Pixel size/focal length

[morls (Stephen)]

Hi,
I'm reading up on imaging theory, and was wondering about the optimal relationship between pixel size and focal length. I've come across the "pixel size x 5 = target focal length" equation, and found a calculator that gives me some numbers.
As I ponder different options, reading too much and getting lost in detail, my goal has become to be able to image at the native f-15 of my 180Mak.
With this in mind, I've attached the calculations for two different sensors. Disregarding field size and considering only pixel-size-to-f-ratio, does one look markedly more suited than the other, or is it really just splitting hairs?
Cheers
Stephen

[Camelopardalis (Dunk)]

Stephen, looking at the numbers it seems you’re comparing a 290 to a 178/183 (the pixel sizes are the clues).

The 290 will be better hands down. It’s more sensitive and gives higher frame rates, which are important for planetary.

For lunar, the 178 may well do a better (or at least, different) job as the Moon is so bright anyway, and the FOV is a fair bit larger.

[morls (Stephen)]

Hi Dunk,

Yes, the two cameras I'm considering are the ZWO ASI290MM and ASI178MM. I realise the fact that they are mono brings extra expense, but it's where I want to end up anyway so I might as well save for a while longer and do this properly.

Am I correct to think that, other considerations aside, a pixel size of around 3um is optimal for this scope (without barlow or reducer) when considering a camera for lunar/planetary?

[Startrek (Martin)]

I don’t know if your after the image scale calculation of your image train but here is mine based on my 8” f5 newt and Canon DSLR


Image Scale or Pixel Scale Calculation - Over Sampling and Under Sampling

P is Image scale in arc sec/pixel

Formula
P= ( pixel size in microns x 206.3 )/ focal length in mm

Main Imaging Scope
New 8” f5 Newtonian imaging telescope (focal length is 1000mm)
Canon 600D pixel size is 4.3 microns
Therefore image scale would be 4.3 x 206.3 / 1000 = 0.88 arc sec/pixel

Over sampling is < 1.0 arc sec / pixel
Under sampling is > 2.0 arc sec / pixel
In good seeing conditions 0.80 to 1.0 arc sec/ pixel is OK stars will be round and sharp
In average to poor seeing conditions 0.80 to 1.0 arc sec/ pixel will produce slightly fuzzy or bloated stars
DSLR’s are generally only useful in telescopes with focal lengths less than 1200mm or > 0.80 arc sec per pixel

Guide Scope
Orion 50mm Guide scope ( focal length 162mm)
ZWO ASI120MM-3 Guide camera pixel size is 3.75 microns
Therefore image scale would be 3.75 x 206.3 / 162 = 4.77 arc sec/pixel

Image ratio imaging camera to guide camera

4.77 / 0.88 = 5.4 x

Guiding Advice
Advice is you should be below 4x image scale from guide scope camera to main imaging camera to guide at an acceptable error

Hope this is of some relevance to your question?
Cheers

[Camelopardalis (Dunk)]

Originally Posted by morls

Hi Dunk,

Yes, the two cameras I'm considering are the ZWO ASI290MM and ASI178MM. I realise the fact that they are mono brings extra expense, but it's where I want to end up anyway so I might as well save for a while longer and do this properly.

Am I correct to think that, other considerations aside, a pixel size of around 3um is optimal for this scope (without barlow or reducer) when considering a camera for lunar/planetary?

Indeed, so 2.9 x 5 puts your scope pretty close to optimum

[morls (Stephen)]

Thanks Martin, that's very relevant.
The reason I'm considering the 178MM is because of the hope that it would be suitable for planetary, but also the larger FOV would let me capture galaxies as well. The two images I've attached are from an imaging toolbox site, and are both of NGC613. The larger FOV is the 178MM. It would be amazing to be able to get images like this. My question is, is this achievable in real-world conditions, which for me would be taking my 180Mak and NEQ6 Pro to a reasonably dark site?
I appreciate that this is not a wide-field setup, but maybe a little galaxy-hunter?

Thanks for the input so far.
Stephen

[Camelopardalis (Dunk)]

Tough call.

f/15 is hideously slow for faint fuzzy imaging

[morls (Stephen)]

Originally Posted by Camelopardalis

Tough call.
f/15 is hideously slow for faint fuzzy imaging

And waaaay beyond my abilities. The 290MM seems to be a good fit.

[Camelopardalis (Dunk)]

For comparison with DSO, your f/15 scope would take 14x longer to expose to the same level as a f/4 astrograph...

[morls (Stephen)]

Thanks Dunk. I'll stick to the (excellent) advice I've received here and read in other posts on this topic, and stick to planetary/lunar with this scope. Horses for Courses!

[Camelopardalis (Dunk)]

Scopes are like shoes...we need several of them for different purposes

Good luck!

[morls (Stephen)]

Thanks everyone for the great advice. I've learnt a lot from this thread, and have a much clearer understanding thanks to your input.
Although the 290MM would be the best planetary camera for this scope, the price is beyond me for the time being. (fiscal responsibility sucks ). However, the current sale at Bintel has prompted me to order an ASI120MM, and at around $300 less than the 290MM I can live with the slightly larger pixel size. I'm sure this will be excellent for my needs, and I'm planning to concentrate on imaging the moon so I can learn to use my gear without worrying too much about things like colour. Light and shade beckon!



Cheers
Stephen