#1  
Old 16-02-2012, 04:40 PM
tilbrook@rbe.ne's Avatar
tilbrook@rbe.ne (Justin Tilbrook)
JHT

tilbrook@rbe.ne is offline
 
Join Date: Jun 2007
Location: Penwortham
Posts: 3,039
8" F/4 atsrograph advice needed.

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.
Attached Thumbnails
Click for full-size image (orion831.jpg)
156.0 KB70 views
Reply With Quote
  #2  
Old 16-02-2012, 05:17 PM
Lester's Avatar
Lester
Registered User

Lester is offline
 
Join Date: Jan 2006
Location: E.P. S.A.
Posts: 4,963
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.
Reply With Quote
  #3  
Old 16-02-2012, 05:22 PM
Tandum's Avatar
Tandum (Robin)
Registered User

Tandum is offline
 
Join Date: Apr 2008
Location: Wynnum West, Brisbane.
Posts: 4,161
This image here 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.
Reply With Quote
  #4  
Old 16-02-2012, 05:27 PM
tilbrook@rbe.ne's Avatar
tilbrook@rbe.ne (Justin Tilbrook)
JHT

tilbrook@rbe.ne is offline
 
Join Date: Jun 2007
Location: Penwortham
Posts: 3,039
Thanks, Lester.

That poses another question.
What should the offset be?

Cheers,
Justin.
Reply With Quote
  #5  
Old 16-02-2012, 05:33 PM
tilbrook@rbe.ne's Avatar
tilbrook@rbe.ne (Justin Tilbrook)
JHT

tilbrook@rbe.ne is offline
 
Join Date: Jun 2007
Location: Penwortham
Posts: 3,039
Thanks, Robin.

Good advice, looks like a few things to check!

Cheers,

Justin.
Reply With Quote
  #6  
Old 16-02-2012, 05:40 PM
Lester's Avatar
Lester
Registered User

Lester is offline
 
Join Date: Jan 2006
Location: E.P. S.A.
Posts: 4,963
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.
Reply With Quote
  #7  
Old 16-02-2012, 05:58 PM
Tandum's Avatar
Tandum (Robin)
Registered User

Tandum is offline
 
Join Date: Apr 2008
Location: Wynnum West, Brisbane.
Posts: 4,161
Should be just over 3mm.
Reply With Quote
  #8  
Old 16-02-2012, 08:06 PM
Moon's Avatar
Moon (James)
This sentence is false

Moon is offline
 
Join Date: Oct 2008
Location: Melbourne
Posts: 1,147
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
Reply With Quote
  #9  
Old 16-02-2012, 10:06 PM
tilbrook@rbe.ne's Avatar
tilbrook@rbe.ne (Justin Tilbrook)
JHT

tilbrook@rbe.ne is offline
 
Join Date: Jun 2007
Location: Penwortham
Posts: 3,039
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.
Attached Thumbnails
Click for full-size image (eta1.jpg)
214.8 KB33 views
Click for full-size image (eta2.jpg)
211.3 KB32 views
Reply With Quote
  #10  
Old 16-02-2012, 10:21 PM
Moon's Avatar
Moon (James)
This sentence is false

Moon is offline
 
Join Date: Oct 2008
Location: Melbourne
Posts: 1,147
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
Reply With Quote
  #11  
Old 16-02-2012, 10:52 PM
Tandum's Avatar
Tandum (Robin)
Registered User

Tandum is offline
 
Join Date: Apr 2008
Location: Wynnum West, Brisbane.
Posts: 4,161
Is that 2nd image cropped? If not then that corrector has a bit of barlow in it
Reply With Quote
  #12  
Old 16-02-2012, 11:25 PM
jjjnettie's Avatar
jjjnettie (Jeanette)
Registered User

jjjnettie is offline
 
Join Date: Feb 2005
Location: Monto
Posts: 16,738
I feel your pain.
They're a damn fiddly thing to collimate.

My 10" dob was a breeze compared to the 200mm F4.
Reply With Quote
  #13  
Old 17-02-2012, 06:06 AM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,837
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.
Reply With Quote
  #14  
Old 17-02-2012, 07:15 AM
tilbrook@rbe.ne's Avatar
tilbrook@rbe.ne (Justin Tilbrook)
JHT

tilbrook@rbe.ne is offline
 
Join Date: Jun 2007
Location: Penwortham
Posts: 3,039
Quote:
Originally Posted by Tandum View Post
Is that 2nd image cropped? If not then that corrector has a bit of barlow in it
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.
Reply With Quote
  #15  
Old 17-02-2012, 08:18 AM
Poita (Peter)
Registered User

Poita is offline
 
Join Date: Jun 2011
Location: NSW Country
Posts: 3,586
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.
Reply With Quote
  #16  
Old 17-02-2012, 02:23 PM
Garyh's Avatar
Garyh
Amongst the stars

Garyh is offline
 
Join Date: Jun 2006
Location: Glen Innes, N.S.W.
Posts: 2,867
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
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 04:20 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement