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Summer Triangle Challenge

Matching Astro camera to telescope

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Afternoon guys, just doing some research on which camera would be best for my scope.

I am using a WO Z73 (430mm) with a 600D at the moment. 

I have decided to get an ASIair Plus, so ZWO cameras will accompany it.

For my 430mm focal length, am I correct in thinking that this;


Would be a good match? If I'm not mistaken, the combination achieves 1.8 "/pixel, with between 1 & 2 being the goal to avoid under/over sampling?

I was looking at this originally;


But this achieves 2.22 "/pixel as it has a CCD Pixel size of 4.63 um 

So I am currently thinking about the 533mc Pro given the above?

I have a budget of about £1,000 for the camera, open to any other suggestions of cameras or other factors I should be thinking about.

Thanks for the help.



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It is becoming generally accepted that the online ccd suitability calculators currently available not give you good information with regard to recommended sampling rates.

You're fortunate in that you already have some images, so rather than go into too much theory, you can assess practically: look at some of the images you have already produced, measure FWHM, then divide that by 1.6 - this is your optimum sampling rate in arc secs per px

I have seen @vlaiv advise this time and time again, and if you want the maths behind it, he can explain it in great detail (I hope I got it right!!)


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I'm inclined to say - go with ASI294.

2.2"/px is closer to what you can realistically achieve with 72mm of aperture.

Do be careful with that sensor. Many people were complaining of difficult flat calibration. It looks that sensor is not quite linear at some gain settings, and it does not behave well with duo band filters like Optolong L-eXtreme / l-eNhance

Yes, advice to measure FWHM is a good one - that will give you baseline of what you can expect. Divide FWHM with 1.6 to get close to optimum sampling rate.

We can go into math of it, but maybe it is just simpler to give you an example?

Here is nice image found online that we will use as test sample.


Imagine that this is high resolution image without impact of atmosphere - it is sampled at 0.1"/px (although it is large image it is only 48" x 48" given that it is sampled at 0.1"/px)

Image blurred with 1.6" FWHM looks like this (remember 1.6" FWHM is actually 16px with this image scale):


What is sampling rate that is needed for image like this? It turns out that 1.6" FWHM / 1.6 = 1"/px - we need to sample with x10 less pixels than image has - so in reality, we only need this:


At this image scale - image again looks sharp. But how do we know if that is indeed what is needed to capture all the detail in above blurred version? Let's restore this small version to original size to see what it looks like - we will simply enlarge it:


We have basically the same image as above. Eyesight is really not best judge of the difference between two images - so we will perform actual difference of images (subtract one from another) - to see what we'll get:


Well - nothing, apart from some very faint ripples (which are consequence of resizing algorithm used - mostly due to edges) - there are no detail in difference - blank / gray.

Here is what happens when you under sample:


This was sampled at 1.5"/px instead of above 1"/px, so slight under sampling - but here, image starts to degrade - even visually - you can see the difference between original and resampled version - and difference now starts to show distinct ripples - now coming from contours / high contrast edges in the image (you can make out silhouette of bird and branch in difference image). 

If you over sample - like at 0.67"/px - then obviously there simply won't be any difference between the two images - like  this:


Point of choosing x1.6 factor is simply - lowest sampling rate where image does not degrade (or degrades below noise levels)

Back to original question - again, I feel that your images will have 2" FWHM or larger - you can measure that by measuring FWHM of your raw subs or stacks (still linear prior to any processing). In most cases, aperture that small will simply produce larger scopes FWHM as such small aperture starts to have significant impact (airy disk diameter of 72mm aperture is 3.56" without impact of seeing and guiding, so we can't really expect FWHM to be lower than 2" when seeing and guiding are added to the mix).



Edited by vlaiv
I simply wrote nonsense - now corrected :D
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Thank you for the responses guys, and the detailed reply, vlaiv!

Il try find a single sub soon and have a look!

Il go for the ASI 294.

Those two cameras seem to be the stand out OSC in my £1,000 price range.

Mono will be a later investment I think, what with British weather and all the rest of the new equipment, I think OSC will serve me better for a good while!

Hopefully be a bit of an upgrade from my old battered Canon 600D if nothing else! :D

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533 is a good colour camera. Very little noise, easy to calibrate. It's a square sensor of course, so some find that restrictive. 

I looked at both to go with a GT71, went with the 533 in the end

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