CMOS camera guiding question

[festoon]

If I am using a CMOS camera for deep sky imaging at a focal length of 412mm.  I usually take many exposures of 30s length and stacking. Each sub there is no noticeable drift of the star or blurring.

However after maybe 2-3 subs there is a noticeable shift in the framing of the image. At this point all I would like to do is re frame the image. In this scenario is it possible to use the same camera for guiding and imaging? Probably better described as correcting the tracking and imaging using the same camera.

[Skipper Billy]

Are you using a tracking mount?

You cannot guide with your imaging camera - the guide cam needs to clatter away at a frame every 1 - 3 seconds ie whilst the imaging camera is working.

[vlaiv]

26 minutes ago, festoon said:

If I am using a CMOS camera for deep sky imaging at a focal length of 412mm.  I usually take many exposures of 30s length and stacking. Each sub there is no noticeable drift of the star or blurring.

However after maybe 2-3 subs there is a noticeable shift in the framing of the image. At this point all I would like to do is re frame the image. In this scenario is it possible to use the same camera for guiding and imaging? Probably better described as correcting the tracking and imaging using the same camera.

I don't see why not, but that should not be the feature of "guiding" software - it should be part of imaging software.

As far as I know - no software does this - correct position based on star positions in the image (or previous few subs).

Regular guiding won't help there, I'm afraid, because it needs to work all the time.

Btw - small drift between the frames is a good thing - it is like dithering. Maybe best thing to do would be to leave it have such natural dither and then re frame manually every half an hour or so - just a nudge or two in right direction via scope slew control - while sequence is paused?

[festoon]

Thanks @Skipper Billy. Yes I’m using an tracking equatorial mount. I can understand the point that if I need to guide at a rate of say 1s exposures then they need to be different cameras. In my case my exposure is usually 30s and my stars are still not blurred during this time.

Good point @vlaiv about the software. Yes I don’t know what software would do this, and was part of the reason behind posting here if it would even be feasible. 

 

[newbie alert]

What mount are you using?, no good thinking of guiding if the mount isn't capable .. start from the ground up as they say

[festoon]

Thanks @newbie alert,  the mount is a heq5

[newbie alert]

1 minute ago, festoon said:

Thanks @newbie alert,  the mount is a heq5

No issues with that then.

 Can't see why you can't get more than 30 seconds  @ 412fl thou.. should get far more than that unguided, how are you polar aligning?

[vlaiv]

36 minutes ago, festoon said:

Good point @vlaiv about the software. Yes I don’t know what software would do this, and was part of the reason behind posting here if it would even be feasible. 

How much drift do you get and in what direction?

Maybe we could work out something to reduce drift - but still leave it at desirable level? Like, I mentioned - you might not want to remove it completely as it acts as natural dither and that is a good thing.

My guess is that main drift is in DEC axis and RA can have periodic error - but that will "circle back" to original location and won't cause great shift between first and last sub.

[festoon]

Usually the drift depends on how well I polar aligned. (Which I use SharpCap for). Yes do see drift in dec.

For sure I could probably go longer than 30s but that is what the optimum exposure time is due to light pollution and low read noise of the CMOS (based on the SharpCap smart histogram https://www.sharpcap.co.uk/sharpcap/features/smart-histogram)