Ha Focus - How ?

[johnb]

Hi

Hope this is posted in the right area?

I now have a 7nm Ha filter and focusing is somewhat tricky without the filter I use a good bright star stick on my bahtinov mask and using the live view feature in the Canon software and the 10 x option I focus with no issues, stick on the Ha filter and there is virtually nothing to see, would eb interested to know how other folk get a good focus with the Ha filter (oh I do use Backyard Eos for capturing images and this has a focus thing using FHMW or something like that but I have never used it)

Kit is in my signature

Regards

John Berman

[todd8137]

Use the feature on BYE. It's simple use in the x10 on the cannon is so hard read bye help I use it for focus all the time turn the iso up to get focus and back dimage patimage pat

[IanL]

The FWHM focus in BYE is pretty easy to use (but don't use a focus mask if you are going to use this method). You just need to use frame and focus liveview mode, centre a star in the cross hairs and it should appear in the small zoom box at the top right. Make sure the whole star appears in the small box (i.e. not too near an edge). You just focus visually first to get the star reasonably small, then look at the FWHM number below the box. This will be going up and down quite a lot with the seeing, but it should hover around a particular number.

Now fine tune your focus until you get the smallest number you can. I find that making small tweaks to get the number smaller and then finding the point where it starts to increase works best. You can usually estimate fairly well what the smallest FWHM is for that star (depends on magnitude and current seeing). Just tweak the focus, wait for vibrations to stop then look at the number and wait a few seconds to estimate the actual number.

If you can't get a bright enough star with a filter on to use liveview mode, then you will need to do short exposures instead. I think BYE's FWHM only works with the liveview image, so you'd either have to keep snapping JPEGs and measuring the FWHM in an external package, or use a focus mask. Again in the frame and focus tab, at the bottom right you can set a high ISO and an exposure length (how long depends on the filter and the brightness of your stars). You can either just press the 'snap' button, look at the mask pattern, tweak focus, snap, etc. or I think it has a 'loop exposure' button at the bottom right too. This just keeps snapping one exposure after another automatically so you can concentrate on focussing.

I know it is a lot harder than focussing in liveview mode, but it's how you have to do it I'm afraid!

[Kbramley]

In APT (similar to Backyard EOS) I use the FWHM tool on a bright star while in live view , this seemed to work ok with 7nm Ha filter, although the brighter the star the easier it was .

Not sure how BY eos works but APT defaults to max brightness setting for live view which helps.

[IanL]

Oh and another tip, some (really expensive) focusers have a digital readout so you can find the same focus position again easily. Mine doesn't, but if you buy a set of digital calipers (about 15 quid, e.g. http://www.maplin.co.uk/electronic-digital-calliper-32198 ) you can reliably measure the distance from a point on the back of the focuser to a point on the front of the camera. Once you have found focus for a given filter/camera set up, just measure it and make a note somewhere. Next time out you can set the focus using the measurement, and whilst you may be slightly out due to temperature differences, you will be pretty close and only have to tweak a bit.

(This doesn't work for moving-mirror scopes like SCTs though; even if you have a secondary focuser, the fine focus position will vary depending on the gross focus due to the mirror position inside the scope, which you can't easily measure).

[Zakalwe]

Oh and another tip, some (really expensive) focusers have a digital readout so you can find the same focus position again easily. Mine doesn't, but if you buy a set of digital calipers (about 15 quid, e.g. http://www.maplin.co...-calliper-32198 ) you can reliably measure the distance from a point on the back of the focuser to a point on the front of the camera. Once you have found focus for a given filter/camera set up, just measure it and make a note somewhere. Next time out you can set the focus using the measurement, and whilst you may be slightly out due to temperature differences, you will be pretty close and only have to tweak a bit.

Or draw a pencil mark on the focuser tube to mark the same position

[ollypenrice]

Hi

Hope this is posted in the right area?

