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Hyperstar for photometry?

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jamestscreech's picture
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Hyperstar for photometry?

I'm thinking of upgrading my variable star photometry setup. One of the options I'm considering is a C6 + Hyperstar, with an aperture of 150mm and focal length of 290mm I think I'll be able to get to much fainter stars than with my current ED70 f6 scope.

Has anyone tried photometry with a hypostar system? If so how well did it work?

James

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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. :)

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As seeing will effectively

As seeing will effectively make stars extended objects a low focal ratio should get me fainter stars without very long exposures. Combined with the larger apature to give me more photons and better signal to noise ratio, I thought a hyperstar would be a good choice. However I say a review that said they weren’t suitable for photometry, but it gave no explanation as to why, and I couldn’t think of one. To me it looked like a win, win situaction. How bad would undersampling need to be before it is an issue?

James

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Photometry

With UK seeing being of the order of 3 arc secs its traditional to have pixels half that ie 1.5 arc sec across. The main worry I would have would be the vignetting, but if the flats are good and the guiding good then I don't see it as a serious problem. I  use a Celestron RASA and like the images that provides. Used with a Trius 694 it gives good clean results.

The only thing I have heard against hyperstars is the fun and games of collimating them,  but for photometry rather that pretty pictures thats not a huge issue.

Do let us see some results...

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My current camera on a C6

My current camera on a C6 Hyperstar will give 2.7"/pixel though I might also get a new camera and so this may change to 1.7"/pixel which would be better. My current system (ED70) gives 1.9"/pixel, so is a little under sampled.

This is all only a thought at the moment I need to see what the finances will allow.

Robin Leadbeater's picture
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3 arcsec at 290mm focal length

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

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focal reducer ?

Would the C6 with a focal reducer to f6.3 perhaps be a better bet? (3 arcsec at 950mm focal length = 13.8um)

Robin

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current star size ?

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

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I typically get 2.5" - 3"

I typically get 2.5" - 3" FWHM on my images using a 3.8um pixel size camera (ASI1600MM-C). I only use the middle quarter (area) of the camera due to field curvature.

James 

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undersampled

Then  you are currently slightly undersampled already and would not want to go any shorter in focal length. Undersampling in photometry is much less desirable than oversampling.

Robin

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I think those area

I think those area calculations are a bit out, looks like you did pi * D * D instead of pi * r * r.

However, if the seeing is making stars effectively extended objects surly the gain in magnitude will be related to the change in focal ratio not aperture, and the aperture increase will improve the signal to noise ratio as more photons are collected in total.

James

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relative area

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

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Aperture vs Focal Ration and other thoughts

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

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If anyone can recommend another telscope

If anyone can recommend another telescope for photometry with an ASI1600MM-C camera (3.8um pixels) ideally with all the required accessories (excluding mount) totaling less than £1000, though I might be able to go up a little more. I would prefer something with a focal length of 800mm or less to give me a wide field for selecting comparison stars. It will also need to get to fainter stars than my current 70mm f6 scope without needing longer exposures. Also it needs to be light enough for an AVX mount.

I'm considering the C6 + Hyperstar, Bresser AR102XS/460, StarTravel 120 (with upgraded focuser) & 150mm f5 Newtonian.

James

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Scope

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.

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I did consider the SkyWatcher

I did consider the SkyWatcher version but felt that the weight might be too much for the mount. It's a shame SW stopped selling the CF version as it was lighter.

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replies to #9 and #11 above

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

Robin Leadbeater's picture
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calculator

Michael Richmond's on line calculator can be used to estimate the performance of a given setup for example

http://spiff.rit.edu/richmond/signal.shtml

Robin

Martin Mobberley's picture
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Hyperstar for photometry

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