Hyperstar for photometry?

I’m curious, why do you think it would be any worse than anything else? Theres possibly significant vignetting at the field edge and you ideally want a sensor where the pixels arent grossly undersampled, but otherwise its business as usual. 🙂

3 arcsec at 290mm focal length is just 4.2um so with most cameras you are likely to be undersampled unless you defocus aren’t you ?

Robin

What is your typical star size with the 70mm f6 currently?  3 arcsec is only 6.1um with your existing setup so you are potentially undersampled already so there would be no advantage going to a lower focal length to beat the seeing.  (I think potential gain is only marginal anyway as all you do by moving to a shorter focal length  is reduce the camera noise contribution, the star and sky background counts stay the same).

The extra aperture will gain you some photons though  (Area of C6 is 608 cm2 allowing for the central obstruction compared with 154 cm2  so ~4x or ~1.5 magnitudes)

Robin

Hi James,

The signal to noise ratio will be affected by both the number of photons, and how those photons are spread across the detector pixels (amongst other things). However, the SNR by itself is not everything, for example you also need to keep systematic errors to a minimum.

I think you already know this, but it is useful to state it anyway. Assuming a constant aperture, then the number of photons from a star will remain the same at different focal ratios. The focal ratio will then only affect the size of the star image and so how many pixels those photons will fall onto.

Taking an extreme case where the focal ratio is sufficiently short that the photons from an individual star fall onto a single pixel. In this scenario the signal from the star on that pixel will be at its maximum possible value and it will be as under sampled as it is possible to be. While this might sound good, it means you are relying on a single pixel to tell you everything about the star. While flat fields and dark frames remove the majority of pixel to pixel variations, then do not remove it all, and you will also get random noise. By spreading the photons from the star across several pixels you help to ‘smooth’ out this variation, and particularly any systematic differences from pixel to pixel. For example one technique to improve photometry is to defocus star images slightly so that the photons are spread across a larger number of pixels.

Hence for photometry, getting the sampling right should give you the best results. In some cases this might even mean oversampling (more pixels per star) though I have never tested this myself.

A couple of other point to consider:

• If you use comparison stars across a wide field of view then applying transformations to your measurements becomes more important as you are looking at stars passing through a different length of atmosphere.
• As you go fainter so there are more stars and so crowding of fields can become a problem. In this scenario you want a larger image scale so you can separate stars. Though of course you also need better seeing to do this properly.

This is not to say don’t go for a Hyperstar, but if you do then I would advise an imaging chip with small pixels so you don’t end up under sampling.

Cheers,

Andy

The Orion 8in f/3.9 astrograph (FL 800mm) is less than 500 pounds. I think with a Baader coma corrector it should be a good fit. This combo is on my wish list. The reviews that I have read have been very complimentary.

Yep sorry, factor of 4 out but the ratio (and hence the increased counts) is the same.  Changing the magnification (ie focal length) changes the counts per pixel (both star and sky background counts) but makes no different to the counts in the aperture, either the star counts or the sky background counts. The only difference is if you spread the light over more pixelsthan you need to, you increase the camera noise contribution. In practise this is only the thermal noise, not the read noise as you can compensate for this (in CCD cameras) by binning pixels.  Thermal noise is very low in modern CCD, particularly at typical exposure times used for photometry so the net effect is that changing focal length does not improve photometry SNR, only increasing the aperture does this   

Robin

added replies to #9 and #11 above

(Note to webmaster – we need a quote button so we can keep to a linear thread and not lose replies back up the thread)

Robin

Hello James,

>If anyone can recommend another telescope <

A Sky-Watcher Explorer 130P-DS OTA might be worth considering. They get good reviews and at £169 (FLO price) it’s not a financial disaster if things don’t work out…..

Regards,

Martin