Seeing, Resolving Power and the rest.......

[gorann]

I am running a triple-rig with two RASA8 (f/2) and a small Samyang 135 (f/2) sitting on top. So all at f/2. There is quite a striking difference in resolution. The Samyang with 67.5 mm aperture does not even get close to the resolving power of a RASA8 with 200 mm aperture. However, the Samyang of course wins when it comes to field of view with its 135 mm focal length.

Edited November 16, 2022 by gorann

[vlaiv]

31 minutes ago, scotty38 said:

Thanks and yes maybe not the best question but you did sum it up with "We often say things like "You need to guide at half of our sampling rate"" as I think that's the basis of what I was asking.

Given you essentially said guide as best as you can are you saying that due to seeing etc etc that even with a setup that "could" deliver 0.48" even if I can still only guide at 0.8" with binning in software later I could still make good use of the kit? Maybe not as good as it could be but not a complete and utter waste of time?????

I'm purposely trying to give a bad example to try and make my point by the way 🙂

It's not bad example - it is realistic example. We can never have exact conditions what we want, so there will be variability even if your guiding is spot on and always the same.

What you really want to know is whether you'll be able to make good use of the kit and under which circumstances?

Well, to answer that - maybe it would be best to put it in perspective and compare it to some other piece of kit? Or at least set certain goal?

Maybe you want to image fast and don't really care about resolution?

In that case, just determine resolution that you want to image at, and just make sure you are not over sampling in majority of conditions.

In most cases with such aperture, you won't be oversampling with 2"/px, so you'll need to bin quite a bit, but you'll get very fast system (254*254*2*2 = 258064 - just compare that with any of the "Speed" numbers above to get the idea). However, in order to do that - and still get sensible field size for 2"/px - you'll need very large sensor. Maybe you want to image with at least HD pixel count (1920x1080px) as final image, and with modern cameras that have say 3.76 pixel size you need to bin at least x5 - that means you need almost 10000 x 5000px camera - or ASI6200 (and field flattener to be able to properly illuminate full frame).

Maybe you don't want that, maybe you want some balance - and want to image galaxies at 1.5"/px. Then you need bin x4 (or perhaps bin x3 if you use some sort of reducer) and you can get away with APS-C sized sensor or maybe even 4/3 if you want to image smaller galaxies.

In this case, however, if you expect your mount to perform only to 0.8" RMS, you better only image on nights that are 1.5" FWHM seeing (or very good seeing nights).

If you get your guiding down to 0.5" RMS - then you'll be ok even at 2" FWHM seeing for that resolution.

Even if you don't achieve these - you'll get image, don't get me wrong and you'll get it "fast" (compared to other setups) - but it will be a bit more soft than what is possible at given resolution (now, when I say a bit more soft - that is what I really mean. Most people produce much softer images because they over sample by fair a bit more than this - and this softness is only visible at 100% zoom - it might not be visible when image is looked at full screen if image pixel count is bigger than monitor pixel count).

If you however want to be serious about high resolution work and want to attempt to really capture image at say 1.15" FWHM (this is just number when you bin x3 above pixel size without reducer but it is close to what can be achieved in very good conditions), you'll need to guide very good and have very good seeing (and it will take longer as you are now using "slower" setup than either of cases above because you'll be working with 254*254*1.15*1.15 = ~85000).

You'll need to guide at 0.4" RMS in 1.5" FWHM seeing.

If your seeing is say 2" FWHM you won't be able to achieve that resolution with essentially perfect mount (one that has 0 RMS). When I say you won't be able to achieve - I simply mean in mathematical terms of close to ideal sampling - it is probably best read as "your image won't be as sharp as it is possible given 1.15"/px sampling rate, but a bit softer when viewed at 100%".

Hope this explains things a bit better?

 

 

[scotty38]

Thanks @vlaiv yes that helps it really does and just what I was looking for, much appreciated!

[vlaiv]

By the way - if you look at above numbers - you'll see where guide at 1/2 of imaging resolution rule of the thumb comes from.

You can achieve 1.5"/px with 0.8" RMS guiding

You can achieve 1.15"/px with 0.4" RMS guiding

In general range of sampling rates and usual conditions (here I mean 1"/px to 3"/px and 1.5" to 2.5" FWHM seeing) - it turns out that guide RMS is in ball park of half of resolution, but lower RMS is always better.

[Adam J]

On 14/11/2022 at 14:44, wuthton said:

Colour me confused, my apologies if I've misread your post but isn't the LZOS 130/ASI183 (F6, 0.63"/px) significantly faster than the 10" RC/ASI16200 (F8, 0.38"/px), regardless of the aperture?

No you are forgetting about pixel size in your speed comparison. Pixel size can be altered by using binning. 

[wuthton]

On 16/11/2022 at 13:51, Adam J said:

No you are forgetting about pixel size in your speed comparison. Pixel size can be altered by using binning. 

No, I was wrong at native pixel size and binning just increased my humiliation.

I'd like to say a thank you to the contributors on this thread, I’ve never seen a comparison of different systems with the same field of view, it’s been fascinating. But with that said, I own an 8” RC and for a moment I was looking at it in a different light but the bottom line is… it’s still slow when not binned.
 

[dan_adi]

On 14/11/2022 at 18:06, CraigT82 said:

Actually, no. If we take 'faster' as meaning taking less time to acheive a specific SNR in an image.

Very useful calculator here by @dan_adi.  Plugging in the scopes outlined above and subbing in the 533 in place of the 6200 as it's not in the app database (same pixel size) you can see the 10" RC setup is faster than the 5" frac. Have to make some assumptions about reflectivity/light throughput.

 

http://clearskies.go.ro:8501/

Hello, I've expanded the webbapp quite a bit since then. I've rearranged the graphical user interface a bit, and I added stuff I usually need, like:

1. Searching the Simbad database for stars, extract info and display as table

2. Constructing 3D maps of those stars and also their 2D and 3D orbits

3. Space time diagrams, so I can understand proper distances, comoving distances and light travel time, for a given redshift

3. Searching Simbad database for galaxies, extract info, compute surface brightness ... since it's not so easy to find surfaceB for galaxies

Below are some pics. I haven't had the time to upload the new app to the server .... but I hope I'll find the time soon.