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How to focus in Ha?

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Hi!

I'm nowadays trying to learn how to do narrowband imaging, instead of using a DSLR. I am using an Atik 383l+. And I experience challenges in focusing in Ha-channel, more so for some objects rather than others. And I do have a quite long focal length, 1000 mm, f/9. The focus modul has dificulties identifying reliable stars, and it often lands out of focus. So I am looking for advice on how to manage this. What is a good strategy? Increasing exposure time makes the focusing more vulnerable to guiding errors etc. Is binning a better idea, or does that degrade the stars so focusing are more imprecise? I've been playing with both strategies, not really finding a good solution. I have also tried full frame focusing, but no improvement there.

Any advice from the rest of you, more experienced with Ha imaging?

Magnus
5 years 1 week ago #36726

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Replied by Ron Kramer on topic How to focus in Ha?

I have 1216 FL at F8 - so similar. I love Ha - and love how tight the stars are. I've never focused in Ha. I always have focusers set to lum (or red?) pick one. It focuses then changes to Ha and shoots. You're filters need to be parafocal. (meaning same focus point).

If they aren't. You can set the offset of each filter in the software. (SGP does) and I saw it in Kstars too. In focus settings set Lum or other bright filter. Focus - then swap to Ha - (shoot a sub and check your stars?) If they are in focus you're lucky!

I had ZWO NB filters and they were not - it was a pain... I sold them and replaced them with Astrodons which are parfocal.
If it a shot and let us know!

You can focus in HA but will waste a lot of time. 2 minutes exposures? Up the gain and maybe 1 minute? still who wants to do that.
But you could at least try to compare to a lum filter focus and switch. Myself I've not figured out auto focus yet in Ekos.
I love SGP's auto focus. Always works well. It picks 50-100 stars or so and averages teh HFR of all those and compares to focus steps until it plots a nice "U" or V and best focus is at the bottom. I'll try auto focus in Ekos again tonight if clear.

I googled this up.
stargazerslounge.com/topic/299679-how-pa...lters-an-experiment/
AP Mach1 / CP4 APCC & PEMpro.
EXP SCI - ED152cf APO - Celestron 11" RASA - Stellarvue 80mm
Baader F2 HS NB filters, Lodestar X2 guide camera / OAG - ZWO 290mm mini
ZWO ASI1600MM Pro / ASI174M (solar) / ASI094MC
NEXDome, CLoudwatcher, AVX mount/ASIair and Stellarmate

The following user(s) said Thank You: Jasem Mutlaq
Last edit: 5 years 1 week ago by Ron Kramer.
5 years 1 week ago #36728

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Replied by Jasem Mutlaq on topic How to focus in Ha?

You don't focus in HA. You configure the filter manager settings to use LUM (or whatever you called it) filter as the Locked Filter. This way it always uses Lum for focusing before going back to HA.
5 years 1 week ago #36736

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Replied by Jarno Paananen on topic How to focus in Ha?

I have the same camera and similar focal length, though somewhat faster (1624mm f/8 reduced to 1090mm f/5.3) and haven't really had issues focusing Ha or other narrow band filters. I have Baader LRGB-NB set which is almost parfocal but not quite so I have small offsets defined as well, but usually just focus with the correct filter using 4s exposures for NB and 1-2s for LRGB. The camera has mechanical shutter so exposures should be fairly long in any case to avoid shutter shadow, but haven't had issues with drifting. I'm using OAG so guiding is disabled during focusing anyway as it just gets confused when stars go in and out of focus. I have a fixed setup and my polar alignment is quite good so it might be different in a mobile setup.
5 years 1 week ago #36739

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Replied by Radek Kaczorek on topic How to focus in Ha?

I have never had a problem with focusing in NB. I use 3-5s exposures for NB filters and it works very well (a bit slow though).
Using focus offset for various filters is a good idea (Ekos CCD tab, click an icon located right of filter wheel drop down). Get focus on L and determine offset using Bahtinov mask for your filters.
5 years 1 week ago #36764

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Replied by Magnus Larsson on topic How to focus in Ha?

Hi!

Thanks for your replies! For the moment, I don't have a filter wheel, just fixed filters, so I need to focus in Ha (filter wheel will come), but that should then be doable.

How about binning? Bad idea or could be useful? I cannot really see what it does to focusing. It does increas S7N ration, though.

Best,

Magnus
5 years 1 week ago #36767

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Replied by Radek Kaczorek on topic How to focus in Ha?

I don't know about the others but I use bin2 when focusing. Less noise and faster
5 years 1 week ago #36771

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Replied by AstroNerd on topic How to focus in Ha?


That will only work if your filters are very parfocal, and many are not...!! :)
5 years 1 week ago #36777

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Replied by Ron Kramer on topic How to focus in Ha?


No it works either way... you just have to set the focus difference factor for each filter. The "offset".
But he would need a FW. 199 for a 5 filter why not? I have a 8 in my dome and a 5 on my RASA. Looking for another 5 for my portable setup.
AP Mach1 / CP4 APCC & PEMpro.
EXP SCI - ED152cf APO - Celestron 11" RASA - Stellarvue 80mm
Baader F2 HS NB filters, Lodestar X2 guide camera / OAG - ZWO 290mm mini
ZWO ASI1600MM Pro / ASI174M (solar) / ASI094MC
NEXDome, CLoudwatcher, AVX mount/ASIair and Stellarmate

Last edit: 5 years 1 week ago by Ron Kramer.
5 years 1 week ago #36807

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Replied by Magnus Larsson on topic How to focus in Ha?

Yes, FW is on the top of my list. Next to a new kitchen... ;)

Magnus
5 years 1 week ago #36808

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Replied by Eric on topic How to focus in Ha?


That's an interesting point. Anyone knows how to prove mathematically that binning x2 results in the same focus quality as x1?
I'd expect focusing on images twice the resolution would give better results, but with low exposure, that intuition could be wrong.
I would go further and pretend that stacking a few frames while focusing should even improve the situation (but the focus module is not very helpful in verifying this).
Focusing is already eating large parts of imaging time in our processes, so it all depends on the requirement of the optical setup and it would be interesting to back this with theory.

That reminds me of the discussions over polar aligning setups accurately on a recent AIC show, very interesting.

Last edit: 5 years 6 days ago by Eric.
5 years 6 days ago #36822

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Replied by Chris Rowland on topic How to focus in Ha?

I don't have much mathematical insight but binning will reduce the noise significantly, 2x2 binning will reduce the variance by a factor of four. That has to be balanced against the reduction in resolution, is that only by a factor of two?
Another thought is that if multiple stars can be registered and summed that will also reduce the noise.
And we are going from a 2D star image to a 1D map of signal as a fuction of diameter. So plenty of options to get the noise down.

Another thing that could help is choosing things that are stars, not hot pixels, cosmic rays, galaxies, gas clouds etc. I wonder if plate solving could help to identify the objects that are stars? It would only have to be done once, a sample of stars identified as stars, ideally all of a similar brightness, and maybe their images combined to make a super star. Their astrometric positions could be used rather than the position on the image.

People always seem to get better results with refractors than obstructed scopes because the obstruction makes the star size difficult to determine, especially if it isn't already close to focus. It's difficult to realise how poor the 'vision' of the focus algorithm is.

Once you have a good focus position the hardware has to be mechanically very good to return the scope to the focus position that was determined.

A bit of a brain storm come ramble but maybe some ideas that could help.
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5 years 6 days ago #36831

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