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Old 08-02-2008, 08:23 PM
Cluster
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Seriously disappointing - GSO 6" F5

About a month ago I took delivery of a GSO 6" reflector on a SkyView I mount. It's F5, so I expected some coma. However, the amount visible in my photos blew me away. The photos below were all taken on the same night in late January in reasonable seeing.

The tracking isn't right but it's close enough to demonstrate the point.

http://users.on.net/%7Emmienik/photo...20-%20coma.jpg
Nikon D70 , GSO 25mm plossl via camera adapter. 30 seconds ISO 1600.

http://users.on.net/%7Emmienik/photo...ery%20poor.jpg
It's Mars, but no one would know if I didn't tell them

http://users.on.net/%7Emmienik/photo...ery%20poor.jpg
Saturn, or a very rough approximation of it.

The planetary views look ok but extremely small at 25mm or even 15, but as soon as I try to use 9 or 6mm all I can see if a washed out fuzzy blob. I have a 3x ED Barlow but that just makes things worse with all eyepieces.

I can't see the Cassini division and can barely strain to see the rings themselves at all in a low power eyepiece.

I'm having a lot of problems with collimation. If I line up all the mirrors with a Cheshire so they look ok, my laser collimator says everything is way out of line. If I use the laser collimator and it indicates everything is perfect I get the above results.

I'm not expecting miracles from a 150mm, 750mm length reflector, but the above results are indicative of what I can see in the eyepiece. It's very disappointing and it's making me reluctant to pull the telescope out at all and spend an hour setting up polar alignment. I think I'll need to find a local astronomy group and get expert advice from them on collimation.

So far collimating has been a royal pain and I feel I've completely got the wrong 'scope. I should be using a Maksutov
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Old 08-02-2008, 08:49 PM
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I agree that collimation is the key to clear view/imaging, as you have stated your two methods are contradicting each other.

Even a rough collimation should give you better results than that, that is wicked.

I'm no expert on this, but i'm sure some here will help you out soon.

Leon

Last edited by leon; 08-02-2008 at 09:27 PM.
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Old 08-02-2008, 09:03 PM
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Starkler (Geoff)
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Quote:
Originally Posted by Cluster View Post
I think I'll need to find a local astronomy group and get expert advice from them on collimation.
It does look like you are missing something fundamental re collimating the scope.

If you are confident that you have followed all the online advice and tutorials re collimation it might be time to pull the primary mirror out and check that the centre spot is indeed in the centre. I have heard of some being placed inaccurately at the factory and that may explain whats happening.
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Old 08-02-2008, 09:09 PM
Zuts
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Hi,

I dont have a reflector but if a scope is poorly collimated doesnt the overall image quality degrade. The stars in the photos look sharper in the middle but horribly distorted on the edges. Maybe he was very unlucky and got an extremely poor mirror?

It cant be the eyepiece because you can see the camera gives the same results.

Paul
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Old 08-02-2008, 09:29 PM
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Make sure your laser collimator is collimated itself! If you have V-blocks try rotating the collimator with the laser on and see that it doesn't scribe a circle on the wall, it should remain pinpoint and not wobble off centre. make sure this is right before trying to collimate the scope with the laser!
Alternatively try placing the collimator in the drawtube and rotate it, if you notice the laser wobble on the primary the collimator may need aligning. make sure the unit rotates in the drawtube without wobbling.
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  #6  
Old 08-02-2008, 09:51 PM
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g__day (Matthew)
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I'd ponder if its either badly in need of colimination - and checking the centering of the primary - or wonder is it a badly shaped mirror? Is it new - I'd ask the retailer to check it - it looks faulty to me!
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Old 08-02-2008, 10:05 PM
Cluster
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Hi,

The laser collimator looks fine when rotated in the eyepiece holder of the telescope. I don't (yet) have a v-block to try rotating it in that.

This is my first reflector and the mirrors could be out of alignment, despite it looking fine during collimation. That's why I need to find someone with experience who could probably spot what's wrong.

I have read that shorter focal length telescopes are much more sensitive to misalignment. However, how far does it have to be to ruin the view? Even moving the focuser changes collimation according to the laser (I can see now why people pay extra and get Crayford style focusers).

