C9.25 Collimation Test Image

[asimov]

Hi.

Haven't posted this one before. Took it on 4th June. This image convinced me I had an optical misalignment problem somewhere. But then again, it was 3/10 seeing at the time to my eye.

I'm not sure what to think. This image has detail, but theres something funny/fishy about it. I can't put my finger on it, but it looks like theres 2 semi transparent images superimposed on top of one another, but not quite aligned identically....?

I thought I better get some opinions before I start stripping the scope! :lol:

[Astroman]

Maybe only use one eye when taking the images, that might help....


Other than that I got nothing :D Well how does the scope perform under a star test? does it look misaligned when looking at a defocused star?

[Lester]

Hi mate,

Was that a 90 second capture time? If the moon is Io which we learnt needs 45 seconds to freeze it and no longer, it looks just slightly elongated. So in that case I would say try again before open heart surgery.

Did you want me to drop around after tea tonight to give you a hand?

[asimov]

No idea Lester, deleted the AVI.

No, won't be home tonight, thanks anyway.

[Starcrazzy]

yep..scope is had it..send all your stuffed gear to me...lol

[davidpretorius]

dunno mate, i would say seeing.

The seeing has probably been too crap to collimate on the moons of Jupiter. Chris go in Cebu Phillipines collimates on the moons first before he fires up the runs on his c11.

There is no way i could collimate on a moon at f38.5 with my dob, but how well would it work to tweak th collimation on the c9.25 or the super meade 14"?

[StargazerX1]

asi,

I think it is seeing! Sometimes Registax will include frames that are
sufficiently different to create this illusion, which is seeing based. I
think stack graph allows you to deselect them.

The scope may need collimation, but the image posted does not indicate
that one way or another. Try a star the next time the seeing is at least
fair. Throw away any images that don't resemble a star, Registax will loose
track on a bunch of them anyway (using a 32x32 box), you press 'ignore to
deselect frame'. When stacked you will hopefully see an Airy image!

Do you have Bobs knobs on your C9.25? I have them on my C11, which is much more difficult to collimate than the C9.25, and the knobs make it at least bearable. Unfortunately, seeing has to be at least fair to do it properly. I also collimate using the same image train as I image with (no diagonal!). Slipping a 10mm plossl in there, I'm working at about 840X which makes miscollimation evident. I sometimes also select the red filer which minimizes turbulence. I have to collimate twice every night I follow a planet crossing the meridian! Sometimes I collimate on a Jovian moon, with experience you notice asymmetry in a slightly out of focus moon, but seeing has to be better than average in my experience to do this.

You might also try collimating using your camera. You have a great scope, don't blame it until you verify it has reached equillibrium and is collimated Good focus is assumed - but alas, it also requires fair seeing!

Glenn Jolly
Arizona, USA

[davidpretorius]

Glenn,

would you say colour shift is a collimation issue. I know that in average seeing, i can see the moons of jupiter warping but with say blue to the top and red to the bottom? Also generally, Jupiter will be needed to adjusted with RGB shift in registax accordingly.

Many people have been suggesting not to collimate with the toucam in, ie it may not sit in straight etc, but I am thinking that although it may not be straight, it is still part of the image train. Sure, do not collimate with the toucam in terms of getting "visual" collimation sorted out, but to get "imaging" collimation sorted out, then you must collimate with the complete image train set up.

Any thoughts.

By the way, your posts re exponential deconvolution have worked for me, At the moment, a 3x1.1 to 5x1.1 ME exponential deconvolution seems to be better on Jupiter at the moment - thanks

[StargazerX1]

David,

I image in 3 colors, almost necessary given that Jupiter at my latitude
is not getting any higher than 43 deg. I collimate in red, not just
because of minimizing the turbulence, but the atmospheric dispersion
separates the colors so much at 35 deg that a white image is a mini-rainbow, almost impossible to collimate in average seeing. One should collimate an SCT at an approximate elevation one does imaging. Your dob probably is not so particular as you dont have mirror flop. SCT's are a nice
engineering compromise, but a newtonian should give better images in general.

People have had problems squaring up their cameras, especially if the 1.25
barrel is undersized. A single layer of thin tape solved that issue for me.
The last thing I do before inserting the camera is check collimation with a
short fl eyepiece. Any tilt would of course have focus implications. Probably as important, the ccd should be at the same 'center' you were collimating. On my setup the planet on CCD is no more than 30" away from my crosshairs.

Indeed at low elevations, the RGB shift in Registax is necessary for color ccd's. However if separated out into 3 colors, Maxim DL's align will automatically zero out the dispersion.

