Sub Discarding

[MattJenko]

I have recently made a foray into >30sec subs with the acquisition of a Canon DSLR and have noticed that a decent percentage of the shots I am taking display distinct star trailing. I am putting this down to tracking errors, which are a result from slapdash polar alignment (or just not accurate polar alignment - certainly not sub 1minute) and possibly periodic error. These are the only two things I can think of which would cause the tell tale trailing along the same arc, although as they are not in every shot (although present in some degree in them all), I am not sure that the polar aligning is the main cause. I do not guide as yet.

I use an HEQ5, so would like to know how many subs people are throwing away due to tracking errors as my results seem high, and how much of this would be resolved by moving to a guiding setup for this kind of mount? We have such small windows of opportunity to get a sensor lined up to targets, it seems a tragedy to have to throw away any data at all.

Matt

[szymon]

Well, guiding is not a good solution to bad polar alignment.  Go on, ask me how I know ;-)

Start off by getting good polar alignment, this should give you the potential for subs up to a minute or two (I've had 90s unguided subs without trailing).  Then add a guiding solution and you can get much longer.

[swag72]

Guiding on an HEQ5? I found that with 30 minute subs I wasn't throwing any away. That though was with a short focal length scope.

A good PA is good to have and coupled with guiding you should have a fighting chance of keeping a good percentage, perhaps all of them if you are using the ED80 in your signature

[glowingturnip]

i lose about 10-15% of 5 minute guided subs due to star-trailing on a so-so mount.

[lensman57]

I have recently made a foray into >30sec subs with the acquisition of a Canon DSLR and have noticed that a decent percentage of the shots I am taking display distinct star trailing. I am putting this down to tracking errors, which are a result from slapdash polar alignment (or just not accurate polar alignment - certainly not sub 1minute) and possibly periodic error. These are the only two things I can think of which would cause the tell tale trailing along the same arc, although as they are not in every shot (although present in some degree in them all), I am not sure that the polar aligning is the main cause. I do not guide as yet.

I use an HEQ5, so would like to know how many subs people are throwing away due to tracking errors as my results seem high, and how much of this would be resolved by moving to a guiding setup for this kind of mount? We have such small windows of opportunity to get a sensor lined up to targets, it seems a tragedy to have to throw away any data at all.

Matt

Without guiding coupled to a good polar alignment you will always be throwing subs away. There is no way I am afraid unless you have 10 Micron mount of some sort. Some members get decent results with astro track unguided but the polar alignment has to be pretty near perfect and the FL used on the short side.

A.G

[Astrosurf]

I used to have to discard some when not guiding, but not many, if I was careful. How do you polar align??

Alexxx

[dph1nm]

I have an HEQ5 at 1200mm FL (~1" pixels)  and use 30 sec exposures. I throw away about 20%, maybe a bit less. My periodic error is around 30" peak-to-peak, so it is not surprising really.

NigelM

[MattJenko]

I don't have a permanent setup, so I do a polar scope alignment each time and am currently trying out one or two iterations of the 2 star align/polar-align routine with the synscan controller, although it seems to make things worse in that the offsets reported after the process is finished do not necessarily get progressively lower each time, probably more an issue of stiff bolts and bad estimates from me about what constitutes perpendicular centering in doing the pa routine than a fault with the software routine itself! I have also given drift aligning a go, although the only time I tried it clouded over before I got a chance to try out some imaging. I will be doing that again this evening with any luck and seeing how I fare then as I think I prefer it, as the handset routine just seems a little awkward with the centering it asks you to do.

I do know that guiding is what I need to move to with this setup, and it is on the overall 'plan', but getting the best out of what I currently have will hold me in good stead I feel, as getting polar aligning nailed down each time is all 'good hours' with the equipment, despite it being frustrating seeing a whole host of star trails when the clouds are clear and everything else works a treat.

[Astrosurf]

I always make sure that my mount is level with a spirit level and that the altitude is as close to accurate as possible. This helps me to find Polaris quite easily. If it's a bad night, I get someone to hold my laser pointer at Polaris till I get it! (I'm going to wrangle a holder onto a tripod for the pointer in future.)

I then use Polarfinder to get Polaris on the correct place on the large circle. (I don't then bother with getting it in the little circle). Here's the app:

http://myastroimages.com/Polar_FinderScope_by_Jason_Dale/

There are also some for smart phones.

Then I just make sure the clutches are really tight. I do all this without the scope on to start with then just re-check when it's all on. I mostly to go out with my astro group to a site with tarmac so my set-up tends not to slip.

I use a red dot finder when doing a star align as it's never spot on. Then I know the mount should find the object OK.

[ollypenrice]

If some subs are OK and some are not it isn't PA. Your PA doesn't vary during a run! It's just periodic error and backlash that introduce the variability.

Guiding is a fact of imaging life. Without it you are messing about, quite honestly. Life is too short and the stuff that's out there is too splendid!

Olly

[MattJenko]

Thanks Alex, seems like I am doing as much as I need in terms of PA + aligning. Practice practice practice etc.

[MattJenko]

Now that I have gotten myself a setup which does more than 30 second subs, your comment rings very true Olly

Without it you are messing about, quite honestly. Life is too short and the stuff that's out there is too splendid!

I am glad that I now have someone else to blame when more 'presents' arrive through the postal system