Focusing techniques with filters

[Han Solo]

This may be an obvious question, please forgive me, I am new to all of this! It became very apparent to me very quickly that I need to refocus every time I change to a new filter. I am currently using a mono camera with a manual filter wheel and Baader LRGB filters. The issue I am having is I have no idea how to refocus without slewing to a bright star and using a B. Mask. To me, this makes sense, but how to I then subsequently slew back to my original target and get the framing to match the original subs taken with the first filter especially if I have manually framed up the image how I want using the Synscan controller?

Is there a better way of focusing?

Can I save my imaging asc. and dec. position to allow me to slew back to the target? this may be useful if imaging over multiple evenings too?

Thanks, James

[souls33k3r]

You don't have to focus on a bright star. Focusing on a bright star will get you close but you always fine tune it at the target. You don't have to slew to a different patch of the sky to refocus when filter changes. Use the zoom feature on your acquisition software on your target and you'll find one of the brightest starts amongst the bunch in the same frame that you can focus on. 

[Han Solo]

1 hour ago, souls33k3r said:

You don't have to focus on a bright star. Focusing on a bright star will get you close but you always fine tune it at the target. You don't have to slew to a different patch of the sky to refocus when filter changes. Use the zoom feature on your acquisition software on your target and you'll find one of the brightest starts amongst the bunch in the same frame that you can focus on. 

Thank you for your message. Oh right, so do you choose the brightest star in the field of view and use a bahtinov mask to set focus? Or Sharpcap’s FWHM focus assist? Or some other method?

[souls33k3r]

Yes that's it, choose the brightest star from the field of view. You might have to crank up the binning or exposure. 

[ollypenrice]

If you have to slew away you can do two things to get back on target. Once framed up you can save the position as a User Defined Object and then 'Go To' it again or you can note the framing's RA and Dec and drive back manually to those co-ordinates.

Unlike Souls I frequently find that I have no star suitable for a B-mask in my target FOV, especially in narrowband. However, I only use the B mask for an initial focus while aligning the mount. I then focus using the FWHM feature of my capture software. Full Width at Half Maximumum is a universally popular focusing routine available in most capture software. Over bright stars with saturated cores are not suitable so it works fine on faint ones.

Olly

[Han Solo]

58 minutes ago, ollypenrice said:

If you have to slew away you can do two things to get back on target. Once framed up you can save the position as a User Defined Object and then 'Go To' it again or you can note the framing's RA and Dec and drive back manually to those co-ordinates.

Unlike Souls I frequently find that I have no star suitable for a B-mask in my target FOV, especially in narrowband. However, I only use the B mask for an initial focus while aligning the mount. I then focus using the FWHM feature of my capture software. Full Width at Half Maximumum is a universally popular focusing routine available in most capture software. Over bright stars with saturated cores are not suitable so it works fine on faint ones.

Olly

Hi Olly, Thanks for your insight. I did wonder if I could save the positioning using the User Defined Object feature, but to be honest I haven't really explored the Synscan controller other than the basics to get me aligned and on to a target. Funny, I din't think to just note down the RA and Dec values, I guess I was expecting the need for a more sophisticated solution, but I really like the simplicity of this!

So far I have only been using the bahtinov mask to focus, but maybe next time I will try using the FWHM focusing assist more! 

[steppenwolf]

Olly’s method should work well but you will have to take a little bit of an ‘average guess’ to allow for the variations in FWHM due to the seeing.

[ollypenrice]

On 25/05/2019 at 19:07, steppenwolf said:

Olly’s method should work well but you will have to take a little bit of an ‘average guess’ to allow for the variations in FWHM due to the seeing.

Yes, a tweak based on eyeballing the perimiter stars will be needed. My method also allows you to find out whether or not you can still operate pen and paper...

lly

[fireballxl5]

Although I have a computer controlled motorised focuser I've found myself using the following process...

1. Find bright star and use B-mask to focus

2. Frame target and select guide star using OAG/PHD2

3. Note FWHM value of guide star and temperature reported from focuser

4. As temperature changes up and down, tweak focus in small steps to keep guide star FWHM similar/lower than initial value

Although I should persist with trying to get auto focus to work, this process seems to work well enough, though it does require regular monitoring as my air-spaced triplet is sensitive to temperature.

 

[alan4908]

If you like your sleep and accurate focusing then I would suggest you consider a move to automated focusing.  Seeing is a particularly important consideration when attempting focus. 

I currently:

1. Use filter offsets between my range of RGB and narrowband filters. These are pre-calculated by ACP and are used whenever I change filter. All of these are made with respect to the lum filter.

2. Use the free version of FocusMax for my automated focusing with the exception is that I let ACP choose the focus star.  I've programmed ACP to select the focus star such that it is between 6 and 9 and it's near the zenith.  FocusMax then works out the exposure by taking several exposures, working out the incoming flux and ensuring that the focus star is non-saturated, sufficiently bright and is on the linear portion of my camera. The reason I choose a focus star near the zenith is to minimize the effects of seeing, which will lead to a greater uncertainty in your focus. 

3. Automatically refocus about every 2 hours to allow for changes to seeing/temperature etc. 

The interesting thing about FocusMax is that it attempts to work out best focus from data gathered at near focus, this is again to minimize the effects of seeing.

Alan