M13. Short very sharp image

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timh
Posts: 377
Joined: Mon Aug 26, 2019 5:50 pm

M13. Short very sharp image

#1

Post by timh »

In the process of trying to deconvolute some problems with elliptical shaped stars I first needed to be sure that the telescope optics (PDS200) , alignment and collimation were as near perfect as possible so as to be certain that the mount rather than the telescope was the culprit.

So to do this I made some images of M13 while it was nearest culmination (early am on 16 June) and made an image stacking just the best 1/3 of 600 x short (100 ms) exposures - from a 1 minute Sharpcap SER capture. Gain was 350 using an ASI MM 294 PRO camera in 46Mb mode with 2.315 um pixels and at 12bit.

I was surprised by just how sharp (if not very deep) the resultant 21s exposure of M13 turned out to be. According to PixInsight subframe selector the average FWHM value of stars in the image is 1.25 ! If you count pixels from one star centre to another this does seems correct - there are star pairs clearly separated at a distance of only ~ 3 pixels (0.477 arcsec/ pixel) .

Just thought it interesting that it is even possible to get that sort of resolution under UK skies ....although with the obvious caveat that it is a rather noisy image at only low SNR. Nevertheless !

Tim
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turfpit
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Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
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Re: M13. Short very sharp image

#2

Post by turfpit »

Tim

A good image considering the time spent on capture. If you are experimenting with SER files for deep sky, have a look at

http://www.astrokraai.nl/viewimages.php?t=y&category=7

Produced by the author of Autostakkert and stacked using his software. My favourite is the M51 which is 2000x1s SER files stacked with AS!2. A 16" DOB was used for many of the images.

Some notes from Emil's Cloudy Nights post attached.



Dave
timh
Posts: 377
Joined: Mon Aug 26, 2019 5:50 pm

Re: M13. Short very sharp image

#3

Post by timh »

Thanks very much for that useful and very relevant link Dave. It has gone into my favourites.

It was interesting to see that many of Emil's images were from 1s frames using a 16 inch dob - and very similar to the stuff that I have been attempting but that Emil had already done some 6 years ago. I wondered at first why his 1s dob images were so sharp (15 arcsec of movement per second) but then noticed that the dob itself was placed on an equatorial platform. That must work pretty well although I can see a bit of trailing in his star shapes - probably - just as in the case of my mount - due to RA movement at the subsecond microscale not being entirely smooth. A 16 inch dob is some weight to carry! His set of guidelines also very useful - particularly around getting the gain and brightness (offset) right for the short .SER file frames - and improving the final stack using a master dark.

Frames as short as 1s certainly look to be viable with a 16 inch scope but with my 12 and 8 inch Newtonians I would prefer be working in the 2-5s range in order to get not only sharper luminosity but also reasonable SNR across a wider set of deep sky objects. Surprisingly (to me anyway) rather than 'the seeing' turning out to be the main limitation it looks to be the subsecond 'granularity' of mount movement that limits resolution and star shape. I suspect (but don't know) that this is rather general phenomenon unless you take out a second mortgage and get one of the software bisque mounts or similar?

Tim
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turfpit
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Re: M13. Short very sharp image

#4

Post by turfpit »

Tim
I suspect (but don't know) that this is rather general phenomenon unless you take out a second mortgage and get one of the software bisque mounts or similar?
I know the feeling - serious investment needed.

I did experiment with this technique back in 2017 but a general lack of imaging experience went against me. Processing a large number of SER files with AS!3 is easy:
  • drag and drop all the SER files onto the 'Open' button in AS
  • select a single alignment point, a bright star
  • stack to produce the TIF files
  • drag and drop the TIFs onto the Open button in AS and stack
I think I will revisit this using M51 and my QHY5L-II-M, much improved understanding of the histogram will help with the capture settings. Dark challenged at the moment here at 53°N.

Dave
timh
Posts: 377
Joined: Mon Aug 26, 2019 5:50 pm

Re: M13. Short very sharp image

#5

Post by timh »

Hi Dave,

I note the AS!3 methodology which clearly works well and that I am hoping to use more for planetary -- but thought that PI which I already own must probably be at least as good if not better for deep sky?

The PI work flow then being to use PIPP to convert the .SER file to a set of FIT or TIFF files. Then Subframe Selector to prioritise, select and weight (according to PSF signal). Then calibration - optionally cosmetic correction - and star alignment (based on a number of stars) followed by integration. PI also provides the Blink tool which is not only useful for selection by visual inspection but also useful for cropping the entire set together.

PI does fall down though in images where there are too few stars and star-alignment then refuses to work - in those cases I have fallen back on the manual dynamic tool which is very laborious for large numbers of files.

Tim
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