of improvements go.

Doug

[marcoverstraaten]

+ There's no space to put in a decent mirror cell. Back to the drawing board to design a new backplate & mirror cell, that will do the trick.........

[RobH]

+ There's no space to put in a decent mirror cell. Back to the drawing board to design a new backplate & mirror cell, that will do the trick.........

That's what I'm having done Marco. I'll be away from home working for the next 5 weeks, so hopefully it'll be done by the time I get back.

[silentrunning]

Well all is not lost. Last night was clear for the first time in ages and though twilight was around for the duration I had a bit of an imaging session rather than a tweaking one.

As I was convinced the dodgy stars are due to decentering and tilt I packed the mirror tight in the mirror cell with cork strips and added more cork between the mirror cell and the rear assembly. I tightened everything down and left the secondary hard up against its back plate.

I had a quick look at star shapes and they were elliptical. Loosened of one of the fully tightened primary mirror collimation bolts through about a half turn and the star were circular. Carefully tightened the locking grub screw or that bolt and then checked for any coma in the on-axis star. Slightly loosened the secondaries centre bolt and made some very small turns on the secondary adjusters and bingo! Little or no on-axis astigmatism or coma. Moving the scope between about 20 degrees and 90 degrees altitude no change in star shapes. This was looking good. The extensive cork packing must e preventing the mirror moving/tilting.

I quickly grabbed 4x120s frames of M13....

As slight breeze was playing havoc with guiding I tried 4x120s with my CCDT67 focal reducer (which intrisuces its own aberrations and corrections)

Both images just stretched and color enhanced slightly.

The near centre star shapes are now as much at the mercy of guiding accuracy as anything else although collimation can still be improved as I didn't bother with any barlowing so the tweaks were done at native f8. There is also a bit of tilt in the image but I happy that the star shapes are now very close to what ideal collimation and zero mechanical problems permits. Wether it holds collimation between me lifting the scope on and off and generally knocking it around waits to be seen.

I also got some images of M57 which look good but I have yet to process.

Doug

[RobH]

Nice one Doug....that's more like it!

I put my 6 inch TMB triplet on the mount yesterday and also managed 2 x 30 second shots of M13 before the clouds stopped everything.......staggeringly good.....don't think I'll be getting rid of it any time soon!