Help. Dithering problems with stars

[Desbarry]

Hi Robin,
having problems with frames, not all but blurred stars after dithering every frame in Sharpcap in ra, i wasn't guiding. I have my C925 and Altair 26c on a pier in obs. I did a decent image of M81 at f10 2350mm fl so it can be done. Not sure why ive been getting problems this time. Should i increase my settle time? Or is something else a problem? As I said some frames are fine, others not. To add further insult Pixinsight failed to stack. Ive attached a pic of M81 which was done with same kit. Hope you can offer some fixes. Best wishes Derek

[Desbarry]

FAO ROBIN, I got this feedback from Pixinsight fyi

[admin]

Hi,

I would certainly increase the 'minimum settle time' in the SharpCap guiding settings - that should help as SharpCap will wait longer after the dither movement is complete before starting imaging again.

I'm not sure about the PI issue - I don't have much experience with using it for that sort of stacking. I do wonder if the stacking is perhaps unhappy about being given already dark/flat corrected frames from SharpCap (I presume that's what you are doing), or alternatively using SharpCap master dark/flat frames. Mixed processing (doing some of the processing in SharpCap and some in another application) can work fine, but can also end up giving problems in some cases, so it's usually best to either let SharpCap handle all the processing (creating dark/flats, using them, etc) or to do everything in an application like PI.

cheers,

Robin

[Desbarry]

Thanks Robin. I create about 40 darks in Sharpcap in addition to my light frames so I've no clue why its rejecting them, its probably a PIxinsight issue. I was able to stack an image of 80 light and 40 darks when I rolled back to the previous version of Pixinsight. However it again failed to stack 3 hours worth of M51 which Im pissed off about.

Do you think that the settle time is indeed the problem after the dither? Would anything else cause this in Sharpcap, ie blurry stars as I don't get this in a live preview when the mount is tracking. PS Is there a reason why Sharpcap cant dither in dec also? Excuse my ignorance as I've yet to save up for a Celestron off axis guider and guide camera. Thanks again
Derek

[admin]

Hi Derek,

the default dithering operation is to dither in both RA and Dec, there is an option to dither in RA only - that is there to mirror the option with PHD2 of dithering in RA only if you want. Anyway, if you want a dither in both directions, leave that unticked and you are fine. If you wanted to dither in Dec only, there is no option for that either in SharpCap or PHD2.

With the trailed star issue, it depends on how you are organizing the dither operations - either via live stacking's option to dither or via the sequencer. The key is to make sure that SharpCap stops imaging during the dither - in live stacking this is handled either by having the camera in still mode, or by SharpCap automatically reducing the exposure to 4s (I think it's 4s) during the dither and then turning it back up after the dither is complete. In the sequencer, still mode is the way to handle this so that frames are not captured while the dither is happening. I presume you are doing one of those already though, which makes the settle time the best bet.

cheers,

Robin

[Desbarry]

Thanks Robin, I will give those suggestions a try. I am only using the Sequencer.
cheers Derek