New Mexico Sky: M31 and M33 with Astrotrac

Another post on my 2 week escape to clear skies – Abiquiu, NM … I probably produced as many pictures in two weeks as in 6 months in the San Francisco Bay Area! Another aspect of doing Astrophotography in a Urban environment is that you need to go out very far to access clear skies..  And the Astrotrac travel system helped there – I was able to easily carry the equipment on my flight to New Mexico!

I captured these two great galaxies with two different cameras: A modified Canon XTi for m31, and a Qhy8 CCD for m33, both cameras using my Canon 200mm L lens and the astrotrac travel system. Note the Qhy8 has the same sensor as the Orion Starshoot Pro V2 ( Sony ICX413AQ).

M31 with Canon XTi and Canon 200mm L

This is a cropped picture. It is a combination of 13 frames of 3 minutes at f/d 3.5.

Stars on the image are round till the edge (on the uncropped images) which is a sign of perfect focus (with my Canon 200mm L lens).


M33 with Qhy8 and Canon 200mm Lens

This is a cropped picture.

This is a combination of 19 frames of 3 minutes at f/d 2.8, taken with the Qhy8 CCD Camera.

Comparison between the Canon XTi and Qhy8 for Wide Field imaging: my personal opinion

Having taken a couple of pictures with the Canon XTi and Qhy8 CCD using the same lens (Canon 200mm) I’d like at this stage to do a comparison between the two approaches.

This comparison assumes the CCD or Canon are used with Teleobjectives (not a telescope), and the CCD is a color one, not a black and white …

Mobility: Advantage to the Canon XTi … Non need to plug to a power cord, no power converter… Which is really in the philosophy of the Astrotrac

Filter use: Advantage to the Canon … You can use clip filters in front of the sensor (HAlpha, Anti-pollution – Astronomik) – I cannot see how to use Filters with the CCD given the back focus. The Filter has to be in front of the objective, which is (more) expensive and cumbersome.

Sensitivity and Signal to noise ratio: Advantage to the Qhy8 on a couple of points

  • a) On Red/HAlpha better sensitivity (even though the Modified Canon should be very close).
  • b) Larger chip pixel size on the Qhy8 which leads to better overall sensitivity (7.8 microns for the Qhy8, and about 6 for the Canon XTi)
  • c) Better Signal to noise ratio with CCD Cooling (for the Qhy8 chip which is already a very low noise one even without cooling)

For faint emission nebula there is an advantage with the CCD – for Galaxy / Reflection nebula I think the difference is minimal (as the pictures above show it).

Integration with Lens: Advantage to the Canon  – I did not find a way to control the f/d ratio when the Canon Teleobjective is not connected to the Canon Body. With the CCD I had to use the Lens at F/D 2.8. It is not a big deal if the lens is excellent – but it is a problem if you need to stop the aperture. You have to use a mask in front of the lens in this case.

Focus: great live focus support with the Qhy8 and software shipped with the CCD. I don’t have the latest Canon body with the “Live view” function – but the CCD software is really designed for a very precise live focus and display in real time of the Full Width Half Maximum (FWHM).

Versatility: Advantage to the Canon XTi – I can (and did) use my modified canon for daylight pictures. which is another way to travel light: one camera for everything!

Ease of Use: Advantage to the Canon XTi. However the Qhy8 CCD  – as many Color CCD cameras – is quite simple to operate (vs. Back and White ones with filter wheels).

As a summary, for Wide Field Astrophotography with Teleobjectives and the Astrotrac:  I don’t think the kind of pictures you  do with a color CCD camera such the Qhy8 are drastically better compared with a modified Canon XTi (not modified is another story). Given the versatility of the Canon DSLR  body I think it is still a good choice for Wide field Astrophotography –  if a choice has to be made between a CCD and a DSLR camera.

This said if you already have a CCD camera similar to the Qhy8  (as I do) – then using it can be a tool of choice for wide fields that include faint emission nebulae (e.g. in Cygnus). The overall sensitivity is I think better compared with the Canon XTi sensor (without any doubt for a non modified DSLR), and as a bonus it allows  a very precise control of the focus. As an illustration, I took this picture of Ngc 7000 with the Qhy8 CCD …

  1. #1 by Laurent Combémorel on September 29, 2010 - 12:23 am

    Hi Jean-Christophe,

    A simple trick to set the aperture on a Canon Lens : use a Canon DSLR. Switch it on, set the appropriate aperture, press the aperture test button and switch the DSLR off while pressing the aperture test button. Normally the lens should keep the selected aperture when removed from the DSLR. And you can use it with your Qhy8 CCD.

    Clear skies
    Laurent

  2. #2 by jmeriaux on September 29, 2010 - 7:39 am

    Laurent, This is a great trick ! Thanks for the info ….

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