Impact on telescope focal length with a camera

[Atomic]

Hi all

Total newbie at guided imaging. 

I intend to purchase a second hand mono ccd or cmos camera and guide camera to have a go at longer exposure tracked imaging. I have been reading reviews on some cameras and am puzzled about a cameras suitability for different focal length/ratio of telescopes.  I note that many refer to short to moderate focal ratios, say f4 to f6.

Firstly, I dont understand why this is significant except that a telescope with a focal ratio of f10 would have a slightly dimmer image projected onto the sensor than say a F5 instrument. 

Why is this important?

Secondly what would be the effect of using a telescope with a focal ratio outside the recommended guidelines?

i have a 200mm f5 newt and  285mm f10 SCT here. EQ6 goto mount

Thanks

 

Martyn

[Elp]

Deep sky requires long total exposure time to get detail, or in this case signal. Just like with camera lenses a fast f ratio lens collects light "faster", thus exposing the camera sensor with more signal than a slow lens in the same given time.

As you need to take many photos the difference between a fast system and a slow system becomes wider over time. A slow system will also need better and more stable guiding, given the choice it makes more sense to use as fast a system you can accommodate.

The thing you have to look at when choosing a camera is whether it's pixel imaging scale closely matches your telescope resolving scale so you don't get oversampling or under sampling. I don't read too much into this though, I just do with what I have.

[Atomic]

Thanks for that.  I think that i need to try and understand the pixel imaging scale vs resolving scale. I understand over and under sampling but puzzled why this would have an effect when the sensor is simply collecting photons.. I appreciate i may have oversimplified that last sentence

 

[vlaiv]

51 minutes ago, Atomic said:

Thanks for that.  I think that i need to try and understand the pixel imaging scale vs resolving scale. I understand over and under sampling but puzzled why this would have an effect when the sensor is simply collecting photons.. I appreciate i may have oversimplified that last sentence

 

Essence of long exposure imaging is to attain certain level of SNR for your data.

You can't show the detail in the image if it is polluted by too much noise (all you'll see is noise rather than signal).

Photons that you mention being collected by sensor - are in fact collected by individual pixels and each pixel is like a bucket - it captures certain amount of photons.

More photons pixel captures - larger signal and larger signal - better SNR.

This is directly related to imaging scale. Say you have nebula that is covered by 100 pixels in one case or by 400 pixels in second case. That is the difference between say 4" F/5 and 4" F/10 telescopes with same camera mounted. Double the focal length - and you double resolution in both height and width - you quadruple amount of pixels needed to cover the object.

Aperture did not change. Target did not change - they both emit and collect same mount of photons in total. What has changed is how many pixels those photons are divided among.

In first case all photons are distributed to 100 pixels and in second case - same amount of photons is divided between 400 pixels.

In second case - each individual pixel gets 1/4 of photons compared to pixels in the first case.

Just by simply changing the focal length of instrument - you changed level of signal and hence SNR dramatically.

You say that you understand over and under sampling and that is the part of the story.

Over sampling is bad as it wastes SNR. Under sampling is not as bad and is needed for wide field work.

Matching focal length of instrument to camera has to do with making sure you are less likely to over sample and more likely to properly sample, so that you always get best SNR possible for detail captured.

There are other ways you can "adjust" things - like changing pixel size instead of focal length (or in combination) - but that is advanced stuff.

[The Lazy Astronomer]

To add vlaiv's post (with a real world example), I shoot with a 100mm refractor at ~1.7"/px, and based on my recorded fwhm in a stacked image, I achieve, on average, a resolution of 3 - 4" (this is with what I consider to be a typical suburban sky, and typical guiding of around 0.6 - 0.7' rms). This means, generally, I'm oversampling by around a factor of 1.5(ish), which reduces my SNR. But never fear!  With CMOS sensors, this cab be recovered by binning in poat processing.

[Atomic]

Thank you both for your comprehensive responses.  I think that I am starting to understand the limitations of the sensor and telescope combination. I'll need to read a bit more, I have ordered a copy of every pixel counts and I am sure that will help fill the gaps.  In the meantime, i need to get a camera.. It won't be state if the art, but it will get me started. 

Best wishes 

[DSviewer]

From my experience little that it is shows that the field of view is impacted look here http://astronomy.tools/calculators/ccd_suitability

[Atomic]

Thanks for that link. 

The calculator at the end gave me the assurances that I need. 

 

Really very helpful and bookmarked

[rnobleeddy]

I wouldn't worry about oversampling, as it's easy to work around by binning.

One important point to note is that it's not as if there is a wide range of pixel sizes available. I'm sure someone will correct me, but I don't remember more than a 2x range between the largest and smallest pixels on modern CMOS cameras.

So I'd personally identify my other requirements , such as price and sensor size, then work out what the best, modern cameras are that fit that, and then only at that point would I worry about optimizing the choice of pixel size.

Or what I'm really saying is don't buy an ancient CCD with  massive pixels just because it matches your scope. You will get  worse results than a modern camera!