Filter Factor and Exposure

[purgitoria]

Hi all,

So, i'm quite new at astro imaging and I have been looking into how to go about calculating my exposure times and am getting a little confused. I downloaded a great tool to help me which is the Astrophotography Exposure Calculator by Michael Covington. One of the item I am failing to grasp however is the Filter Factor in relation to exposure time.

I know that the different filters will need different lengths of exposure time due to the ammount of light they let through but how do I figure out what factor each of my filters has? I am currently using the Astrodon 36mm LRGB and Ha filters as well as the SII and OIII from Baader planetarium but do not seem to be able to find any information from the manufacturers websites as to what factors each of their specific filters have.

[NGC 1502]

Hi, I don't do imaging, but was brought up with film photography, and used filter factors to calculate the extra exposure needed.

Perhaps this may help http://en.wikipedia.org/wiki/Filter_factor

Regards, Ed.

[purgitoria]

Thanks for the response Ed. I did have a quick look there but it is very confusing along with information i can also see in other places. I know there are also other factors that will have an effect on the exposure time but what i was looking for was more precise details on the specific filters that i actually have as the factor can vary immensly with each of the colors and also on the specific wavelength range that the filters are letting through.

[NGC 1502]

Hi again. You may get some help if you post to the imaging section, more folk that actually do imaging will be more likely to be on there.

Hope you find the info you are after.

Regards, Ed.

[cantab]

I would guess that things are more complicated for nebula filters. They have a narrow bandpass so block a lot of light, which would mean a high filter factor. You could estimate how much light they block by seeing how much they reduce your stellar limiting magnitude. However, nebulae emit most of their light in the specific wavelengths the filters pass, so the "filter factor" for a nebula source will be lower than for a stellar source.

[sgazer]

unless you are really keen to do some complex calculations (based on object brightness, sky glow, sensor sensitivity and noise, filter response etc etc), I don't think it's necessary and have never even considered it during my 4 years of imaging. Asuming I've understood your intention correctly; it's not really important to calculate whether you need 2min 34s on one object or 5min 12s on another, with/without various filters. Generally most objects are faint enough, that with any filter, you will want as long an exposure as possible and will be limited by your camera, tracking and light pollution. It is very rare that you will get an over exposed image apart from something like M42 or M31 which have bright cores.