#1  
Old 08-10-2010, 09:58 PM
stanlite (Grady)
Registered User

stanlite is offline
 
Join Date: Jan 2010
Location: Brisbane
Posts: 344
Confused about strange drift.

I have been imaging for a little while now and i have noticed that although i can guide (through a guide scop) with no problems for hours my images slowly drift in one direction over time (not sure but say around an two or three arc mintues over 2 hours). My question is ... how can phd say everything is sweet according to my guidescope but on my imaging scope i get this drift??? Guide scope 102mm refrac. Primary scope 8" newt on a NEQ6 pro. I haven't done PEC training yet. Nor have i set backlash could it be these that are my problem ??? could it be flex? although given that the drift is in one direction and at a steady rate i wouldn't think so. Your thoughts would be welcome.

Grady

P.S. both scopes are F5's

Last edited by stanlite; 08-10-2010 at 10:53 PM.
Reply With Quote
  #2  
Old 08-10-2010, 10:28 PM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,049
welcome to flexure!
Reply With Quote
  #3  
Old 08-10-2010, 10:49 PM
stanlite (Grady)
Registered User

stanlite is offline
 
Join Date: Jan 2010
Location: Brisbane
Posts: 344
oh goodie ... i have been waiting for this one so what are my options for fixing it? maybe someone comes up with something good before i find something.

Seems i might need to increase the FL of my guide scope??
Reply With Quote
  #4  
Old 09-10-2010, 08:15 AM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,049
Changing the guide scope FL won't help if it's flexure. You are looking for sources of differential movement between the imaging camera and the guide camera. Some possible sources are:

imaging scope focuser flop
guide scope focuser flop
imaging scope primary or secondary mirror flop
movement between the two otas - do you have them mounted side by side or piggyback?
Reply With Quote
  #5  
Old 09-10-2010, 09:19 AM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,188
An OAG would sort any kind of flexure problem. If you feel your imaging scope primary may flop a bit or move with temperature then it would be the easy way out.

Otherwise if it is the guider then revise its attachement in relation to the main scope. I could also be as simple as your guiding camera sag into the focuser or the imaging camera sag in the scope focuser.

My gut feeling is your refactor focuser shifts the guider. I had the problem with a barlowed ED80 piggy back on my newt. To verify stick your imaging camera in the refractor and guide through your newt.
Reply With Quote
  #6  
Old 09-10-2010, 09:56 AM
Rigel003's Avatar
Rigel003 (Graeme)
Registered User

Rigel003 is offline
 
Join Date: May 2006
Location: Adelaide, South Australia
Posts: 1,037
If your images don't have trailed stars within any subexposure (do I correctly gather that from your original post?), then I'd say it indicates that your polar alignment is a little off. The guider manages to keep the stars centred on the chip within any exposure, but successive frames move slightly.
Reply With Quote
  #7  
Old 09-10-2010, 01:25 PM
marki's Avatar
marki
Waiting for next electron

marki is offline
 
Join Date: Mar 2008
Posts: 2,427
I would have to agree with Graeme here as I doubt flexure would be a great issue @ F5 unless your focuser is flapping around in the breeze. "Perfect" polar alignment is very elusive and as long as the shift (read field rotation) is not too extreme within any given subframe it doesn't really matter. In your case it sounds like it would only be a problem if you were taking 2 hour exposures and if thats the case good luck to you .

Mark
Reply With Quote
  #8  
Old 09-10-2010, 01:28 PM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,049
I don't agree Mark. I have a 10" f5 Newt and flexure is definately a problem I have been working on for some time.
Reply With Quote
  #9  
Old 09-10-2010, 02:07 PM
marki's Avatar
marki
Waiting for next electron

marki is offline
 
Join Date: Mar 2008
Posts: 2,427
Quote:
Originally Posted by peter_4059 View Post
I don't agree Mark. I have a 10" f5 Newt and flexure is definately a problem I have been working on for some time.

