8" F/4 atsrograph advice needed.

[tilbrook@rbe.ne]

Hi,
I would like to get some opinions on possible problems with my 8' f/4 astrograph.
It's a standard off the shelf BINTEL job, runnung on a HEQ 5 pro mount.
When I recieved the scope I collimated with my laser, seemed slightly off so corrected that.
I'm using a 2" coma corrector, again BINTEL stamped, don't the real identity of it.
The image below was taken using my canon 1100D, and focused with a bahtinov mask.
As you can see the star images to the left of field are not good. I was wondering if this is.
A. Collimation problem.
B. Coma corrector not quite up to sctrach.
C. Poor mirror.
I think C is unlikely, because it showed good figure in star testing.
Any suggestions?
Advice would be appreciated.

[Lester]

Hi Justin, the first thing that comes to mind is, do you have the secondary offset? Some months ago I did a collimation on my Takahashi F3.3 astrograph and had the secondary centred; the results were aweful, till I offset it.

All the best.

[Tandum]

This image here (http://www.iceinspace.com.au/forum/attachment_browse.php?a=101860)is from a newt in which the collimation shifted as the altitude changed. It is worse on the right instead of the left, but generally looks very much like your shot if not worse.

What you should try is setting up your laser and adjust it as best you can, then hang the camera off the focuser with a loop of string and watch what the weight of the camera does to collimation.

Also have a read of Moon's write up on these scopes here (http://deepspaceplace.com/at12in.php).

[tilbrook@rbe.ne]

Thanks, Lester.

That poses another question.
What should the offset be?

Cheers,
Justin.

[tilbrook@rbe.ne]

Thanks, Robin.

Good advice, looks like a few things to check!

Cheers,

Justin.

[Lester]

Hi Justin, I don't know what the offset should be in your case. I'd say a phone call to Bintel should help you.

In my case I refered to the diagrams in the manual.

All the best.

[Tandum]

Should be just over 3mm.

[Moon]

Justin,

Probably collimation and slightly off focus. Maybe the mount got bumped a bit too?

Which way was the camera mounted? across the scope or along?

For collimation, it pays to go back to basics and double check everything from scratch - eg center spot, secondary under the focuser etc etc. there are plenty of guides around.

I have 2 of these (200mm and 300mm) and one thing that has been bugging me lately is the secondary adjustment screws are only useful to position the secondary under the focuser. You still need to rotate the mirror by hand for collimation. If you use them for gross collimation, it actually swings the entire assembly out of position, so it's no longer correctly located under the focuser.

Also as Robin said, check nothing is moving around.

good luck.

James

[tilbrook@rbe.ne]

Hi, James.

Thanks for the tips.
I rechecked the collimation, seems good.
Then decided to take an image without and with the coma corrector.

Eta1 shows an even coma right around the field.

Eta2 with the corrector shows reverse coma, and in one direction more than the other. It looks like it's overcorrecting?

Any thoughts?

Cheers,

Justin.

[Moon]

I still think its mostly collimation. I agree the first image looks more even, but according to CCDIS, stars are smallest in the top LHS,so it still can be improved.
Spacing for the coma corrector could be off, but first get it collimated before you can judge that. keep at it.
James

[Tandum]

Is that 2nd image cropped? If not then that corrector has a bit of barlow in it :)

[jjjnettie]

I feel your pain.
They're a damn fiddly thing to collimate.

My 10" dob was a breeze compared to the 200mm F4.

[alistairsam]

Hi Justin,
I've seen similar issues posted here and it was attributed to the ccd plane not being perfectly square to the optical axis.
Have you checked for slop in the focuser or the 2" tube ring that hold your camera.
Also, check the vertical offset using a sight tube. I use a 2" extension tube as a sight tube. It needs to be centered in the sight tube.
Laser collimators don't reveal all.

[tilbrook@rbe.ne]

Hi,

Thanks everyone for the advice.
Robin the 2nd image isn't cropped. I have read that can be the case with some coma correctors.

Cheers,

Justin.

[Poita]

My first thought is that the camera is not square, if there is some tilt relative to the scope it could look something like your first picture.

[Garyh]

Hi Justin,
Looking at the eta images the first looks like it is pretty well centered and square, the second looks offset a little and slightly magnified but showing a lot of coma still, I would be checking if you have the right coma corrector like a baader or paracorr and not a field flattener for a refractor also check if the spacing is correct to the sensor plane and that the T-ring is all nice and square. I don`t think there is a problem with the scope it`s self.
cheers Gary