Guide/image pixel ratio

[Anthonyexmouth]

At the moment I'm guiding at an image scale of 1.9ish and imaging at 1.5 .

Would I be better binning the guide to increase the ratio. 

Only just reading up on this so I might have misunderstood it.

[vlaiv]

I would leave it as is.

What do you mean by increasing the ratio? Ratio between guide scale and imaging scale? Why increase that one? It is better to have it lower rather than higher.

[Anthonyexmouth]

so a near 1:1 ratio is ok? whats the deal with the recommended 2-4:1 i keep reading about? 

[vlaiv]

I think you are reading about upper not lower limit.

Guide resolution is related to how well can star position be established from guide exposure. I have not done any research on that and go by figures I saw quoted on internet. Precision is star position is about 1/16 to 1/20 of guide resolution.

This means that if you have 2"/pixel guide resolution, best you can hope to establish true guide star position is about +/- 0.1". This is error in measurement due to coarseness of pixels. This has a consequence to precision of RMS in guiding, which is about x3 of that. So if your RMS is bigger than 0.3" in above case it will be "properly" determined (with small error). If you are getting RMS less than 0.3" it is very likely that it is not true value.

We have one more thing - recommended RMS is half of imaging resolution. Let's put all that together.

Let's say that your imaging resolution is X. Needed RMS is then X/2. Star position precision is then further 1/3 of that - X/6. Now we have those two mentioned figures 1/16 to 1/20, so for needed star position precision of X/6 you need guide resolution that is 16 to 20 times bigger, and that gives ~2.66*X to ~3.333*X

This is "explanation" for guide resolution being in 2-4:1 to imaging (we showed that it should be in range 2.66:1 to 3.33:1 but people probably round things up to 2-4:1 when giving recommendation).

Your ratio is better than "minimum needed", so I would leave it as is. If you are having trouble finding guide stars or getting good SNR on them then you can bin, because after bin you will have ratio of ~2.533 - and this is still within recommended range, so you will not be loosing precision in guiding due to low guiding resolution.

[Anthonyexmouth]

thats good to know. maybe i should stop reading for a while and just use and learn what i've got.