Photometry

Discussion of using SharpCap for Deep Sky Imaging
Post Reply
Geoff_OPELS
Posts: 11
Joined: Tue Jun 29, 2021 7:29 pm

Photometry

#1

Post by Geoff_OPELS »

Using a QHY600M (16-bit) camera, I'm trying to do photometry using SharpCap with Gain=0 and Offset=10, and writing FITS files. A histogram of the ADU values in the FITS images shows that the ADU values in the pixels are being changed, i.e., "stretched" (for astrophotography folks). What SharpCap parameters must I set to ensure that the ADU values in each pixel remain unchanged, i.e. not stretched?

Under "Image Controls" I see parameters "Gamma", "Brightness", and "Contrast". Do these affect the ADU values in the stored FITS image?

Thanks,
Geoff
User avatar
turfpit
Posts: 1779
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Photometry

#2

Post by turfpit »

Geoff

Under File > SharpCap Settings > General tab see:

photometry-setting.PNG
photometry-setting.PNG (12.63 KiB) Viewed 956 times

Checking this should provide what you need.

The settings under 'Image Controls' only affect the displayed image and nothing from here finds its way into the FITS files. I fell for that one a long time ago.

Dave
Geoff_OPELS
Posts: 11
Joined: Tue Jun 29, 2021 7:29 pm

Re: Photometry

#3

Post by Geoff_OPELS »

Thanks. I tried checking that box, but it didn't help. The histogram of ADU values were still stretched. But I will try again, using a different Gain setting.
User avatar
turfpit
Posts: 1779
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Photometry

#4

Post by turfpit »

Geoff

I am sure Robin will be able to throw more light on this.

Dave
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Photometry

#5

Post by admin »

Hi,

if you already have a 16 bit sensor then that check box should be irrelevant, but do make sure you are capturing in MONO16!

Gamma, Brightness and Contrast should be left at default values (1.0, 0.0, 0.0) to avoid stretching. The same for the colour balance controls on a colour camera. You should be safe adjusting exposure, gain and offset.

cheers,

Robin
Geoff_OPELS
Posts: 11
Joined: Tue Jun 29, 2021 7:29 pm

Re: Photometry

#6

Post by Geoff_OPELS »

I use a QHY600M camera (16-bit) to take images that enable photometry.

1. I check that box about “Save 10/12/14 bit images in FITS files without scaling . . ."
2. I set "MONO16".
3. I set gamma, brightness, and contrast to (1.0, 0.0, 0.0), because Robin states it avoids stretching.
4. Set Gain = 0 and Offset = 0.

That Gain=0 gives Full Well = 51k e- and read noise = 3.7 e-.

(Alternative: At Gain=57, one gets full well = 23k but lower read noise of 1.7e-, useful for faint sky and objects.)
Post Reply