Help please - Ritchey Chretien driving me mad

[Skipper Billy]

Apologies for posting in the 'getting started' section as I am not a total newbie but felt this was the most appropriate place to post my problem as this scope has made me feel like a total beginner.

Its a 6" Ritchey Chretien - Altair Astro - bought used. 

Its on a HEQ5 and attached to it is a Moonlite focuser with electronic stepper motor, Atik EFW2 and Atik 460EX

The guiding is spot on (ST80 with Lodestar) and well proven with another scope.

I have tried every trick in the book to achieve a decent star shape to absolutely no avail - this is a brief summary of what I have done. (Based on reading every document and watching every YouTube video under the sun) and having bought a Howie Glatter laser collimator and diffraction grid to assist - which cost as much as the scope!)

1. Made sure that the laser collimator is collimated
2. Made sure that the primary mirror is not pinched or too slack and that the drawtube is orthagonal to the primary mirror by laser test
3. Collimated the primary so that the laser exits exactly through the central mounting hole for the secondary. Later checked that the laser lines up with the middle of the circle in the middle of the secondary.
4. Collimated the secondary so that the laser returns exactly to the source point.
5. Platesolved the resulting images to check that the focal length is correct (1370mm) and adjusted the distance between the two mirrors until the focal length is spot on - then repeated all the above steps as this alteration changes the collimation.
6. The diffraction pattern as per the many videos looks spot on. A peek through a quality Cheshire confirms everything is spot on.
7. Fitted the AP CCDT67 reducer. (I have also tried it without the reducer and the results are just the same but a narrower field of view)

I expected after all that effort to have tight sharp stars.

Below is what I have got (after just a rough and ready stretch of single 30 second images with Luminance filter) - really poor star shape and cannot achieve good focus. The variance between the images is down to trying to make minor adjustments in real time on a defocused star. Image 'd' is the best I could achieve. (The Airey rings were extremely narrow - huge central obstruction?? or a pointer to the issue ???)

Focusing with Focusmax also appears to be an issue - probably down to the lousy star shape - I cannot achieve a FWHM less than 9. If I focus manually I can do no better - the stars look like they are about to pop into focus then before that happens the focus goes away again. The images below are the best focus I could achieve (granted the seeing wasnt very good last night)

I have had to use all 3 extension tubes to achieve focus (75mm in total) - not sure if thats relevant but the camera is now a LONG way from the back of the OTA !!

I am now coming to the conclusion that there is a serious problem with this scope that cannot be sorted out by collimation.

My appeal is to anyone that might be able to shine any light of wisdom on this mess. Having spent countless hours adjusting and readjusting then wasting two whole clear nights I am ready to scrap this scope and write it off as a bad loss and dust off my trusty ED80 - I just wanted a longer focal length for the smaller targets.

Any suggestions no matter how far off the wall or whacky will be most gratefully received!!!!

Apologies for the lengthy post but at least the laughter that the attached images will generate will hopefully brighten your Sunday morning !!!  

 

 

 

 

[alanjgreen]

Off the cuff idea.

Did it come with the moonlite or have you added it afterwards?

is the focuser in alignment with the scope?

[Skipper Billy]

I added the Moonlite to replace the standard focuser.

It is orthagonal to the primary mirror (which it is attached to) as far as I can tell by laser test as per http://www.hnsky.org/RC_collimation.htm

[alanjgreen]

have you got results whilst using the standard focuser to compare against?

[Skipper Billy]

No - I never fitted it as its just the standard GSO focuser and I need to be able to focus remotely - besides the draw tube had significant play in the body.

I can always fit it to try it though as remote focusing is merely a pipe dream at the moment !! Are you thinking faulty Moonlite ??

[alanjgreen]

I was thinking that the moon lite may need a collimation tweak.

i had a hotech SCT laser collimator once and it showed that my baader steeltrack was not aligned with the optical path.

the striking thing in the image is the double diffraction spikes -must mean something

[Dave In Vermont]

Is it possible it could have 'pinched-optics?' I'm not quite sure how the RC has the main-mirror seated though. But that's what I'd think might be culpable in a reflector.

An aside: Did you get this for visual or AP or both?

I'll be watching this thread. Those scopes are an enigma to yours' truly. Good luck!

