LRGB and Autofocus

[DaveS]

I've been thinking (Possibly overthinking) about autofocus within a LRGB regime. Up til now I've been focusing manually with a Bahtinov mask, so making a new focus for each filter. Now with autofocus I have initially been using the Red filter and measuring offsets for each other filter for the autofocus routine.

However, if one is planning a LRGB image, then the Luminance layer is going to be an average position between the other three, so will have a blur due to uncorrected chromatic aberrations, even with a triplet.

Therefore, if one is going to use the Luminance filter for autofocus (As I think I should), then is it necessary to use individual filter offsets for the other filters? For narrow band, where the L filters isn't used then I think offsets will be needed.

Help! I think I may be overthinking this.

[Laurin Dave]

I use SGPro and just focus on whatever filter I’m using (focusses on filter change and every 1 deg C..  ) takes a while with the CCD on narrowband but for LRGB I can’t see any time advantage using L (or another filter)and an offset..  

[Adreneline]

I have wasted so much time trying to get autofocus to work (in SGPro) that in the end I spent one night just establishing the best focus position with a BM.

With my Baader filters and a ZWO-EAF I got these focus positions:

OIII 28302   -50

G    28312   -40

B    28312   -40

SII  28322   -30

L    28352   0

R    28372   +20

Ha  28382   +30

The mean position is 28342 so close to L but far enough away from G and B (and OIII) to give bloated stars.

Adrian

[steppenwolf]

My system seems to work well for me, I have Astronomik LRG and B filters, 7nm (Baader) and 3nm (Astrodon) Ha filters, a 3nm (Astrodon) OIII filter and an 8nm (Baader) SII in my filter wheel - I ALWAYS focus with the L filter (even for NB images) and use offsets for ALL the other filters. I determined these offsets using a Bahtinov mask and simply noted each offset from the L. I haven't changed those offsets for several years.

[DaveS]

Thanks both.

ASA Sequence (NOT SGpro) insists on using just one of the filters for both plate solving and autofocus, then applies an offset (Which I have to determine) for the focus position of the other filters. A slight complication is that currently in my ZWO wheel I have Baader LRGB filters, which are 2 mm thick, and an Astrodon H-alpha which is 3 mm so there's a focus shift due to the wavelength plus a shift due to the increased optical thickness.

I think I will have to run some autofocus routines tomorrow night before any propper imaging.

[steppenwolf]

20 hours ago, DaveS said:

A slight complication is that currently in my ZWO wheel I have Baader LRGB filters, which are 2 mm thick, and an Astrodon H-alpha which is 3 mm so there's a focus shift due to the wavelength plus a shift due to the increased optical thickness.

I don't see that as a problem - focus position is focus position so using a single filter (in my case 'L') as the 'master' for both plate solving and focus with offsets for all the others will work fine irrespective of their physical makeup.

[DaveS]

OK, I take your point. I already have offsets from the Red filter, so I could recalculate them for the L.

[rubecula]

Steve, When you determining your filter offsets using a BM did you take any account of the temperature at the time?

[DaveS]

Oh, I did recalculate the offsets from the Luminance filter, which I set as the one for Sequence to use for plate solving and autofocus, and the results look to be fine. I may get autofocus results for the other filters, but not sure how much better they would be, if at all.

[jimjam11]

On 12/04/2020 at 20:14, Adreneline said:

I have wasted so much time trying to get autofocus to work (in SGPro) that in the end I spent one night just establishing the best focus position with a BM.

With my Baader filters and a ZWO-EAF I got these focus positions:

OIII 28302   -50

G    28312   -40

B    28312   -40

SII  28322   -30

L    28352   0

R    28372   +20

Ha  28382   +30

The mean position is 28342 so close to L but far enough away from G and B (and OIII) to give bloated stars.

Adrian

Once I got autofocus working in sgpro I ran the routine, noted the position and then interated through the filters. I did this three times and then calculated the average offset before confirming with a b mask and b grabber. It would have been easier to just use a b mask but I used this process as a way of testing autofocus was configured correctly and would work reliably with all filters. I generally let sgp focus with whatever filter it chooses at the time (dependent on sequence) and the offsets seem to work very well.

[Xplode]

On 12/04/2020 at 21:14, Adreneline said:

I have wasted so much time trying to get autofocus to work (in SGPro) that in the end I spent one night just establishing the best focus position with a BM.

With my Baader filters and a ZWO-EAF I got these focus positions:

OIII 28302   -50

G    28312   -40

B    28312   -40

SII  28322   -30

L    28352   0

R    28372   +20

Ha  28382   +30

The mean position is 28342 so close to L but far enough away from G and B (and OIII) to give bloated stars.

Adrian

Setting up autofocus in SGP nowadays is usually pretty easy, i'd love to help if you'd like

[Adreneline]

2 hours ago, Xplode said:

Setting up autofocus in SGP nowadays is usually pretty easy, i'd love to help if you'd like

A very kind offer Ole and one I will take you up on once I've got my ED80 system up and running again. My ED80 has a ZWO-EFW and HitecAstro focus system but at present no camera attached. I'm sure my failure to get SGPro to work is purely down to operator error so any guidance would be appreciated. I'll get back to you

Adrian

[Jkulin]

Dave, not sure what make filters you have, I use Chroma and when I first got them I spent a couple of nights checking and rechecking the focus on the LRGB and NB and they were all par-focal, apart from the green which was 20 units out over 11000 movements, enough to say I ignored the difference.

I always focus using my Lum, and I never had an issue with focus.

@RayD posted this video over a year ago of how to set up focusing with SGP and it has proved invaluable for me and once setup you will never look back: -

 

[DaveS]

My LRGB are Baader, and the offsets are only due to residual colour in the 'scope but the H-alpha is an Astrodon (I have the others waiting for some NB imaging) and is 3mm thick against the Baaders which are 2mm, so there is an optical path difference. Slight, but enough.