Image Scale and Guiding for round stars

[AlistairW]

Hello,

I went to this site http://www.astropix.com/wp/2011/03/23/image-scale/ with my scope etc ...

Image Scale per Pixel  P = ( 8 x S ) / (FL)

I have a Atik 383l and WO GT81 with 0.8 reducer, so ...

"S" = size of pixel in microns = 5.4um

FL (WO GT81 with 0.8 Reducer) = (478mm x 0.8) = 384.4mm or 15.13 inches

therefore

P = (8 x 5.4) / 15.13 = 2.86 arc seconds per pixel.

I also read (on a thread somewhere) that a rule of thumb is that if .. the Total Guiding RMS (in arc seconds) is less than the image scale (for the scope and camera) then I should be getting 'round stars'.

Is this true. (I am happy with my guiding, and I can achieve better RMS values than 2.86), but .... is striving for greater accuracy really worth it ?

Thanks

Alistair

 

 

[Droogie 2001]

I would interested in other peoples response to this question. My Arc Sec P/P is 3.17 and I have an average total RMS of around 0.75 Arc Seconds. Whilst this gives me round stars for say 5min exposures, if I stretch this to say 10mins I then start getting stars which are stretched. This is because of my guiding not being optimal, something I am aiming to improve.  So I am not sure what you are asking?

Obviously the calculations above have some value as a starting point but the longer you guide the more likely you will suffer from not having round stars, unless you have no issues at all with guiding?.

I would have thought an average RMS of 2.86 would be causing some issues for you?

Apologies if I have misinterpreted your question.

 

 

[AlistairW]

Good point, I am not sure there was an actual question :-) . For myself I also have a RMS for guiding of around 0.75. I can guide for around 10 minutes before stars losing shape. I guess that having relatively high arc second per pixel for my setup 'should' mean that I have more "room for error (?)" than say someone more interested in longer focal lengths. Maybe this is why the information I was told was an 80m refractor is a good initial staring point to get in to AP.

Thanks

[libraryman]

I think the general answer to this is that in a perfect world one would hope to achieve as low a rms as possible, because though the general point is true...re rms lower than the imaging resolution is good, it's not true to say it would give either round stars or small stars!

if the Dec rms matches RA rms ...yes round stars are probable, though a small difference will have no appreciable effect!

a larger than achievable rms will give bloated stars.

 

Ray