Dave

[Skipper Billy]

15 minutes ago, alanjgreen said:

I was thinking that the moon lite may need a collimation tweak.

i had a hotech SCT laser collimator once and it showed that my baader steeltrack was not aligned with the optical path.

the striking thing in the image is the double diffraction spikes -must mean something

I adjusted the focuser collimation as the focuser is directly attached to the primary mirror - then the whole assembly is aligned to the optical path - only 1 screw needed about 1/16 of a turn. I have never seen double diffraction spikes before !!!

15 minutes ago, Dave In Vermont said:

Is it possible it could have 'pinched-optics?' I'm not quite sure how the RC has the main-mirror seated though. But that's what I'd think might be culpable in a reflector.

An aside: Did you get this for visual or AP or both?

I'll be watching this thread. Those scopes are an enigma to yours' truly. Good luck!

Dave

The primary isnt pinched - when I got the scope it was loose - it is now just about firm enough not to rotate or flop but only just. The secondary I think is bonded in to its holder so there doesn't seem to be any way to adjust it.

I got it purely for AP.

[MARS1960]

Have you tried using an RC focuser collimating ring?

[Skipper Billy]

4 minutes ago, MARS1960 said:

Have you tried using an RC focuser collimating ring?

The Moonlite has a built in collimating feature - I used this to align the focuser with the primary mirror instead of a collimating ring.

[alanjgreen]

You could try an image up at the zenith to rule out mirror flop?

not all your images have the same double diffraction so maybe something is moving depending on the scope angle/gravity.

take a much shorter exposure to reduce chances of something moving?

 

need more coffee to get more ideas now...

[kens]

 

39 minutes ago, Skipper Billy said:

I have never seen double diffraction spikes before !!!

I get double diffraction spikes on my RC8 when it is not in focus. I used that to help focus till I got a Bahtinov mask.

[steppenwolf]

Image 4 looks in focus as the 'double' diffraction spikes are aligned with one another and the star shapes are good so the telescope can physically focus - are these sample images crops or reduced size full frames?

[Oddsocks]

I had an early version of the Altair Astro 8" RC for a couple of years and as delivered it had similar issues to yours though not so severe.

Things I learned were that the focal length specification is only a guide, due to production tolerances encountered when making the two hyperbolic surfaces the distance between them has to be adjusted to match both primary and secondary "actual" figure, in my scope on delivery it suffered from severe spherical aberration at the nominal focal length of 1600mm and using a Ronchi eyepiece I found adjusting the distance between primary and secondary for the best Ronchigram and minimum spherical aberration resulted in a focal length of 1545mm and I was able to drop from three extender tubes to two extender tubes.

My early RC was delivered without the tilt adjuster plate on the rear cell for the focuser mounting and it was impossible to achieve a good collimation when trying to align the optics axially to the centre of the camera, the best collimation resulted in image tilt at the sensor, this is due to the way the primary is mounted, only fixed by a thin ring nut on the centre baffle tube and no other support results in a tilted primary that can not be corrected, you have to adjust for perfect collimation and then find a way to tilt the camera, I used an add-on tilt T2 adjuster to correct for this defect.

The secondary and primary not only have to be the exact distance apart to suit their individual figure but also rotation of the secondary has an effect, when eyeballing using the Ronchi eyepiece you can rotate the secondary to find the "sweet" spot that gives the straightest interference pattern of the Ronchigram.

Summary, use a Ronchi eyepiece to set the distance between primary and secondary, not the expected focal length specified by the maker, you can make your own Ronchi quite easily, I originally made my own Ronchi filter with a laser printer on clear acetate and fitted inside an old collimation cap, no actual lenses needed, but now I have a bought Ronchi eyepiece with a higher line count for setting up my very similar Dall Kirkham telescope), rotate the secondary while eyeballing the Ronchigram to find the "sweet spot", use a T2 tilt adjuster to correct for residual image tilt, do not try for perfect axial alignment while collimating, it is very unlikely to be achieved because of the central primary mirror mounting system.

Although you have probably read enough about collimating an RC you might find the linked webpage of interest.

http://interferometer-tests.blogspot.co.uk/2013/06/2542000-10-ritchey-chretien-gso.html

 

[Peter Drew]

My impression of the first image is that something in the imaging train at that moment was tilted, the asymmetric images seem similar across the FOV. Subsequent images as posted seem to have corrected that to a great extent, the stars are round which rules out collimation issues but still "blobby" by expectation standards. My limited experience of using focal reducers on SCT's leads me to suspect that this might well be a contributory cause.   

