image guiding ratio

[velaproxima]

hi guys just wondering about the image guiding ratio .

i have calculated mine to be at  ,,image guiding ratio 1:5.36..

does this look about right ? 

[vlaiv]

Hi and welcome to SGL.

I don't really subscribe to notion of having certain ration between imaging and guiding resolutions.

In the end - one might end up with certain ration - but these two things should be considered separately.

For example - given your stats in screen shot above - you'll be imaging at 0.85"/px, In my view that is very high sampling rate. In 99.99% of cases you should not go below 1"/px in long exposure astrophotography.  In fact, with 6" scope - you should probably keep above 1.2"/px.

What mount do you have and how well it guides?

Guide resolution is more related to how well your mount guides and how accurate you want your centroid calculations to be rather than imaging resolution. Guiding at 4.58"/px is probably on the low side of guide resolutions.

Say you have mount that can be guided at 0.6" RMS total. By The way - that is poor guiding if you want to attempt resolutions below 1"/px - but otherwise quite decent for budget mount (like EQ6 type mount - in fact in order to get there - you'll need to tune EQ6). Anyway - in order to reliably measure that sort of RMS - you need your precision to be at least 1/3rd of that - so centroid precision needs to be at least 0.2". Centroid algorithms have precision to 1/16th of pixel - which means that pixel can be max 16 larger than 0.2". So we have 0.2 * 16 = 3.2"/px.

You need 3.2"/px or higher guide resolution if you want to reliably measure 0.6" RMS total.

You might be able to get reading of 0.6" RMS with 4.6"/px guide resolution - but you can't be sure if that is actual 0.6" or maybe 0.8" RMS or 1" RMS - as measurement itself is not precise enough.

If we go the other way - 4.6 / 16 =  ~0.86" RMS

So that guide resolution that you now have is good enough if your mount does not usually guide below 0.9" RMS. In turn - you don't want to image at 0.85"/px with such mount as you'll just get blur at that resolution. That sort of guide RMS is ok for 2"/px or above resolution (depending on seeing).

General rule of thumb is that your guide RMS should at least be half of intended imaging resolution of not less.

I'd recommend that you bin your images x2 and get OAG for RC6" type scope.

 

[velaproxima]

my mount is a skywatcher eqm35 pro

 

[vlaiv]

9 hours ago, velaproxima said:

my mount is a skywatcher eqm35 pro

 

In that case - don't worry much about guiding, your guiding setup is fine.

With that mount, I think you'll be happy with about 1" RMS or so. Don't think you'll be able to go much lower than that as it is lightweight mount that is not very precise - it has single step 0.28125" in its stepper motors.

Look into binning of your camera data. Not sure what camera are you using - but you can bin either in hardware (CCD) or software (CMOS usually, but CCD can be binned in software as well).

There is relationship between imaging resolution and achieved SNR in your image for given time and aperture. Higher resolution you image at - lower SNR you can achieve. This is because light is effectively spread over more pixels and image gets "dimmer" per exposure (similar to using higher power eyepieces  - image gets dimmer with increased magnification). Lower signal -> lower SNR.

At some point, you enter domain of empty resolution - meaning increased resolution won't help you capture additional detail because things are blurred out - due to seeing and mount and aperture of the telescope. In long exposure astrophotograpy it happens at much lower resolution than the scope is capable of in comparison to lucky type planetary imaging - because of mount performance and atmosphere.

In that case - increased resolution is not bringing you anything - and it is costing you SNR for your image. For this reason, it is not good to over sample your data, and if you over sample your data - then you can use binning to recover SNR.

How to know how much to bin? Well - that is rather easy - take your linear subs and measure average FWHM you get with your setup. Take FWHM in arc seconds and divide that with 1.6 to get sampling rate.

If for example your average FWHM over the course of the night is about 3" - you should sample at 3 / 1.6 = 1.875"/px. If your FWHM is 2.2" then you should sample at 2.2 / 1.6 = 1.375"/px. After you figure out - what your skies and setup is capable of - then it is just matter of "dialing" in as close as you can with binning - with keeping in mind that slight under sampling is better than over sampling as far as SNR is concerned.

You can bin x2 to get 1.7"/px - which is nice medium resolution to work with and requires FWHM of about 2.7", or you can bin x3 to get x2.55 - which is nice lower resolution that is good for FWHM of about 4".

Hope this helps.