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peroni

Ha focus problem?

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I'm taking some photos of NGC7000 tonight using an Ha filter.

Seeing isn't great but I'm not sure if I have the focus a bit off.

I focussed using Bahtinov mask without filter, then move filter wheel round carefully to Ha

post-15911-0-13708300-1449516896.pngpost-15911-0-38292900-1449516894.png

Any thoughts? 

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Looks a bit out, can think of 2 possibilities:

The initial focus was I assume in white light so the focal plane for that - likely the green region - and the Ha will be different. The lens is a doublet and there are reports of CA being present.

Next the filter has a thickness and that will alter the focal length by either half the filter thickness of a third of the filter thickness. Think it is 1/3 but not overly sure - Olly knows which of the 2 options, I can just recall it is one or the other.

One option is to have a glass disk in the wheel that is the same thickness of the real filters, preferably the same refractive index but that may be a little difficult to get, and to focus with this in position. Better still is to focus with the filter in and so use the Ha wavelength.

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My thoughts are the focus is a long way out... ;-)

It is a problem with Ha filters because everything is so much dimmer than using the Luminance filter (or no filter). You have two choices - either focus using the filter (and CCD camera) or calculate in advance what the focus offset is from another filter that offers a brighter image (say, Luminance or Red for example). You cannot expect the scope to remain in focus if you focus with no filter, or even another filter supposedly 'parfocal' with it, you must either use the Ha filter itself or calculate the offset.

For myself I use the actual filters (including Ha, OIII and SII) and use 15 second exposures. I also use an autofocuser too but that isn't required if you are prepared to sit with the scope and turn the knob manually whilst observing the FWHM values of a particular star. Definitely consider an autofocuser though, it makes for a much easier life ;-)

ChrisH

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I think the focal point alters when a filter is used, my filter wheel needed 1mm extra spacing when installed with filters.......

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You can't expect your Ha to be parfocal with other filters. It will need to be focused individually. Whle you might be able to measure the offset from a more transparent filter and re-use that offset I have never done that. I focus the filter as it is being used that night. Ha can be hard to focus because it holds down the stars very effectively so you can either slew to a bright star at a similar elevation (elevation/altitude affects focus) or focus on a star in the field if possible. This may need long subs. It is often impossible to use a Bahtinov in Ha because you don't get enough signal so using FWHM is the answer provided your capture software offers that.

Olly

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I found that a bright star and 2x bin, along with a 5 second exposure always gave me enough of a view to use the Ha filter with the bahtinov mask.

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I focussed using Bahtinov mask without filter, then move filter wheel round carefully to Ha

Any thoughts? 

Indeed, there lies your problem, you need to focus with the filter in place!

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Thanks to everyone for your input.

I've had to move back to using a Bahtinov mask as my FWHM in APT broke when I moved over to windows 10. Lots of troubles this season with new OS and remembering the setup / config I had on Win 7  :huh:

The slot I have in my filter wheel for focus is empty. So no filter at all, not even a piece of glass or and IR filter.

I hadn't thought of using a bright star and then slewing over to target. I'll give that a go next.

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Went back out. Moved to a bright star (Vega). Stuck the Bahtinov mask on with Ha filter in place. Hey presto! Totally out of focus.

Off I go to update my workflow. Again  :grin:

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And here we have an Ha filter in focus  :cool:

post-15911-0-12019700-1449523044.png

Thanks everyone for setting me off in the right direction. Again!

Edited by peroni
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Thanks to everyone for your input.

I've had to move back to using a Bahtinov mask as my FWHM in APT broke when I moved over to windows 10. Lots of troubles this season with new OS and remembering the setup / config I had on Win 7  :huh:

The slot I have in my filter wheel for focus is empty. So no filter at all, not even a piece of glass or and IR filter.

I hadn't thought of using a bright star and then slewing over to target. I'll give that a go next.

Hi

Have you posted on the APT forum or spoken to Ivo/Yoddha? I'm sure he can help! There is a v2.98 beta available for registered users and I think v3.0 isn't far off!

Louise

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Hi

Have you posted on the APT forum or spoken to Ivo/Yoddha? I'm sure he can help! There is a v2.98 beta available for registered users and I think v3.0 isn't far off!

Louise

I'm running v2.80

I had a quick search and a look for an update but I'll need to buy it again as I purchased my version over a year ago. I went back to using the Bahtinov mask so my clear sky session wasn't wasted. 

I'll have a look at updating the software and searching/posting on the forums when the clouds roll in again.

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I'm running v2.80

I had a quick search and a look for an update but I'll need to buy it again as I purchased my version over a year ago. I went back to using the Bahtinov mask so my clear sky session wasn't wasted. 

I'll have a look at updating the software and searching/posting on the forums when the clouds roll in again.

Try your s/n - Ivo extended licences for 3 months or so until v3.0 is released :)

Louise

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I'm running v2.80

I had a quick search and a look for an update but I'll need to buy it again as I purchased my version over a year ago. I went back to using the Bahtinov mask so my clear sky session wasn't wasted. 

I'll have a look at updating the software and searching/posting on the forums when the clouds roll in again.

You have to be careful focussing on a bright star in APT as I discovered to my dismay the other night. If the star is too bright (Betelguese in my case) the histogram has peaks at both ends and the new auto-stretch cannot function correctly!.

It's not Ivo's fault, but it caused me no end of confusion at the time !! ;)

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You have to be careful focussing on a bright star in APT as I discovered to my dismay the other night. If the star is too bright (Betelguese in my case) the histogram has peaks at both ends and the new auto-stretch cannot function correctly!.

It's not Ivo's fault, but it caused me no end of confusion at the time !! ;)

yeah, you normally need to find a relatively dim star to focus on using fwhm/hfd and to fit with a nice peak without clipping. Of course, with an ha filter you need a fairly bright star to get a decent peak response :)

Louise

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With your ED 80, a nearby bright star (in this example, Deneb would be ideal), Ha filter, Bahtinov Mask and a 6 second binned 2 x 2 test shot should be ample for getting the focus nailed.

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In FWHM you have to be sure that your star is below saturation (65,000 counts in my case.) FWHM measures the width of the bell curve half way to its top. Once it's saturated you are decaptiating the bell curve so the 'Half Max' cannot be meaningfully defined.

I'm envious of those who can obtain a meaningful B mask image in 5 seconds in Bin 2. I can't, other than on a bright star! (3 nan Astrodon. The 7 nan Baader is easy enough.)

Olly

Edited by ollypenrice

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With your ED 80, a nearby bright star (in this example, Deneb would be ideal), Ha filter, Bahtinov Mask and a 6 second binned 2 x 2 test shot should be ample for getting the focus nailed.

I'm envious of those who can obtain a meaningful B mask image in 5 seconds in Bin 2. I can't, other than on a bright star! (3 nan Astrodon. The 7 nan Baader is easy enough.)

My filter is the 7nm Baader Ha. I managed to just about see a usable star in APT with the mask and filter in place and 5 secs at Bin 2.

I have to use the windows magnifier though. Not sure if it is normal but since I've been using my 460ex camera in APT the preview window displays a very small picture.

I'll have to RTFM before posting a query though. A quick look at manual has thrown up a few solutions to the problem  :rolleyes:

Edited by peroni

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