I'm glad the exp PSF is working for you. Any telescope with a central obstruction will see the airy disk fall off faster than a gaussian. The Maxim DL exp is not as sharp a singularity as posted by janoskiss, but the profile needs to be determined -- at some point when I have time, I'll plot it out.
On some images the gaussian will be better. It's a strange thing, but night to night an image will yield to a different method. Some images like VC decon for reasons that are a mystery to me. It will take a lifetime to figure all this out!

I'm going out now to set up. Temperature here is 109F (43C) even as the sun is setting! I'm not sure the optics were designed for this!

Glenn Jolly
Arizona, USA

My usual image train:
C11 / JMI NGF-CM / 3x Meade imagemate / Sirius RGB filter wheel / DMK 21BF04

[asimov]

Thanks Glenn.

I found the culprit: I had the secondary holder retaining ring come loose on me. A slight noise from somewhere in the OTA as I was slewing across the meridian was the give away, apart from my suspicions from the submitted Jupiter picture.

All sorted now & my collimation is close to perfect at 800 mag.

[Lester]

Hey Asi ever since you super glued my secondary in place collimation has stayed spot on.:whistle:

Thanks, :thumbsup: but now I feel I should return the favour.:lol:

[matt]

Interesting, John.

Don't suppose you could post a pic/s of exactly where the secondary holder retaining ring is to be found?

As you know I've had a few suspicions about my collimation too, and wonder whether I've got the same prob and could fix it by doing the same.

Did you have to remove the corrector?

Cheers:thumbsup:

[Lester]

Hi Matt,

Asi isn't on line, so I will try and explain.

The outside ring on the secondary holder on the Celestron 9.25 is threaded. This is the ring that is around the secondary where the 3 collimation adjustment screws are. Its like a big nut, and holds the whole secondary assembly. Well his was loose. I think he tightened it by hand after recentring the secondary with calipers.

[asimov]

It only needs to be just finger tight Matt....Is your secondary holder loose? Just grab the external secondary housing & wriggle it slightly & see if it moves, if it does not, everythings cool!

I thought you fixed your problem??:shrug:

[asimov]

Matt, a word of warning.

The secondary is indexed at the factory. If it is not loose, don't muck with it. If it is loose...you have nothing to lose. The secondary holder itself must not be rotated...Only the retainer!

Heres a pic..

[matt]

Thanks lester.

So you don't have to remove the corrector to tighten that up then???

Sorry. Don't have the 9.25 at my place today.

Asi. I'm still seeing slight shifts in the position of the central obstruction when I do the star test. I want to eliminate every "correctable" mechanical possibility before I put this down to mirror flop and something that's just "normal" to SCTs.

Thanks guys

[matt]

Ahh. Ok.

Thanks for the pic Asi:)

Sorry for the stupid question, but how did you tighten yours? I can't see anything to grip it by

[asimov]

Well, if thats what your seeing (slight shifts in the central obstruction) This has to be the problem I would suggest.

The 3 collimating screws are tricky too. All 3 must have the same (or very close to) the same amount of tension on them by the time you have FINISHED getting your collimation spot on. An example: I had collimation spot on, but 1 screw was tight, 1 screw had medium tension, & the 3rd was only finger tight. All that = no good! Pinched optics & slight secondary movement.

Not having your SCT at your place today is a good thing! Can't have you jumping in boots & all without letting you know all this! :)

Correct, no pulling the corrector out to do any of this.

[asimov]

If your right handed, grip the 3 collimating knobs with your left hand, grasp retaining ring with your right, & twist the retaining ring clock wize.

Just make out its a big nut & bolt...the housing being the bolt, the retaining ring being the nut. The object here is to keep the housing stationary here while doing up the retaining ring! Only has to be just past finger tight....just enough to stop the bugga from flopping around in the corrector plate.

[matt]

Thanks John.

How do you know you've achieved equal tension on all 3 collimation screws?

By the way... is that your 9.25 in the pic??

[asimov]

No mate, not mine. I still use a screw driver on the standard factory screws..

You have bobs knobs don't ya? I just went by the 'feel' of them Matt. All 3 just past finger tight. I'm used to working with screw drivers to get the feel of how much tension there is on a screw. To get 3 screws with the same amount of tension on 'em comes easy to me...plenty of practice. It's not overly critical really, just as long as all 3 are firm, not loose, not overly tight.

[asimov]

Youv'e probably already read this, but just in case you haven't. Omit some info in this, it's not specifically for the C9.25 only SCT's in general.

http://legault.club.fr/collim.html