Well I have finally got to bottom of my guiding (drift) issues. As you all probably know I have issues where although guiding works fine but I get shift between the subs. I had tried everything to eliminate the problem which although didn't really effect images too much the stars were marginally mis-shaped which obviously means I must also be loosing detail in what I am imaging. Some of the things I tried were different telescopes, different mount methods (ontop V's side-by-side), all the different guide progs going, polar alignment, drift alignment, guide rings & probably other things as well.
Well last night I borrowed Jim's (thanks mate) OFF AXIS GUIDER. I attached to ED80 with my imaging camera. I had only ED80 on my mount. I did a very quick polar alignment (didn't bother with drift or anything) & then set up focus on the OAG (a little tricky at first but not too bad). I then pointed at a random area of sky calibrated my guide prog (Guidemaster) & off I went. I did 4x5mins & 3x10mins. I couldn't believe difference every sub downloaded right ontop of previous (no drift) & had perfectly round stars. Below are stacks of the 7 images I took (50mins total). The first stack I have aligned the individual subs with 2-star alignment & the second is with no alignment at all - I just stacked the subs one on top of each other. If I had done this previously with my setup I would have had horrible eggshaped stars.
Anyway I am well chuffed - Can't wait to try an image something with the new setup.
Cheers, John
(http://www.jpastronomy.co.uk/graphics/web/Stackx7xaligned.jpg)
(http://www.jpastronomy.co.uk/graphics/web/Stackx7xnonaligned.jpg)
Sounds like the problem may have been flexure then.
What is the OAG model you used?
Yep definitely even though I had what I thought was a rock solid setup something was still moving. I even eliminated the mirrors by trying different scopes for guiding - It really did my head in. Anyway if last night wasn't a fluke then the OAG is definitely the way to go. The one I borrowed off Jim is the BrightStar one from Bern - He is selling them for £94 - You need something like 30mm of back focus to insert it but no probs for ED80 (might be for reflectors).
John
I think a lot of trouble is with the focuser, even the Moonlite's have some movement though I wouldn't have expected that much with a light camera like your's. Anyway, whatever the cause, the results speak for themselves.
Well sorted !!!
Robert
PS. Of course we'll be expecting even better images... ;)
QuotePS. Of course we'll be expecting even better images...
I flippin hope so the amount of grief it has given me. Even though my images are OK the mis-shapen stars were really bugging me & were obviously loosing detail in image. At least now I know image quality should be down to seeing/ light pollution & not hardware.
Cheers, John
Nice result John
Cheers Mick - Yep think it is the best I have EVER had for 10min subs... fingers crossed next time I image something. Good luck for tonight - John
Nice to finally get to the bottom of things,
Mac.
Ta Mac - John.....
Out you go then its clear tonight :-)
Excellent news John. I'm just a bit concerned for Jim - what will he do without his OAG?
Cheers Mark.
Jim doesn't use his OAG so no probs... :lol:
Seriously though I am going to get one - Jim's one is <£100 so may get same model. If it is clear tonight I may test it on a DSO.
Cheers, John.
PS - Any joy down in France - did you manage any imaging at all?
There's one on uk astro buy sell for £65... :surprised:
Oh cheers Robert will take a look - John
I think he's marked it as Pending now as it looks to be the same as this one on eBay which has just under two hours to go (he can't withdraw with less than a day to go):
eBay link (http://cgi.ebay.co.uk/Brightstar-Off-Axis-Guider-Orion-Deluxe-Off-Axis-Guider-/330561941325?_trksid=p4340.m263&_trkparms=algo%3DSI%26its%3DI%252BC%26itu%3DUCI%252BRTU%252BUA%252BFICS%26otn%3D15%26pmod%3D190516520970%26ps%3D63%26clkid%3D9009712493947024422)
Robert
(Edit - URL inside tags, hopefully -- Rick)
oh yes you are right... not worth it for £75+£5 delivery - Bern is selling for £94 delivered so may as well get new.
Thks anyway, John
How much backfocus does the OAG take up and are you using it with your Atik or a DSLR ?
Thanks
Robert
The brightstar takes up 32mm. I am using with Atik (I don't image with DSLR).
I did some more tests on Tues night on DSO's. I did some quick images of m101 - total 6x10min subs - When I stacked them the log file shows total movement between first & last frame as following:
dX = 0.08, dY = 0.74, Rotate = -0.03 Deg, Scale = 1.00
Measurements in pixels - so avg per frame in Y was .74/6 = 0.12 pixels - this may have been down to Polar alignment - Anyway it was essentially no movement & I could easily stack all 6 frames with no star alignment & stars were still round.. :-)
For a laugh I also moved to m57 (totally different part of sky) - again finding a guidestar was no problem & did 2 subs of 20mins each (my first time to ever attempt such long exposure) & they downloaded straight on top of each other again. Again no alignment necessary to stack.
Hope that helps,
John.
PS - The only thing I'll say (I think I have already said this once) is that the guidstar is not so much a star as a straight line - terrible coma because it is on edge of light cone. However, the guide prog still seems to be able to find centroid & guide successfully.
Those figures look pretty impressive John. I'm assuming your measurements are in pixels, what was the pixel scale in arcsec/pixel ?
32mm is quite short. Is the Brightstar very strong, also what length is the 2" push fit adaptor and can it take different camera adaptors?
Thanks
Robert
Robert will take a picture of setup & send to you. Not sure about the lengths I will need to measure. I have just ordered one from Bern - I am currently borrowing Jim's - so if you speak to him nicely you never know he may let you borrow to try out on your setup... :-)
With regards pixel scale as you know my atik16ic is only 640X480 pixels - In fact this was the main reason for trying to sort this - I always have to display my images full size because there are so few pixels so any egg shaped stars etc. really show up whereas people with DSLR's or 3,000 pixels sensors normal reduce image size for display so it is not as noticebale. 10 pixel drift to me is a lot to loose around the edge of a frame as well :-(
Sorry getting back to field of view/ resolution my chip images at 2.54 arcsec/pixel which yields a FOV of approx 28 X 21 arcmins.
Cheers, John