Taking mulitple Subs

[drumsolo]

Hi again and here comes another questions from a newbie. If i was to take multiple subs of a DSO using my HEQ5 goto, lets say 20 of around 1 minute for example without autoguiding. Is it ok to recenter the object as it will inevitabaly drift across the eyepiece after a while and keep taking subs? What I am trying to understand is if the images shifts slightly in the cameras view and i take more subs will they still stack ok afterwards in registax?

hope that makes sense!

thanks

stuart

[rocketandroll]

The idea of a tracking mount like the HEQ5 is that you shouldnt have to manually keep adjusting it, it should track pretty accurately by itself.

From experience (a few months of using a HEQ5 for unguided DSO imaging) the amount of movement you should get between subs will be a few pixels at most. Even on a three hour long session on M51 I had a hot pixel that showed a total movement of about 60 pixels distance over the entire 3 hrs. Thats about 1/50th the total width of the image.

Yes, DSS and Registax will easily cope with registering and stacking subs with those kind of small movements between them.

You shouldn't ever have to manually re-center an object when imaging, not unless there's something very wrong :-)

Ben

Sent from my GT-I9100 using Tapatalk

[Shibby]

Stuart, sorry if you knew this already but I thought I'd post just in case:

Further to what Ben has said, the key to keeping the tracking accurate is good polar alignment. With poor tracking you can get away with very short subs and stacking software will cope no problem, all it means is that your final image will start to become cropped as the common image area is reduced.

[roundycat]

There is no problem with re-aligning between sub exposures except for the fact that it takes time. Time is something you will soon realise there is not enough of owing to the weather. The most advanced way of re-aligning is automatic and is called dithering and is recommended for all imaging runs. With this technique and proper aligning/combining you can eradicate most noise without shooting darks.

I would never advocate sloppy calibration but if you try it be sure to use a combining or stacking algorithm that is based on Sigma reject. Never use average or sum.

Dennis

[Shibby]

When you say recommended for all imaging runs, Dennis, does this include unguided imaging? Also, how is dithering a method of re-aligning? I've heard of it in reference to noise removal and also drizzle. I'm not arguing, just trying to understand? Stuart's goal at the moment seems to be to keep his target in the FOV, for which decent polar alignment should be enough.

[drumsolo]

Thanks guys for your responses. The reason I was asking about keeping an object in the FOV is when looking at Saturn I noticed it drifting slowly across the eyepiece rather than staying centered. I thought I did an accurate polar alignment, Polaris in the small circle using the hour angle that the synscan gave me. Should it be drifting slowly? I reckon I could get a few mins before it disappears from the eyepiece all together.

[Shibby]

It depends on the magnification really. You were looking at Saturn so probably had quite a high magnification (small FOV) - some drift will inevitably occur but even so you should be able to keep it centred for longer than a few minutes with a polar scope alignment. For imaging DSOs at prime focus, the drift will be much less noticeable.

Try and get your polar alignment down to a tee; this will give you better sub lengths without trailing! Check the reticle is aligned well on your polar scope.

[Tim]

When you say recommended for all imaging runs, Dennis, does this include unguided imaging?

Be tricky to do it unguided

As regards things moving out of the eyepiece, worth stating again that good polar alignment is the key. When you have good alignment, things will stay more or less centred. As regards Saturn and other planets, they are moving at a different speed to the rest of the cosmos, you can likely set the tracking speed accordingly in your handset.

30 secs unguided is a good starting point. You will start to get useful data on brighter DSO's, and can quickly gather enough subs to stack nicely and get the noise down. Go for 20 or more. You will start to get experience in stacking and processing. As you get used to polar aligning, and get more accurate, then you can increase the sub length, up to 60 secs, maybe up to 90. Anything more is tricky, and other factors start to come into play, such as a periodic hiccup in the mounts gears known as periodic error. In my observatory I have my mount pretty much perfectly polar aligned, and sometimes take 10 or 20 minute unguided subs whilst setting up, but they generally have stars which show the inherent movements of the mount.

Good luck with your imaging, keep asking questions

Tim

[roundycat]

The original question was 'is it alright to re-center the scope on the object throughout an imaging run'. The answer is yes. Regardless of the reason for the drift it is ok to do this.

My reference to dithering being a kind of automatic re-aligning was a bit cheeky. It does not strictly re-align in relation to the camera frame which is what the questioner referred to. Dithering constantly moves the camera frame to a new position between images so an image is never taken on the exact same part of the chip every time. This randomises all sorts of aspects of the chip and when the frames are combined with an outlier rejection routine an awful lot of the noise and muck disappears.

Dithering is recommended for all imaging runs but you cannot dither without guiding in the first place if the software is set to dither via guider. MaxIm offers dither via mount which sounds like a dither option without guiding but if the object moves slightly of its own accord that can be a good thing, provided it does not run off the chip.

Good alignment software such as MaxIm or CCD Stack is robust enough to align images that have moved a lot. If your chosen software cannot do this or tells you 'there are not enough stars' (you only need two) then you might have a problem.

Dennis

[todd8137]

there is a limit to what dss can do if it miles of then heavy crooping resulting in a small area