good focus? FWHM

[Daniel-K]

what sort of numbers should i be looking for>?

[swag72]

I focus using a Bahtinov mask some nights and other nights using FWHM. I think it's good to be able to use both ways, incase one stops working!!! Belt and braces approach!!

In Artemis I aim for a brightness of about 20-35k ADU, for me that is a mag 5 star when I am using narrowband filters. I have it set on a 3 second exposures and then start to play! There is no specific FWHM figure I have in mind, some nights I can get the lowest readout down below 1 and other nights I can't get it much below 2.5!! You will get a feel for when you are getting best focus and generally you'll go past it to then have to come back to it, if that makes sense.

If that HRF-1.21 is a FWHM reading then I'd be fine with that, as long as either side of that focus point I got a larger figure! It's also worth looking at brightness in conjunction with FWHM. If it is jumping wildly about then that indicates your focus isn't quite right. You are looking for as small a change in that figure as possible - A little tricky as it does move quite a bit. But again you'll soon know what is a small change in your setup.

Hope that helps.

[Daniel-K]

thanks sara

[Martin-Devon]

I do a similar method to Sara - I magnify the Artemis capture screen aboout 5X, then pick a small star to make sure it's not saturated on 2 second exposures, then run looping subs tweaking the focus. Sara explained nicely the process above. Most nights I can get to a FWMH value of below 1.0, on a few nights I have to settle for about 1.3, but so far I've rarely had nights when it's been higher. Poor seeing/conditions will definitely push your FWMH figures up. The initial focusing is done with the filter that I plan to start the imaging with, so usually H-alpha or Lum. I don't know why but I've never had much luck with a Bahtinov mask, even using the Bahtinov grabber software to supposedly get it spot on, - I get different focus values compared to the FWHM method, and when I looked at them head-to-head the FWMH subs were definitely better focussed. I'm sure it's just me though, since clearly others have a lot of success with the Bahitinov, maybe I should try another mask...

[Daniel-K]

thanks martin do you think im trying to focus on too bright of a star?

[Martin-Devon]

I'm not sure Daniel since the graph you showed is not the Atik Artemis readout that I'm famiiar with, but if the 19964 figure on there is the ADU, and assuming that on your set up full saturation is 65000 (which it is on most CCDs) then you would be just fine

[Daniel-K]

my full sat is 60000 thanks foe advice martin

[Cath]

The star looks VERY saturated to me, even though it's saying it's not?

[ollypenrice]

Pixel scales vary from setup to setup so you can't just compare numbers across setups. Mine go from less than one in the Tak to more like 2.8 in the 14 inch.

I think it's important to take longish subs though. I go for 3 seconds in Bin 1, sometimes longer in poor seeing.

Olly

[Cath]

Yup, listen to Olly .. all you can do is focus for minimum FWHM, absolute numbers you can't really use for comparison across different software programs etc. And an FWHM much below around 2.3 or so is not a good measure anyway, you really ort to use at least a good enough sampling rate to have stars at least 2 pixels across (preferably 3 or more).

[Ant]

I'd be happy with 1.2, as others have mentioned sometimes you can get lower other times you cannot. Two sessions ago I couldn't get below about 2.2. So just had to live with it.

cheers

ant

[ollypenrice]

I'd be happy with 1.2, as others have mentioned sometimes you can get lower other times you cannot. Two sessions ago I couldn't get below about 2.2. So just had to live with it.

cheers

ant

When seeing is bad (and you are using a mono CCD) shoot colour. Save the luminance for a stable night. Some softness in the colour has no importance at all.

Olly

[Black Knight]

When seeing is bad (and you are using a mono CCD) shoot colour. Save the luminance for a stable night. Some softness in the colour has no importance at all.

Olly

Good tip Olly