I figured my dismal results are not typical of 6 reflectors. Just wanted to make sure. Returning the telescope to the retailer for checking isn't really practical, as I bought it from Andrew's Comm. They're in Sydney and I'm in Adelaide. Ahh, the pitfalls of mail order.
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  #8  
Old 08-02-2008, 10:16 PM
Zuts
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Hi,

You could try emailing your seagull photo and see what they say?

Paul
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  #9  
Old 08-02-2008, 11:16 PM
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I can sympathize with you, having experienced the 14" Allira from Anssen Technologies which had incredibly bad optics.

Can you post intra and extra focal images of a bright star. Not only will it provide a quick assessment of collimation but it might also identify any optical defects in the system. If you do this it is absolutely essential the telescope is at thermal equilibrium.

Regards

Steven
http://users.westconnect.com.au/~sjastro/small
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  #10  
Old 09-02-2008, 11:08 AM
Karls48 (Karl)
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Hi Cluster. I got exactly same scope and I must say that it is quite good scope for price. There must be something seriously wrong with your collimation. I donít use it much for imagining and can not find any decent pictures on this computer but here is snapshot with it when I was testing GStar camera.
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Click for full-size image (Nunki 6.jpg)
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  #11  
Old 09-02-2008, 11:37 AM
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if you are new to this get sumone locally to have a look at it... say at the local astro soc. If you were local to me i'd quite hapily check your collimation for you.
it doesnt take for it to be out by much to get bad results, expecially in a fast scope like yours.

try centering a star and posting an inside and outside focus image back here.
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  #12  
Old 09-02-2008, 01:06 PM
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MrB (Simon)
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Hi Cluster,
While I don't have the GSO, I do have a 6" f5(Orion optics I think?) and it is very sharp when freshly collimated. Coma really only appears at the extreme edges of the FOV.
Have you tried observing after collimating with the Cheshire only?
If not, I say ditch the laser collimation and start over with the Cheshire and see how it is.
Or even just eye-ball it 'till it looks right and see how that goes.
As the saying goes, 'if it looks right, it usually is'.

Quote:
If I line up all the mirrors with a Cheshire so they look ok
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Old 09-02-2008, 01:51 PM
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Those images, are they taken afocally or at prime focus?
Because, collimation seems to me OK or close, the stars are reasonably close to dots in the centre of the image (perhaps tracking and/or alignment was not ideal).
If the image is taken through eyepiece (indicative of this is chromatic aberration on planets.. also I think I can see the round FOV on Orion, see attached image, I increased contrast and luminance to make it more visible), you should not blame the mirror (yet). Some time ago I bought a cheap plossl on ebay (Meade 20mm) and it was absolute #$@#, (especially at the edges) so I had it returned.
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Click for full-size image (Orion's Nebula - coma.jpg)
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  #14  
Old 09-02-2008, 02:33 PM
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Well from what you describe it is quite possible it's a mirror problem, i would have thought a foucault test would show up any mirror imperfections. I would phone Lee Andrews @ Andrews Comm and talk to him about it first, at least ask if it will void any warranties by getting someone to test the optics and eyepieces.

Last edited by Outbackmanyep; 09-02-2008 at 03:08 PM.
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  #15  
Old 09-02-2008, 03:04 PM
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extra assistance.....

Hi Cluster - for what it's worth I checked the Astro Soc. of Sth Aust's website yesterday and noticed they've got a "get your gear checked/adjusted night" coming up in the next week. I'm sure it'd be worth rocking up.....

Regards, Darryl.
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Old 10-02-2008, 12:33 AM
Cluster
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Quote:
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Hi Cluster - for what it's worth I checked the Astro Soc. of Sth Aust's website yesterday and noticed they've got a "get your gear checked/adjusted night" coming up in the next week. I'm sure it'd be worth rocking up.....
Thanks, that seems ideal. I will ask about becoming a member and hopefully get my telescope checked as well.

