Focusing with narrowband filters

[alcol620]

Hi folks

 

Advice required. Started my first imaging last night with HA, SII and OIII filters. Started by getting focus with the HA filter and did a run with the other two filters to check if they were parfocal. Still not sure.

With the focus set on the HA filter the OIII image had much smaller stars than the images taken with  the HA filter.

Is the difference in star size due to the OIII filter producing smaller stars or didn't I achieve focus with the HA filter??

Comments welcome.  Used focus aid in APT software to focus the HA filter.

Thanks

Alec

[DaveS]

Are you using a Bahtinov mask to determine focus?

Oh, sorry I see you used focus aid, but I'd still use a Bahtinov, tells it like it is, and which way you're out.

[alcol620]

Got one for the 8SE but not the 80ED

Saw this list 

1. By Eye
2. Magnifier / Right-Angle Finder
3. Digital Zoom Trial and Error
4. Hartmann Mask
5. Diffraction Spikes
6. Bahtinov Mask
7. Star Trail Test
8. Hybrid Method - Star Trails with a Hartmann Mask
9. Autofocus
10. In-Camera Focus Indicator
11. Parfocalized Eyepiece
12. Groundglass and Magnifier
13. Hybrid Ronchi Screen
14. Knife Edge
15. Ronchi Screen
16. Live-View Real Time Video Display
17. Software Metrics

at http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM and software including FWHM which APT focus aid uses comes top of the list with the B Mask well down.

[ollypenrice]

The B mask is a great way to get very close very quickly and may be the only non-robotic way on some scopes, notably big refelctors. Once close I like FWHM on refractors. When I was imaging with a 14 inch ODK, FWHM was totally, 100%, useless. It gave values which danced about all over the place.

Olly

[alcol620]

Hi Olly

 

I'm using the 80ED DS Pro. I noticed last night that FWHM did dance about. I homed in on focus by taking images and stretching and viewing in Pixinsight for different focusing positions. Is the FWHM Eccentric tool in Pixinsight any good for judging best focus?

[DaveS]

Well, I started out with my CCD focusing using FWHM in AA5 but I had stars boiling / dancing all over the place, probably due to London's horrid seeing. I only started getting pinpoint stars when I went the B mask route. Also if you go through focus it might not be obvious from FWHM but will be clearly shown with a B mask, something I'm finding with my 80 f/4.4.

[Jessun]

14 hours ago, alcol620 said:

Is the difference in star size due to the OIII filter producing smaller stars or didn't I achieve focus with the HA filter??

Yes, that is my experience. Just less OIII emission. That said, by the time you swap filters it's probably time to refocus anyways.

/Jessun

[AndyUK]

+1 for Bahtinov mask here... but I also use it with Bahtinov grabber for tweaking in a few more microns. 

Having just moved to 3nm Astrodon's, I found them quite a bit more tricky to focus, even though using 3x3 binning and 10s exposures(!) until I remembered to a) change the gamma and b ) use ROI to get in closer .  However, just FYI, I was surprised how much tighter the stars were in OIII than Ha - Maybe it was seeing/transparency conditions(?) or maybe there's simply less OIII signal in the stars...(?)

[alcol620]

Thanks folks for helpful comments. B mask ordered for the 80ED.

 

I have one for the 8SE scope that I have, but I didn't get on with it when I used it 12 months ago. The images were always too faint and too small. Maybe now I understand imaging a bit better I should give it another try, especially after your positive comments.

 

Thanks

 

Alec

[alcol620]

A further question if I may.

When using APT or similar software to view the image from the B mask, can you use bin and/or stretch the image from the B mask to get a brighter view?

Thanks

Alec

[AndyUK]

Yes - I always use 3x3 bin and also stretch the image... (In Maxim I also set a "Region of Interest"(ROI) box which speeds up the downloads further )

[alcol620]

Thanks Andy will give this a try as soon as I receive the B mask

[alcol620]

By way of feedback. Received the B mask and had a practice the other night, and I must admit, once I got the hang of it the B mask was easy to use, much easier than watching the FWHM reading jumping about all over the place. The proof of how effective the B mask is will be in future images. No chance to take any yet as no clear skies here unfortunately.

 

Thanks for all your inputs