I now have a 7nm Ha filter and focusing is somewhat tricky without the filter I use a good bright star stick on my bahtinov mask and using the live view feature in the Canon software and the 10 x option I focus with no issues, stick on the Ha filter and there is virtually nothing to see, would eb interested to know how other folk get a good focus with the Ha filter (oh I do use Backyard Eos for capturing images and this has a focus thing using FHMW or something like that but I have never used it)

Kit is in my signature

Regards

John Berman

Start using it immediately!!

However, a lot is to do with your sub length. If you don't get anything visible in 2 seconds just try 10 seconds. The night is young!

Digital readouts or measured drawtube extensions? I very much doubt it. That's a silly William Optics gimmick. If these were going to work then your focus in an untouched telescope would be the same on the second night. Fat chance!! It does happen but not often. Focus is sensitive to altitude above the horizon, air temperature, scope temperature and Lord knows what else. Bahtinov for quick, FWHM for refinement, though it works best at shorter focal lengths. At long FLs you battle against the seeing and generally lose!

Olly

[Gina]

I'm now using FWHM in Artemis Capture with the 314L+. I find it brilliant I have no problem focussing Ha - manually ATM but with remote motor drive. It does fluctuate a fair bit with the seeing and you have to watch things for several seconds. At the same time I zoom in to the star image until I see the individual pixels and you can see the star get smaller as you get into focus. For instance, this evening after roughly focussing, I reduced the FWHM figure from around 3 down to around 1 via 4 as I went the wrong way. I've tried a Bahtinov mask and it's hopeless with Ha on such a weak image.

[IanL]

measured drawtube extensions

Didn't claim that it would get you back to focus, just that it would get you near to focus for a given set-up. I don't particularly want to be marking my equipment with a pencil or anything else/ Using a digital calliper avoids wasting time wondering if you are anywhere near focus or if there is some other problem when you don't get anything on the first exposure (lens cap, software settings, operator stupidity, etc.). Bear in mind that round here, the number of useful clear nights with no moon since the start of the year can be counted on the fingers of no hands, and not many more fingers needed since the start of winter, and you maybe would appreciate why not wasting time is such a big deal for me.

It a piece of kit I need anyway, very useful for all the DIY jobs I have on the go for one thing. For another it helped me to confirm my left-hand-only coma problems were due to the FR/LP Filter/Camera not being screwed together properly. Turns out the retaining ring in the filter holder wasn't screwed in properly at the factory, and now I am certain the whole thing it is as square as I can get it, even though I haven't been able to image since December.

[Gina]

I find digital callipers extremely useful.

[Luke]

I use a Bahtinov mask, maybe I should try the FWHM, I got a bit scared by the way the FWHM value goes up and down with the seeing, but maybe I just need to get used to it.

If I can't see the Bahtinov spikes clearly in the live view (things are a bit better with my new 1100D, thanks to its much higher ISO than the 450D) then I take an exposure of say 5 seconds and if the spikes aren't right, I adjust focus and photo again, and keep going until it's in focus. I keep an eye on the distance markings on the focuser and move it slightly in or out. I can see from the placings of the spikes whether I need to move focus in or out. A bit fiddly and not my favourite part of the setup. I would guess it takes me on average about 10 shots until I get it in focus.

Could be I should be using FWHM, but this is what I've been doing.

[ollypenrice]

Sorry, I'm all for a short cut to get near but I was distinguishing between this and obtaining fine focus which I saw as the OP's question. What I do is have a simple feeler guage from a margarine tub lid cut to the right length to fit in the space exposed by the drawtube. That's for one scope. The other has a simple mm scale printed on it which, again, gets close. I'm one hundred percent in favour of saving time. Noboby has anything like enough of that.

Olly

[Tim]

You can use your mask. Take a second or two exposure on a really bright star, one of the main constellation stars, using a high iso.

When you examine the picture you will see the bahtinov pattern displayed as dots, just adjust as normal.

I don't know if Bye let's you preview and stretch live images on a laptop? If so stretch it to make it brighter and you might get away with shorter exposures.

Cheers

Tim