Hi Peter

I would imagine flexure would be the least of your problems if you are imaging with a 10" newt with mounting and coma being far more problematic. Perhaps the OP can shed some light on this by telling us what type of scopes he is using. If he is using a newt I would agree with you but if he is using refractors then I would not agree.

Cheers

Mark
Reply With Quote
  #10  
Old 09-10-2010, 02:57 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,509
Original post states 8" Newt is the primary OTA
Best thing I did to help with this was move to a 9x50 guidescope arrangement. Plenty of threads on it here. MInd you my guidescope was very basic with wobbly plastic focuser. Peter can probably comment on improvement a small refractor guidescope made to his guiding on the 10" too. Good cable management is a biggy too for eliminating guiding noise but probably not likely to be causing the systematic drift described here.

One other comment - its always nice to get rid of any movement, whether flexure or polar alignment or both, but remember too a bit of movement (dither) between frames actually helps with image processing later. Always better if you're chosing when the dither comes in though if possible
Reply With Quote
  #11  
Old 09-10-2010, 03:28 PM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,049
I agree Rob - it is nice to be able to control the dithering rather than have the edge of your well framed image chopped off due to drift.

I've been keeping a spreadsheet log (using Deepsky Stacker's calculated dx and dy numbers) for each change I've made to my setup as I've tried to eliminate the source of my flexure. Being a side by side setup there are a few more variables and I'm now down to dx (RA drift only). The guide scope I now use has a helical focuser, my newt now has a moonlight focuser, I've changed to heavier bob's knobs springs and losmandy dovetails throughout and to a heavier Starstuff SBS bar.

I started up with 0.6-1.0 pixel per minute drift and now have this down to 0.1-0.3. Changing from 0.8 pixels per minute to 0.2 means each sub can be 4 times the duration without the stars becoming elongated. Previously I was constrained to 1 min subs but now routinely use 5 mins.

Most recently I've changed to heavier Parallax rings for the newt however the weather has been poor since they arrived so I don't know if this has had any impact yet.

I polar align using Alignmaster and then check with k3ccd tools drift alignment analyzer so I know my issue is not polar alignment.

Peter
Reply With Quote
  #12  
Old 09-10-2010, 03:29 PM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,049
Quote:
Originally Posted by marki View Post
Hi Peter

I would imagine flexure would be the least of your problems if you are imaging with a 10" newt with mounting and coma being far more problematic.
Mark,

The Televue Paracorr does an nice job dealing with coma!

Peter
Reply With Quote
  #13  
Old 09-10-2010, 06:22 PM
Alchemy (Clive)
Quietly watching

Alchemy is offline
 
Join Date: Jul 2007
Location: Yarra Junction
Posts: 2,949
Given the initial description.... Guiding frame remains unchanged for 2 hours but image moves.

If it rotates it's polar, if it drifts in one direction it's flexure.

My guess is it's flexure, this can be due as previously stated a number of factors.

Some common ones are......

Focuser is not rigid enough for the weight of the camera, a dslr is much heavier than an eyepiece, the barrel of the focuser in cheap scopes will have some movement, hatbis why people buy precision focusers such as the feather touch

The tube of the scope itself is not rigid enough to support the weight and as it rotates through then equatorial axis changes in the angle of the camera causes movement

Less likely is the rings are not solid enough

The plate/ bar whatever you use is not rigid enough, I use a solid alloy plate not a thin bar.

Wherever you have some form of adjustment, there is always opportunity for some movement, be it a focuser, scope anchor points, rings, you would be surprised at how some things just seem to come loose, check your entire imaging train EVERY time you image, don't assume because it was tight last time you used it , that it remains so.

I speak from experience in these matters and some scopes.... Particulary those designed as Dobs were not specifically made for imaging and whilst they're fine for their intended purpose they need quite some tinkering to get to perform as an imaging scope.

Post an image of your setup and maybee it will be more obvious.

