Diffraction spikes

I will be interested to see what response you get to this, as I am a relative novice myself.  But I have been surprised how little difference image quality in general makes in photometry.  Most imperfection on the subject star is reproduced on the comps stars, so it washes, though any defect that spreads the light reduces the S?N ratio and none of us like that.  

Some defects, like coma can vary across the field.  I use a focal reducer that is also a field flattener on my SCT system, but I have never seen any discussion about coma being a big concern. In part this may be because if your inner annulus is a multiple of your FWHM, so even some coma becomes trivial in the scheme of things and most people try and keep the comps near the subject star anyway.  

The only way I an concieve of defraction spikes impacting photometry would be if a comp star was quite near the subject star and the spike from one happened to fall on the image of the other, but again there would be at least a partial wash as the reverse would be true, spikes being reciprocal features.  In any event, if the stars are sufficiently separated to do differential photometry on in the first place, it is hard to image that a distant refraction spike could involve enough energy to make a material impact. 

My impression is the RC is probably thought of as the optimal design for photometry, lacking any lenses (which can filter certain colors differentially) and being well corrected across the field, but in the end nearly any scope you can put the instrument on seems to be useful.  The scope selection concern I have seen regularly is that of matching the focal length to the pixel size on your camera - minimum of two pixels, preferably three or more per average FWHM image. 

I have frequently wished I had employeed an RC and may eventually change, the reason being mirror flop problems with the SCT.  I have had to go to considerable trouble to secure the primary mirror and even so I have not fully licked the problem  Again though, even trailed images seem to generate valid data, just the S/N ratio is impacted, raising the err range and a small err is a common goal for us all.  ;)

Thanks for the reply.

I was thinking about the 8 inch Ritchey-Crètien by Astro-tec until I saw the diffraction spikes on images it produces. Surly the light from the 'spikes' would bleed from the aperture to inside the annulus...I'm sure it will be difficult to contain the diffracted light inside the aperture, at least not in fairly bright stars?

Doug.

Hi Doug,

For aperture photometry, the important aspect is that you contain the same fraction of a star's profile inside of the aperture, for both the target and the comparison star.  Diffraction spikes occur on all stars in an image, and so the profile will be identical (within reason).  Even if part of the spikes are excluded for the target, they are also excluded in the comparison star.  So the photometry will be fine.

As mentioned before, the one exception to this is that visible diffraction spikes means that some light from the usual Gaussian profile has been scattered into the spikes.  This means that less flux will be included inside of the aperture, and therefore a slightly lower signal/noise will occur for the measurement.  I bet that this is a really minor change, as the eye is really good at seeing extended features like spikes, but that they are low-level and contain little flux.

More important is having nearly identical profiles across the field, and the R/C configuration does a good job of this (especially if a field flattener is added).

Arne

Thanks Arne,

That makes a lot of sense, and now choosing my next scope will be much easier 

Doug.

Doug, following Arne's comment, secondaries also scatter light in to the defraction rings, they just aren't visible on an image; the inner most because they are absorbed in to the seeing smeared image of the star and the outer because they fall below the threshold for being shown on in a digitial display, as determined in your contrast settings etc.  This threshold is why dimmer stars appear smaller on an image, when in fact their light distribution is the same as brighter stars.  What you see is not necessarily what you are getting and visa versa.