recently i was setting up for a project and did a subexposure calculation which yielded an optimum subexposure of around 1-2min. i noticed my background adu after a 3 min test was high, around 1-2000. i'm using an sbig st8xme camera which is in your drop-down menu in the calculator so i believe the only thing i need to do is enter the adu and test exposure time into the program? my main question is, in a case like this is a 1-2 minute exposure a real answer or should i just interpret this as saying the sky conditions really stink tonight and i should wait until the seeing gets better?

Unless your scope is f/2 then a 2 min sky limited exp is not likely correct.

A common mistake is forgetting to dark-subtract the frame before measuring the ADU (and then it is usually necessary to subtract 100 ADU from the measurement because most software adds a 100 ADU pedestal after dark subtraction - except for CCDStack which does not use a pedestal).

The image as it comes out of the camera has a "bias level" between 100-2,000 ADU (and can be different for individual cameras). So you must remove (subtract) that bias level in order to know the "starting point" for photon/electron accumulation.

Stan

thanks stan. how is the bias level determined?

ok. is bias value the same as pedestal value under the fits header info? when i examine the file in ccdstack under fits header info it says 100. so i would subtract then 200 (including the 100 that ccdsoft uses for pedestal as well) to get the correct adu value to use??
thanks,
dave

ok. so i took a 3 min just now, dark subtracted, then got 1000 for background adu, subtracted 200 from that (reasons above), entered into calculator and result is 1.8 minutes? camera is st8, ccdsoft. scope is a 10 f/7 newtonian. seeing is poor by the way...again

Hi Dave,

I know it is confusing - bias and pedestal are 2 different things, though sometimes they are the same value.

The pedestal may be documented in the FITS header or may not be or may be erroneously valued. But regardless of that the true bias (including pedestal) is easily measured as the median value of a short-exp dark frame.

If you dark subtract an image then the bias is automatically removed because it exists equally in bot the dark and the light frames. *But* the software may add a 100 ADU pedesatl after the subtraction.

So there are several ways to deal with this, but here is an easy method for obtaining the value you need for the exp-calculator:

1) measure the median ADU of a dark frame. This is your avg bias and it will not change significantly for the life of your camera (unless you install different drivers or such). So memorize it!

2) measure the median ADU of a background area in a RAW light frame (no dark subtraction).

3) subtract the bias (#1) from the raw background (#2).

Stan

ok. got it! thanks again stan