How to focus with Ha Filter

[Stub Mandrel]

Not quite an idle question...

As Ha filters require exposures of minutes rather than seconds, how do people focus when using one? And surely with a Bahtinov mask you will just get tiny dots rather than rainbow streaks?

[Astrosurf]

What filter are you using? I hear that the smaller the wavelength the less easy it is to see stars. I can't remember what mine is but I have no problem seeing stars with my Atik and its software. They're very bright. I also didn't have problems focusing when doing Ha with my DSLR.

I'm sure someone with more knowledge will be along soon!

Alexxx

[cfpendock]

I use a Baader 7nm Ha filter, and I know exactly what you mean.  Focussing on dim stars can take a long time, but there is always the possibility to slew to a nearby bright star for the focus.  My other filters are also Baader are claimed to be par focal.  In fact according to tests I carried out using Focusmax at 530mm focal length there is no difference between the Luminance filter and Ha, so now I focus using Lum and then switch back to Ha.  But you need to confirm this with your own filters.

Chris

[johnrt]

If your filters aren't parfical, like mine - I use an Astrodon Ha & Baader LRGB, find yourself the biggest brightest star you can to focus on, then slew to your target ?

[Stub Mandrel]

Thanks all - I will be screwing the filter onto the end of my coma corrector for use with DSLR, filter wheels are far, far away.

I already focus on the brightest star so I should be able to manage then, I was worried it might be 1-minute exposure, tweak and repeat...

[Gina]

I use an Astrodon 3nm Ha filter and with SharpCap and the ASI1600MM-Cool camera I use an exposure of a few seconds and find a bright star and make that as small as possible using a zoom of 200 to 400 to see individual pixels.  Seeing makes the stars wobble about and change in size so it needs watching for several exposures to find the best setting.  I guess I could be cleverer and use FWHM in one of the apps that support it - such as APT...

[SkyBound]

3 hours ago, Gina said:

I use an Astrodon 3nm Ha filter and with SharpCap and the ASI1600MM-Cool camera I use an exposure of a few seconds and find a bright star and make that as small as possible using a zoom of 200 to 400 to see individual pixels.  Seeing makes the stars wobble about and change in size so it needs watching for several exposures to find the best setting.  I guess I could be cleverer and use FWHM in one of the apps that support it - such as APT...

Do you use sharpcap for capturing images too, or just for focusing.... ? As it's not really known for long exposure work

[Gina]

Just for focussing and framing - I use APT for capturing images.

[Stub Mandrel]

I thought Sharpcap did FWHM?

[Gina]

Maybe it does   Somewhere...

[SonnyE]

Focusing has been one of the very hardest things for me...

I've been focusing by getting close with my Badder Sky and Moonglow filter under my Bahtinov Mask, then turning my filter wheel to my Badder HA7nm filter and ramping up in time and magnification (Zooming in) to sharpen the focus. But anytime I change my filter wheels position, it of course requires focusing checks.

Once I get as sharp as I think I can at ~ 300-400%, I will try my imaging. I image by ramping up my exposure time, with either of my cameras (CCD, or DSLR). I'm usually working with Nebulae. It's what I like to image.

A friend explained his technique of zooming in for focusing and it has helped me greatly with mine. If I get as good as I can zoomed in, then back out at 100% seems to look pretty good.

I cropped the following examples:

Hope this helps.

[Pompey Monkey]

1 hour ago, SonnyE said:

Focusing has been one of the very hardest things for me...

I've been focusing by getting close with my Badder Sky and Moonglow filter under my Bahtinov Mask, then turning my filter wheel to my Badder HA7nm filter and ramping up in time and magnification (Zooming in) to sharpen the focus. But anytime I change my filter wheels position, it of course requires focusing checks.

Once I get as sharp as I think I can at ~ 300-400%, I will try my imaging. I image by ramping up my exposure time, with either of my cameras (CCD, or DSLR). I'm usually working with Nebulae. It's what I like to image.

A friend explained his technique of zooming in for focusing and it has helped me greatly with mine. If I get as good as I can zoomed in, then back out at 100% seems to look pretty good.

I cropped the following examples:

Hope this helps.

Both of those examples are a bit off-focus: The center spikes need to be further up in both. Exactly (or as close as you can get) focus has the center spikes slightly "staggered" up on one side, down on the other. It's hard to describe, but many have noticed this effect.

Have you tried capture software that incorporates the Bahtinov Grabber routine? I use APT, but there are others available

@Stub Mandrel: It really is worth attaching a computer to your camera, even if it's for focusing alone!

[Stub Mandrel]

Those photos are really useful in helping me know what to expect.

[SonnyE]

2 hours ago, Stub Mandrel said:

Those photos are really useful in helping me know what to expect.

Then I hit the mark, Neil.

My point was to demonstrate how it could be done. Not to profess perfection.