Multiweb has given you a good tip, the off axis guider will solve the problem as the scope will move but so does the guider.
Reply With Quote
  #14  
Old 09-10-2010, 07:52 PM
stanlite (Grady)
Registered User

stanlite is offline
 
Join Date: Jan 2010
Location: Brisbane
Posts: 344
i will get a image up as soon as the rain stops for long enough for me to snap one. As for some more info. the focuser on the primary scope is a crayford one, and on the refractor it is a rack an pinon. The guide scope is piggybacking. The mounting plate and guide plate are both losmandy and the rings on the primary are also quite solid construction. The guide scope is held by rings with plastic screws (should i change to metal ones or just make sure the plastic ones are tight enough). As for polar alignement i have done a fairly decent (well i believe so) drift alignement and can leave it for 30 mins with minimal drift (not noticable to my eye) only drift i see is PEC related on the east-west axis (as i beleive a polar misalignment would result in north south drift i have been led to believe). Also on closer inspection at high zoom you can see slight trailing in the stars of subs from the DSLR.
Reply With Quote
  #15  
Old 09-10-2010, 08:47 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,188
Quote:
Originally Posted by stanlite View Post
The guide scope is held by rings with plastic screws (should i change to metal ones or just make sure the plastic ones are tight enough).
There lies your problem. Right there. Fit proper rings around the refractor and get everything fitted rock solid.
Reply With Quote
  #16  
Old 09-10-2010, 09:19 PM
Alchemy (Clive)
Quietly watching

Alchemy is offline
 
Join Date: Jul 2007
Location: Yarra Junction
Posts: 2,949
Your drift will depend on how it is out, it can be in any direction, if you want to find out how bad it is there's a program called, I think pecpro, download it and it will show , or just calibrate in phd the guidescope, then switch off guide pulses but leave your 50 pixel guide box up..... How long til star is out of box, under 30 mins you could do better.

As for solid, this is the plate I have mine setup on, never moves.
http://www.iceinspace.com.au/forum/a...se.php?a=81157
Reply With Quote
  #17  
Old 10-10-2010, 01:01 AM
Alchemy (Clive)
Quietly watching

Alchemy is offline
 
Join Date: Jul 2007
Location: Yarra Junction
Posts: 2,949
One I hadn't mentioned, if your scope is unbalanced and you have the drag set very lightly on your mount it may not guide properly or even drive properly...... Giving trailed stars.
Reply With Quote
  #18  
Old 10-10-2010, 08:15 AM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,049
If the guider is locking on to a star and guiding properly it can't be the mount clutch slipping. Deep Sky Stacker gives you the drift amount in dx and dy on the main screen when you register the images. If you order the images in the order you took them (ie by time), make the first frame the reference image and register them you can see the total drift over the imaging session. Divide this by the total integrated imaging time (minutes) and you get drift in arc seconds per minute.
Reply With Quote
  #19  
Old 10-10-2010, 09:35 AM
stanlite (Grady)
Registered User

stanlite is offline
 
Join Date: Jan 2010
Location: Brisbane
Posts: 344
provided i have done it right and understood your instructions peter. DSS give me 1.24 arc seconds of drift a minute over an hour on the DY axis and 0.33 arc seconds a minute in dx axis.
Reply With Quote
  #20  
Old 10-10-2010, 09:43 AM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,509
That's a great methodology for keep track (no pun intended) of your tracking over time Peter. I've tried to keep notes on my performance from within the guiding app, but have changed software and rig configuration so many times this has lost continuity.

Good idea to monitor the pics themselves over an imaging run. Had never paid attention to that in DSS.
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 03:32 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Celestron Australia
Advertisement
NexDome Observatories
Advertisement
SkyWatcher Australia
Advertisement
OzScopes Authorised Dealer
Advertisement
Lunatico Astronomical
Advertisement
Bintel
Advertisement
Meade Australia
Advertisement
Astronomy and Electronics Centre
Advertisement