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How do Scope Abberations Affect Photometry?

msheald
Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 02/23/2019 - 14:37

Hello! As I think (dream?) about a new telescope, I wonder how the various aberrations affect photometry performance.

    Commercial SCTs have coma and field curvature. I've read comments about varying amounts of spherical abberation induced with focusing from the prime focus point, though the significance of this was questioned.

    Meade ACF elimiates coma.

    Celestron EdgeHD ges rid of coma and field curvature, (but seems to have a tight backfocus requirement to meet this requirement?)

    Do any other these abberations affect photometry? If so, at what FOV/chip size would the effects become noticeable? Thank you and best regards.

Mike

hambsch
Affiliation
Vereniging Voor Sterrenkunde, Werkgroep Veranderlijke Sterren (Belgium) (VVS)
Sat, 02/23/2019 - 16:01
Abberations

Hi Mike,

since you can do good photometry on trailed stars, not focussed stars (sometimes you want deliberately defocusing a bright star) etc, I do not think that abberations do affect photometry in general.
You can do perfect photometry even with the standard SCT's (non Edge and ACF models). If you opt for s CCD of huge size (budget much bigger than for the scope) than maybe you get deformed stars towards the outer parts of the CCD chip.
Still I would think you could do photometry of those stars. Anyway normally you put the variable closer to the centre of the field and you have enough comp stars around it.
So I would not worry about abberations and would go for the scope I have budget for. You also need a decent mount and CCD.
I hope that helps.
Best,

Josch

Eric Dose
Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 02/23/2019 - 17:56
Abberrations can be managed

Hi Mike. I agree with Josch on all counts. A couple of additional comments:

  • An ill-selected mount can introduce as much spreading as the optical system does.
  • Edge HD OTAs give flat, coma-free images only with a short and unforgiving backfocus, but that can be had by using a flat-style focuser (I use an Atlas; there do exist others).
  • Besides smeared stars, a too-distorted field can cause problems with image plate-solving. All software packages handle linear fields pretty well, but they differ widely in their ability to solve distorted fields.
  • Any chromatic abberations (including atmospheric) in CCD photometry using filters are not very important, as the filter greatly reduces the wavelength range. By contrast, for clear or filterless photometry (CV etc), I would definitely check any system for chromatic (or low in the sky, atmospheric) abberation.
msheald
Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 02/23/2019 - 20:00
Image Aberrations and Photometry as Chip FOV Increases

Thank you!

    I was thinking how a 14 inch Meade ACH (or 14 inch Celesctron) on an appropriate mount would match a FLI ML1001 very well with its 24 um pixels.

    At 24 arc minutes FOV, I did not think the edge stars would show significant abberation that would affect photometry, but I wanted to get folks' opinions. Best regards.

Mike

spp
Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 02/23/2019 - 23:20
telescopes, cameras, mounts

Mike,

I agree with the previous comments.  A plane vanilla SCT works just fine for photometry.  

The KAF 1001e chip is a very nice match for the C14.  I have a 1970's era C14 and a camera with the 1001E chip.  The images are very good across the full width and in the corners.  Of course there is some vignetting, but I think that is the case with any of the telescopes you mentioned, and it is easily corrected with the flat.   I don't believe the extra glass in the light path of the Edge or ACF SCT's gets you anything for doing photometry.

I'd recommend buying a used C14, then put the money you save toward buying the very best mount you  can.

Phil