I shone a powerful torch down my telescope and took a photo with a digicam.
http://users.on.net/%7Emmienik/photo...ollimation.JPG

My laser collimator indicates the above setup is fine, although visually it appears wrong. If I line up everything according to my Cheshire then I have to wind the primary mirror's adjustment bolts a _long_ way one way (two very tight, one very loose) and everything is centered. But put the laser back in (which is collimated) and alignment is way off (about 2cm on the primary mirror).
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Old 10-02-2008, 10:13 AM
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Hi Cluster,
it seems your focuser may be off a bit.
You should remove the secondary out of the way (just rotate it 90 degrees) and check with laser if the beam hits the tube at the point opposite the centre of the focuser. If this is not case, then you can explain what you are experiencing.
However, for your troubles I am sure the mirror is not the culprit, it will be eyepieces and/or barlow. The chromatic aberration visible on the planets shots is proof of what I am trying to say (mirrors do not have CA)
And, besides, the focal length is only 750mm and I think you are expecting too much from 780mm FL in terms of magnification - I would not go beyond 150 or so.
Anyway, at that Astro. Soc. meeting you can try the eyepieces other people have.
My mind was blown away when I tried a really good eyepiece on my 10" mirror, I never suspected it is that good...
Affordability of such eyepieces is another story hough....
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Old 10-02-2008, 11:41 AM
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Looking at your photo I notice two things.

Firstly, reflection of the primary as seen in the secondary is not centred. I can see one and only one of your primary mirror clips. This suggests that your secondary is out of whack.

Secondly, the centre spot donut appears off centre toward the left of the photo. This might be related to the above.

I strongly advise you put your laser away until this issue is properly sorted as it is only confusing the issue. Assuming your centre spot is indeed in the centre, a long cheshire/sight tube tool is everything you need to collimate your scope and get it right. The same cannot be said of the laser. What type of cheshire do you have? Is it a long one or a shorty one like Andrews sells?

Try seating your cheshire in and rack the focuser in far enough until you can see all of the primary mirror in the secondary. Now adjust the tilt of the secondary to centre it, and then align the cheshire crosshairs over the centre spot. Rotate the cheshire in the focuser 180 degrees and look again. This will check how well centred the crosshairs are in your cheshire tool.
How are we going so far? Can you still see the entire primary mirror fairly well centred in the secondary? If not you maybe have a poorly centred centred spider. I had this issue with my gso dob until i identified and fixed it.

If thats all good, adjust the primary to put the centre spot in close alignment with the illuminated reflective ring of the cheshire.

You should now have a fairly well collimated scope. If the laser doesnt agree to within a few mm, try rotating the laser. As you move it does it trace a big circle around the desired centre? If so your laser is out of collimation and needs adjusting. If it traces a circle way off the desired centre, I would suspect your primary mirror centre spot is misplaced from the physical and optical centre of the primary mirror.

Good luck and let us know how it goes.

Last edited by Starkler; 10-02-2008 at 11:56 AM.
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  #19  
Old 10-02-2008, 12:07 PM
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The M42 shot looks like you had an eyepiece in the imaging train.

Not a good idea for deep sky.

Remove the eyepiece and use a prime focus adapter. You should see a world of difference.

Cheers
Peter
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Old 10-02-2008, 09:11 PM
Cluster
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Quote:
Originally Posted by Starkler View Post
I strongly advise you put your laser away until this issue is properly sorted as it is only confusing the issue. Assuming your centre spot is indeed in the centre, a long cheshire/sight tube tool is everything you need to collimate your scope and get it right. The same cannot be said of the laser. What type of cheshire do you have? Is it a long one or a shorty one like Andrews sells?
I'm not using the laser anymore as it's useless for the issues that I'm having. Unfortunately I have a short Cheshire (from Andrew's, for $49... eek) and it doesn't even have crosshairs. I've been browsing web sites and am currently looking at an Orion long collimating eyepiece on Bintel's site for $69. Looks like a worthy purchase.

I think I'll order it and try collimating again. It's now clear to me I don't have all the tools needed for collimation.

I am also going to attend the SA astronomical society's telescope checkup day on the 17th to get hands on advice. After the meeting I will post an update.

This reflector is my first 'real' telescope so I'm a real newbie
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