[Skipper Billy]

55 minutes ago, alanjgreen said:

You could try an image up at the zenith to rule out mirror flop?

not all your images have the same double diffraction so maybe something is moving depending on the scope angle/gravity.

take a much shorter exposure to reduce chances of something moving?

Thanks Alan - I was having to move around the sky to find clear patches so the scope was in various orientations - I hear what you are saying - the primary mirror is only held around the central hole by a compressed 'O' ring - when I got it it was visibly flopping about - now the 'O' ring is just snug but not tight so maybe it is moving - will do as you suggest - forecast to be clear (ish) tonight.

Many thanks.

[Skipper Billy]

25 minutes ago, steppenwolf said:

 

Image 4 looks in focus as the 'double' diffraction spikes are aligned with one another and the star shapes are good so the telescope can physically focus - are these sample images crops or reduced size full frames?

 

Reduced size full frames - the stars are massive !!!!

[Skipper Billy]

16 minutes ago, Oddsocks said:

 

I had an early version of the Altair Astro 8" RC for a couple of years and as delivered it had similar issues to yours though not so severe.

Things I learned were that the focal length specification is only a guide, due to production tolerances encountered when making the two hyperbolic surfaces the distance between them has to be adjusted to match both primary and secondary "actual" figure, in my scope on delivery it suffered from severe spherical aberration at the nominal focal length of 1600mm and using a Ronchi eyepiece I found adjusting the distance between primary and secondary for the best Ronchigram and minimum spherical aberration resulted in a focal length of 1545mm and I was able to drop from three extender tubes to two extender tubes.

My early RC was delivered without the tilt adjuster plate on the rear cell for the focuser mounting and it was impossible to achieve a good collimation when trying to align the optics axially to the centre of the camera, the best collimation resulted in image tilt at the sensor, this is due to the way the primary is mounted, only fixed by a thin ring nut on the centre baffle tube and no other support results in a tilted primary that can not be corrected, you have to adjust for perfect collimation and then find a way to tilt the camera, I used an add-on tilt T2 adjuster to correct for this defect.

The secondary and primary not only have to be the exact distance apart to suit their individual figure but also rotation of the secondary has an effect, when eyeballing using the Ronchi eyepiece you can rotate the secondary to find the "sweet" spot that gives the straightest interference pattern of the Ronchigram.

Summary, use a Ronchi eyepiece to set the distance between primary and secondary, not the expected focal length specified by the maker, you can make your own Ronchi quite easily, I originally made my own Ronchi filter with a laser printer on clear acetate and fitted inside an old collimation cap, no actual lenses needed, but now I have a bought Ronchi eyepiece with a higher line count for setting up my very similar Dall Kirkham telescope), rotate the secondary while eyeballing the Ronchigram to find the "sweet spot", use a T2 tilt adjuster to correct for residual image tilt, do not try for perfect axial alignment while collimating, it is very unlikely to be achieved because of the central primary mirror mounting system.

Although you have probably read enough about collimating an RC you might find the linked webpage of interest.

http://interferometer-tests.blogspot.co.uk/2013/06/2542000-10-ritchey-chretien-gso.html

 

Thanks Oddsocks - I must admit I have never heard of a Ronchi eyepiece - lots of research to do !!!

Understood about the focuser mounting.

Right - Google 'Ronchi' - thank you!!!

[johnrt]

Afternoon,

I full understand how frustrating this can be! I think you have had some decent advice above, but I would double check the primary is secure, I screwed mine down nice & tight so it will not flex at all. Check also the secondary is secure, mine was also loose. It also has a locking ring to hold it it in place around the edges. Looking at the images above I think something is tilted somewhere, I also needed the tilt plate adjuster to get my 6" RC in to an acceptable collimation.

Don't give up!

[steppenwolf]

2 hours ago, Skipper Billy said:

Reduced size full frames - the stars are massive !!!!

In that case, yes, the stars are massive! Frustrating for you but hopefully the good advice from the folks above will set you on the right course. I would be concerned about the adjustment that you have made to 'match' the quoted focal length as this will just be a nominal figure within manufacturing tolerance so there may be some distortion introduced here?

[Peter Drew]

Provided that the detail that the primary mirror rear surface bears against is also a small diameter the mirror can be done up fairly tightly without distortion. This is contrary to the normal advice with mirror clips which, if tight, certainly can introduce distortion. Being as the star shapes in the better images seem round it's probably some anomaly with optical spacing that is spoiling the focused star size as already mentioned.

[Skipper Billy]

Thanks Peter - will try tightening a little more.

Its driving me insane !!!