Skip to main content

Commisioning advice.

andrew s
andrew s's picture
Offline
Joined: 2012-11-07

I am in the process of commissioning my BVR Photometry rig. So far I have measured the linearity of my CCD and it is linear to better than 0.4% from the bias level 5100 ADU to more than 64000 ADU. What I was wondering would be the best way to assess the rest of the set up and processing? I could not find anything in the CCD Photometry Manual so I propose imaging a number of Landolt fields and treat on of them as a variable to assess the performance of the filters etc. and my processing skills (using VPHOT). By doing Photometry on a star at different locations across the CCD I can assess the flat fielding.

Does this seem adequate and or what else should I do? Thanks for any advice you can give.

The filters I have are from FLI and are Johnson/Bessel which I assume are ok but I am not clear about if the difference between R and Rc is significant.

Andrew

Commissioning a new Photometry system
WBY
WBY's picture
Offline
Joined: 2010-07-24

I am not sure what you mean by bias level. 5100 ADU seems terribly high for the average count in a bias image. For comparison, my venerable ST7e has the following statistics:

Mean 104.6 ADU, Median 105 ADU, STDEV 5.3 ADU, Min 83 ADU, Max 258 ADU

 

What are the published full well e- depth, gain and A/D bit depth? I expect the bit depth is 16 bit (65,565 ADU) but the actual full well ADU depth will probably be somewhat less. Using my ST7 as an example, it has a FWD of 100,000e and a gain of 2.3. So  FWD in ADU is about 43,000 but linearity falls of significantly above about 30,000 ADU even though it is an NABG camera. Some cameras have multiple gain settings.

In addition to a linearity check I think you want to Make a pixel map locating bad pixels and you want to check the camera gain. If you make a histogram (#of Pixels vs ADU) of, say, the central 80% of a bias frame, The read noise in e divided by the ADU at FWHM of the histogram = the gain of your camera. They can vary from the published "typical" gain. You might also image a reasonably bright star in a matrix of,say, 12 or 15 positions evenly distributed over the camera FOV. Pick a star that you can image in say 15 to 30 seconds with high SNR and make sure that you focus carefully before taking the sequence. This allows you to check flatness/othagonality of field, and quality of flat frame across your field of view.

Also. you will find it helpful to make a record of the SNR (or CCD error formula) vs. exposure time you get through each filter for stars of different spectral classes and magnitudes. That data will allow you to make a quick reference for determining exposure durations (sky background permitting) you need for various targets to achieve desired SNRs in the future. NGC 7790 is well positioned now and offers large standard star field for calculating transformation coefficients. AAVSO has an archive with the map and sequence for the field and you can find it in my recent post in the Signal to Noise vs. Brightness thread of the photometry forum. It is in an attachment called NGC7790 Standard Star Field.

As far as the difference between Johnson and Cousins/Bessel filters, please see the attached pair of articles. They shed a lot of Light. If you have a red filter with the Is curve, my understanding is that you may get CCD results with very red stars such as Miras that are difficult to transform to the J-C standard which is based on photometry with photomultiplier tubes. My I filter has an Is profile and I don't use it on very red stars.

I think Arne can give a much more complete answer as to pitfalls to be avoided between the two filter sets and the standard. I don't think you have a problem with either formula R filter. Either transforms OK as far as i know. It's good to know which you have, however. The manufacturer should publish a typical transmission curve.

Brad Walter, WBY

Thanks Brad - I will do as
andrew s
andrew s's picture
Offline
Joined: 2012-11-07

Thanks Brad - I will do as you propose and follow up on your various points.

I did measure the gain at 1.66e/ADU and the full well depth is over 100,000 e. I know the bias is high but that is what FLI set it at and is the same as on the data sheet I got with the camera. I also plotted the SN ratio as you proposed. The camera is linear right up to 64000 ADU as far as I can tell from the results I got and the regression analysis had very high confidence limits. The CCD is an FLI MaxCam with an E2V 47-10 back illuminated chip which has 13 micron pixels and is NABG.

 

Thanks for the input - Andrew

 

Commissioning a new photometric setup
WBY
WBY's picture
Offline
Joined: 2010-07-24

The bias settings seem very different for back illuminated cameras. I wonder if that is due to Etaloning or other factors. I have never owned one. Our club just received a new Roper Pro-EM back illuminated camera to assist in research that McDonald Observatory is doing. I checked the specs in the manual but it doesn't list the pedestal. It uses an E2V 201B chip. I would suspect it has a similar pedestal to yours. I assume they don't list it because, according to the manual it varies with camera settings (e.g. gain and read out rate). 

Congratulations on getting such a good camera. I am sure it will be a pleasure to use.  

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484