Dithering with PHD2 and QHY294C pro

[ScottD]

Hello!

I'm sure this has been exhaustively covered, but I cannot find the exact information I'm looking for. I use CPWI as my main connection software. I am running SharpCap and PHD2 for my imaging and guiding. My question is regarding dithering.

My imaging camera is a QHY294C Pro and my guide camera is a ZWO120 mini. Dithering is taking so much time during my sessions and I'm trying to figure out if I'm doing something wrong. Currently, the dither exposures are the same length as my light subs. In an attempt to shorten my dithering time, I checked the box to shorten exposure time in the dithering section. However, even with the shorter exposure time set, my QHY camera cycles through an entire exposure time before it starts live stacking again. I don't know if this makes sense or not and QHY has told me it's the software I'm using to capture, not the camera.

Let me try clarifying - Let's say I'm shooting 2 minute subs. When I first set my time, SharpCap and the camera cycles through two or more full 2 minute exposures, without capturing. The time bar in the bottom right of SharpCap just freezes and counts the seconds down. If I have everything set and decide to change the exposure time to 90 seconds, it starts all over again, cycling through one, two and sometimes three exposures. I can see the timer at the bottom right just sitting there, counting down the time. With that said, if I set the box to "shorten exposure time" in the dithering section, my camera will then start all over and cycle through a few times, without actually exposing. This actually increases my dithering time.

I've done my best to get my thoughts down here, but reading it sounds confusing and convoluted. I'm very sorry. If you think you understand and can advise me, I would greatly appreciate it!

Thank you,

Scott

[admin]

Hi Scott,

well, it should change the exposure to a shorter one without a long delay, so there is likely a bug there with the QHY camera support. However, the simplest work around should be to make sure you are using the latest SharpCap 4.0 and then put the camer into still mode (rather than live video mode) before live stacking (see https://docs.sharpcap.co.uk/4.0/#Live%2 ... ill%20Mode). With the camera in still mode, SharpCap can simply stop capturing frames during the dither procedure, sidestepping the whole problem.

cheers,

Robin

[ScottD]

Thank you Robin! I'm still a bit confused. If I don't ask SharpCap to save the dither frames, how does dithering remove walking noise and such? In other words, what should my settings be if for example I'm taking two minute subs? Do I save the dithered frames or no? Also, my camera is a QHY294C Pro and my telescope is an 800mm - 8" - f3.9 Newtonian. Is there a formula to determine the optimal pixel size and min/max settle time?

Thank you so much!

Scott

[admin]

Hi,

no, you don't want to save the frames during dithering - they will likely be messed up by the mount movement.

Suppose you have one pixel that is a bit brighter than the others - without dithering you either get a hot pixel in your image (perfect tracking), or a warm line (walking noise) as that hot pixel drifts slowly across the view due to tracking errors.

Throw in dithering, which moves the mount around a random bit every few frames - now the contribution from the hot pixel is spread around randomly over a fairly large area of the final image - remember that the frames are aligned on the stars - that means it's much harder to notice any effect in the final image.

cheers,

Robin

[ScottD]

Thank you Robin. I appreciate your generous time and advice.

[ScottD]

Robin, I'm so sorry to bother you with yet another question about this. I have downloaded 4.0 and understand the differences between live and still modes. However, when launching "Start Capture" I don't understand how to integrate dithering with this. As far as I can tell, the only way I can get to the dithering section is to go into Live Stacking. Am I missing something?

Thank you,

Scott

[admin]

Hi Scott,

if you want to dither without live stacking then use the 'Deep sky sequence planner' - one of the things it can do is automate dithering (along with lots of other things - use as much or as little as you want). https://docs.sharpcap.co.uk/4.0/#!2!Dee ... %20Planner

cheers,

Robin

[ScottD]

Got it! Thank you Robin!