Newbie "fits" question

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Rfi
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Joined: Wed Oct 31, 2018 12:17 pm

Newbie "fits" question

Post by Rfi » Fri Nov 09, 2018 4:51 pm

Hi,
I've been using SharpCap for about 6 months and have found it really useful. I'm a retired Lecturer / Professional Engineer, and have some background in Astronomy and Computing. My question concerns some fundamental concepts associated with the fits files saved from the SharpCap "Live Stacking" function. I append some images:

* Image A is a jpg of the png from SharpCap.
* Image B is a jpg of the 32 bit file from SharpCap as portrayed on "Irfanview" (one of the "normal" image viewers that handles fits).
* Image C is a jpg of the same 32 bit file as seen on "avisFT" (a fits viewer that I am trying out).
* Image D is a jpg of the same 32 bit file with the white threshold altered.

Using image A as a reference point being comparable to what I observed in the sky, my questions are these:

1 I can see that image B is vertically flipped, but why is the default rendering so different from image A ?

2 I can see how image B becomes image C, but why are the 4 stars from A portrayed as discs, whereas most of the remaining features are arcs? (I can appreciate that the 4 stars were "locked on to" by SharpCap, but why are the arcs even visible when they are invisible on A? And how can SharpCap generate a visible background between the "locked stars", such as a deep sky object, if non locked entities are permitted to rotate on the image?

3 You will note that 4 stars are circled in red on D. These correspond to stars identified on planetarium software, but which I do not believe were visible by eye at the time of capture. Why are these discs, rather than arcs, and how can one make more arcs become discs?


I apologise for the length of my post, but I am struggling to comprehend what is actually going on, and how I can make use of it.


Many thanks for reading this far, Rfi


Image

Rfi
Posts: 5
Joined: Wed Oct 31, 2018 12:17 pm

Re: Newbie "fits" question

Post by Rfi » Fri Nov 09, 2018 4:54 pm

Whoops, missed the image link:

Code: Select all

https://1drv.ms/f/s!Ans5VxreT9yTgzJRNfxDEMiU3UR2
https://1drv.ms/f/s!Ans5VxreT9yTgzJRNfxDEMiU3UR2

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admin
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Re: Newbie "fits" question

Post by admin » Fri Nov 09, 2018 8:50 pm

Hi,

The streaks in your images are actually caused by hot pixels from the camera. It looks like you have an altitude azimuth mount which means the streaks a curves as you have field rotation as well as a bit of drift.

So what's going on is that the actual stars appear as disc sore spots because sharpcap is aligning them, but the hot pixels appear as streaks because they get moved around by the alignment process that is lining up the stars. You need to start subtracting dark frames to remove the hot pixels which will make this problem not entirely go away but much better.

Hope this helps, you can find lots of info about taking using dark frames in the documentation.

Cheers, Robin

Rfi
Posts: 5
Joined: Wed Oct 31, 2018 12:17 pm

Re: Newbie "fits" question

Post by Rfi » Sat Nov 10, 2018 3:18 pm

Hi Robin,

Thank you for such a quick reply. Yes, that helps a lot. I had never though of that as being the issue - there are so many of them. And your answer makes complete sense - a rather new angle on relativity!

Many thanks, Rfi

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