Selecting a camera

[isoparix]

A table that compares the technical capabilities (and prices) of Atik cameras would be useful - does it exist?

Trying to work from first principles:

I want a camera whose pixel size matches or lies below the diameter of the Airy disk at the focal plane.   This diameter by convention is 1.22 x wavelength x f-number, so for 550nm light and and f10 lens, this would be 6.7 microns.   For an f4 lens/mirror then we'd need <= 3.3 microns (bigger aperture, sharper picture, yes?). 

Then I want a sensor big enough to accept the size of the image I wish to shoot, which is the angular diameter of the celestial object (just a number, no arc-seconds, please) x the focal length of the lens/mirror.   So for the moon (angular size about 1/111) and a 750 mm focal length lens, we have an image size of 750 mm/111 = 6.76 mm. 

So, now I have my desired resolution and size - where's that table?!!

[NickK]

Hi - there are technical parameters for each of the cameras on the product part of the site (www.atik-cameras.com). However I agree that a simple reference table to aid selection would be advantageous.

The calculation of the airy disk differs for refractors and reflectors due to the central obstruction. Also it's good to understand the top and bottom wavelengths - so if your camera's pixel sizes are between the sizes- you'll end up with some wavelengths being affected and others not in the same image!

An example of a standard CCD LRGB filter set :http://www.firstlightoptics.com/rgb-filters-filter-sets/baader-lrgbc-ccd-filter-set.html indicates from below 400 through to over 700.

From you post you're looking at the smallest pixel where the airy disk fits the pixel.. a simple way of thinking about that would be taking the top wave length (i.e. 750nm) then applying the same formula as this is the largest airy disk you'll have in the image.
For narrowband and specialising on specific forbidden lines this can be tuned even further: http://en.wikipedia.org/wiki/Astronomical_spectroscopy onto specific narrow bands (ie Ha or Ca-K for example for solar scopes).

However - in reality many DSOs are large so dropping some resolution in favour of getting the entire thing in one shot is also an important consideration. Mosaics mean serious imaging time. Additionally the "seeing" limits the maximum resolution (related to your airy disk calculations) due to the atmosphere - many say that 2 secseconds is the "rule" however I don't have too much issue with 1.67 arcseconds.

So another thing you need to consider is "Dawes Limit" (or Raleigh's Limit) - also related to the airy disk. It's the smallest resolution you can achieve before the defraction of the aperture starts softening the image. I use a Pentax 105 SDP, with an aperture of 105mm and fl of 670mm. The manual indicates the limit is 1.1 arc second - however with a 383L+'s 5.4um pixel size, this gives me 1.67 arcsecond/pixel which means the pixel size is over the maximum (good) and with a 2x Powermate (fl 1400mm) results in 0.88 arcseconds/pixel which is below 1.10 limit.
Imaging at 0.88 is possible, although the image is softer but can be restored to a reasonable level of detail (this is my m57 at 1340mm fl with bad tracking on the EQ6 attached).

[isoparix]

Nick,

Thank you for that.  Of course,  it's possible to push these analyses to unhelpful limits - I'm just looking for some guidelines to help bound my choices.   And I'd like what I choose now not to constrain me in the future.  But as I say, thanks for your input.