SharpCap Forums

Hello
I have a nexstar 6se with standard mount with asi462mc; so far I've only ever done planetary astrophotography, but 2 weeks ago i was able to do my first deep sky experiment, while being absolutely aware that my setup is not the right one (i don’t have equatorial mount ,my camera is a small asi462mc etc )
Therefore, obtained pic is nothing special , but believe me, it was a real emotion when the nebula , which is distant 1344 light years, appeared on the screen here my questions (apologize if are long )
a)I was using Live stack with standard options : Anyway, having Sharpcap pro version i would try Sigma Clipping,that looks interesting as i understood from the documentation wondering if, considering i have an AZ mount there are special settings that could be helpful or instead if i can use with default options.

b) as i understood LiveStack create (at least in my case , i really tried without testing ) a "final"stacked .fits file, that i can then convert in .tiff for elaboration; is that correct, is best way, or is there a way to obtain directly a .tiff instead?

c)About saving file, unless i will schedule it, i will need to click save button , deciding which format i want ; A 16 bit image with the histogram stretch applied – this will appear roughly as viewed onscreen/ or with no adjustments applied for later processing
A 32 bit FITS image containing the raw stack values for later processing
Last time i have saved with rush in any kind of format (including .png "exactly as seen) cause i was so happy , but i am wondering if there is any difference between 16/32 bit considering i take the image with raw16 format - so main question is which format i could save? sorry if question appears silly but i am a DSO beginner

d) about frames
i see there are many options to save frames collected by Sharpcap in Live stacking, this is absolutely wonderful ; wondering if , in case i would do stacking experiments, to save all frames, or only stacked - again sorry for the silly question thanks so much to anyone would spend a word on that
Fabio

Hi Fabio,

great to see your first nebula images - one of my goals with live stacking was to try to make this sort of thing easy (or at least much easier), especially with hardware that would otherwise make things difficult.

In answer to your questions:

a) There are no special options that are specific to an Alt-Az mount, but you should think about exposure lengths to make sure you do not see trailing of stars caused by poor tracking or field rotation in the individual frames. Remember that the real purpose of sigma clipping is to rule out bad data (satellite trails, aircraft lights, etc) and that this does not happen until after the first few frames have allowed SharpCap to get an estimate of the average value and variability of the brightness of each pixel.

b) The save formats for live stacking are set to PNG for the bottom two (images designed to be viewed directly) and FITS for the top two (images designed for further processing). I am afraid that if you want a TIFF file, you will have to convert. However more software is now capable of opening FITS files (GIMP, Affinity Photo, etc)

c) Again going back to the four save options - the first two are saving the stacked data with no stretch. The 32 bit version is only really helpful if you have taken a very long stack, where the noise levels have dropped to the point that there may just be data worth having beyond the most significant 16 bits. For most purposes the 16 bit save will be fine. As I say, this is unprocessed (no stretch), so is designed for loading into another application to process.

d) If you intend to test, saving all the frames is a good idea, since then you can look at the bad ones along with the good ones and learn how to tell the difference, and how you might exclude the bad ones in future (FWHM filtering perhaps).

Hope this helps,

Robin

Thanks a lot Robin!
Thanks to your suggestions i was able to do a better stack this time (i used 2 focal reducers lol )
this one instead has been done with 0.63 reducer and another focal reducer (0.5x) connected with the asi462 ( Exposure=10.853s, TotalExposure(s)=263 sec, StackedFrames=44) no dark, flat, nothing, just pure live stack About my previous questions

a) "Remember that the real purpose of sigma clipping is to rule out bad data (satellite trails, aircraft lights, etc) and that this does not happen until after the first few frames have allowed SharpCap to get an estimate of the average value and variability of the brightness of each pixel" --> does it mean that if hypothetically no aircraft or satellite passes by, the result would be the same as using the default option?,

d) "If you intend to test, saving all the frames is a good idea, since then you can look at the bad ones along with the good ones and learn how to tell the difference, and how you might exclude the bad ones in future (FWHM filtering perhaps). --> Premise; i would always use only live stack, but my intention was eventually to save fits stacking them with another program (dss, for example) - in this case maybe should i save all but using FWHM so i would have in folder only good frames, am i right?

and last but not least beginner and probably stupid question: wondering Is there a limit to total exposure time, (not talking about exposure i set in sharpcap, that obviously due to my AZ mount mus be max 20 sec) I mean, can i for example do same live stack for 1hr or instead after some time i will get star trails?

thanks so much!

Hi,

nice to see that you are making progress and are getting better images now

You are quite correct that if there are no planes/satellite trails then the results of the two different stacking methods will be very close to identical. Default stacking keeps track of the sum of all the pixel values from the different frames and sigma clipping tracks the average, but of course those will be the same except for one being the other multiplied by the number of frames. It is possible for sigma clipping to misbehave under some circumstances - for instance if there is a drop in transparency due to thin cloud coming over then sigma clipping can start rejecting a lot of data because it is too different to the established values in the stack so far. Something to keep an eye out for.

You can certainly choose to filter on FWHM and then save only the frames that are added to the stack - that will help select only the better frames.

Finally, in theory you can stack for as long as you like, but there are some practical limitations

* Long term drift of the mount - if the mount is drifting then the area in view will slowly change. Live stacking mostly corrects for this, but some of the initial in view area will drift out of view. Those areas will not receive any more new image data, and will become comparitively darker in default stacking, giving a black border on one or two sides that slowly grows.

* Rotation of the image when using an Alt-Az mount can give the same effect as drift, but with the darker areas being triangular shapes and at the corners of the frame.

* The longer you stack for, the more chance you have of a bad frame (plane, satellite, cloud, etc) ending up in the stack and spoiling the image.

With a little trial and error and experience you will get the idea of how quickly the above issues become a serious problem for your setup.

cheers,

Robin