Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Thu, 11/09/2023 - 20:39
Linearity yes. That is THE most important word in photometry. Without it your photometry may be useless. I have to assess where it lies between zero and max ADU. I think I have to take some exposures and plot a graph.
How is it practically done?
Of relevance to this is another oddity I have noticed.
I have a ZWO 1600 MM Pro cooled camera which is 12 bit.
On the AISAIR histogram when taking flats I had to be aware to try and get the maximum between 1/3 and 1/2 of max ADU which on the screen is coming out at about 65000. I think this is the max ADU for a 16 bit detector (2^16). I thought my camera has 12 bit detector so should only be 4096.
I'm confused over this.
Kevin
How to measure linearity is laid out in the Photometry Guide.
AA must be scaling the ADUs to a 16bit range (multiplying each pixel value by 16).
Keith
duplicate comment deleted
"How to measure linearity is laid out in the Photometry Guide."
Kevin,
You won't find "linearity" listed in the Table of Contents. Look for the section on "Saturation". The old Photometry Guide (V1.1) section on linearity testing has some practical advice on page 16.
Phil
Thanks Phil,
Is that the "AAVSO Guide to CCD/CMOS Photometry"
The version I grabbed off the AAVSO website is Version 1.0 (July 2022) so maybe this is some other guide.
Regards
Kevin
Kevin,
The latest version of the Photometry Guide is officially named Guide to Photometry With Cooled Monochrome AstroCameras. It will also say Ver 2.something. The version I have is Ver 2-Rev BV. (There might be a slightly later version.) This covers linearity testing in the "Saturation" listing, Section (3.3).
The older Version is officially named The AAVSO Guide to CCD Photometry. It will be version 1.something, usually V1.1. This covers linearity testing in Info-Box 3.1 on page 16.
You might find it helpful to read both the new and the old versions, not just the testing information but both sections on saturation, FWD, and Analog to Digital Converters.
Phil
I have to admit to being very confused about the different guides and where to find them. A search on the AAVSO website turns up no results for "Guide to Photometry With Cooled Monochrome AstroCameras." What I have been able to find is "AAVSO Guide to CCD-CMOS Photometry with Monochrome Cameras" Version 1.0 (July 2022), which I downloaded from https://www.aavso.org/ccd-camera-photometry-guide. I found the same manual at https://www.aavso.org/observing-manuals. Please let me know where I can find "Guide to Photometry With Cooled Monochrome AstroCameras"
Thanks to all.
Wayne
...my mistake. The Version I have of the "new" Guide is a late draft. You have the final version which has the new name "AAVSO Guide to CCD-CMOS Photometry with Monochrome Cameras" and is now also identified as Ver 1.
The link in the "Publications" section of the AAVSO web site is also confusing. The link to
"AAVSO Guide to CCD Photometry"
which is the name of the "old" photometry Guide now points to the new Guide (AAVSO Guide to CCD-CMOS Photometry with Monochrome Cameras ). I can't find I working link to the "old" Photometry Guide.
I think the "old" Guide might be helpful to beginners who find the "new" Guide at times to be rough going. If anyone wants a copy of the "old" Guide contact me off line and I'll email you a copy.
Phil
Keith wrote:
"AA must be scaling the ADUs to a 16bit range (multiplying each pixel value by 16)."
Keith, by AA do you mean AISIAIR?
If so, it is relevant to note that the ADU values for saturated pixels in ZWO ASI 12 bit cameras are always in the 60,000s whether or not AISIAIR is in use. The ADU for each pixel value is always internally multiplied by 16.
Roy
Yes Info Box 3-B walks through the procedure for linearity in the newest version of the Guide
By AA I meant AsiAir. I don't use ZWO cameras but as you say it must be the driver doing the scaling to 16 bit.
Keith
Thanks Keith