Narrowband Focussing

[Thalestris24]

Hi all

As I've just started making tentative steps with narrowband (just Ha so far) So I'm looking for advice on how best to get focus. I'm hoping to get a filterwheel very soon but for my first go I just put the 7nm filter on the end of the coma corrector. I did manage to get focus but more by luck, I'm sure! I'm presuming you can't just switch from no filter (or lp filter) to NB and expect to still be in focus? I had the idea of getting a 35nm Ha to focus with then switching to the 7nm to image. Is that a reasonable thing to do, or is there a better way?

Thanks

Louise

[RikM]

I use a Bahtinov mask and Bahtinov grabber software to get focus. I usually use my last alignment star and 6 sec exposures and tweak it in from there. With OIII I have had to use 20-30sec exposures!

[ChrisLX200]

You are correct - adding or changing filters will affect focus so you really need to focus with it in place. NB filters result in very dark images as you found out, and SII and OIII can be even worse than the Ha. You're only option without an autofocuser is to use the brightest nearby star to focus with then move back to your target afterwards. That might be doable with a GOTO mount if you synch on the target after framing it in Luminance and before moving off target to the bright star for focusing. It would save a lot of angst to get equipped with an autofocuser though, I simply would not be without mine - and if it broke now I would stop imaging until I could replace it, I find it is that essential.

ChrisH

[Thalestris24]

You are correct - adding or changing filters will affect focus so you really need to focus with it in place. NB filters result in very dark images as you found out, and SII and OIII can be even worse than the Ha. You're only option without an autofocuser is to use the brightest nearby star to focus with then move back to your target afterwards. That might be doable with a GOTO mount if you synch on the target after framing it in Luminance and before moving off target to the bright star for focusing. It would save a lot of angst to get equipped with an autofocuser though, I simply would not be without mine - and if it broke now I would stop imaging until I could replace it, I find it is that essential.

ChrisH

Hi Chris

When you say 'autofocuser' I'm thinking you mean something more sophisticated (and more expensive!) than a simple electronic focuser? 

What about using the 35nm to let more light through?

Thanks

Louise

[carastro]

What camera are you using Louise?  You have a DSLR in your signature, a QHY8L and a mono rebel T3

I found trying to even see stars with an Ha filter on a OSC extremely difficult as it's very faint.  With a mono CCD less so.

The best way I have found (I do manual focus), is to use a bright star and focus on that.  If necessary I try to get close to focus without the Ha filter by using the space in my filter wheel so at least I stand a chance of "seeing it" with the ha filter.  I know you don't yet have a filter wheel, but if you are careful you might just get away with putting the filter on and off to get somewhere close to focus and then just tweak it with the filter in place.

I use a Bahtinov mask mostly.  

Carole 

[ChrisLX200]

Hi Chris

When you say 'autofocuser' I'm thinking you mean something more sophisticated (and more expensive!) than a simple electronic focuser? 

What about using the 35nm to let more light through?

Thanks

Louise

Yes I'm talking about a stepper motor based absolute position Automatic Focuser, with a PC-based controler. They aren't cheap but with a cost perhaps similar to that of a good eyepiece, more than worth the price if you are trying to image though. With an autofocuser you can focus on the actual target using dimmer stars with (say) a series of 10sec exposures. The critical focus point will be selected for you and the focuser moved to that position - no guessing needed. The ease with which you can repeat this process means you'll be able to focus more often as temperature changes and thus get far better results. (A general comment) but I really don't know why folk spend a fortune on expensive optics and then not get the best out of them by focusing it properly - and keeping it focused throughout.

Anyway, with a wide-band filter you begin to lose the advantages of using NB in the first place, you won't get the contrast you seek.

ChrisH

[Thalestris24]

Hi Carole

Um, possibly both cameras! But mainly the qhy8l at the moment as the mono 1100d is being repaired . I do plan using it with the Canon lens + filters when I get it back. I have a Bahtinov mask but never use it! I find it easier and better to use the APT fwhm aid. Having said that, in normal use the 150pds focuser is very stable and I don't have to readjust it very often But obviously switching between filters might be more demanding anyway.

I found it really frustrating to focus when I first tried the 7nm because of the long time lags - and I'm not the most patient of people these days! Bright stars are only an option if one happens to be visible. That was why I was thinking of using a 35nm Ha to help focus with - with the assumption it wouldn't change on switching to a 7nm? I have a feeling nobody has ever tried this?

Cheers

Louise

[Thalestris24]

Yes I'm talking about a stepper motor based absolute position Automatic Focuser, with a PC-based controler. They aren't cheap but with a cost perhaps similar to that of a good eyepiece, more than worth the price if you are trying to image though. With an autofocuser you can focus on the actual target using dimmer stars with (say) a series of 10sec exposures. The critical focus point will be selected for you and the focuser moved to that position - no guessing needed. The ease with which you can repeat this process means you'll be able to focus more often as temperature changes and thus get far better results. (A general comment) but I really don't know why folk spend a fortune on expensive optics and then not get the best out of them by focusing it properly - and keeping it focused throughout.

Anyway, with a wide-band filter you begin to lose the advantages of using NB in the first place, you won't get the contrast you seek.

ChrisH

Hi Chris

Can you recommend such an autofocuser (that I could use with the 150pds)?

Um, I was thinking of just using a 35nm just to focus with then switching to a 7nm to image.

Thanks

Louise

[carastro]

I found it really frustrating to focus when I first tried the 7nm because of the long time lags -

Have you tried binning x 3 while you are still trying to focus as this should speed up the download.  I use Artemis on my Atik cameras binned x 3 to frame and focus and this works a treat.  I did try APT once but found the "delay" time prolonged the ordeal, but at that time I was not so au fait with CCD imaging in APT.  I think there might be a frame and focus option you can use, hopefully you might be able to bin in this too.

Make sure you remember to take off the binning option before you start imaging though   

I tend to these days use Artemis to frame and focus as it's so easy in Artemis and then switch to APT for capture so I can dither, so I am using a bit of each (I know you won;t have the Artemis option).  Thanks for the FWHM tip in APT, I must take a look at that.  

I also think you can also stretch the image in APT to show the detail better when you are trying to frame and focus:

Open the Histogram panel and right-click to move the white point in left to see more of the faint detail.  this should also help with the stars.  

However not sure if all these features are available for DSLR imaging.  Ask Ivo the author he is very helpful. 

Carole 

[Earl]

Hi Chris

Can you recommend such an autofocuser (that I could use with the 150pds)?

Um, I was thinking of just using a 35nm just to focus with then switching to a 7nm to image.

Thanks

Louise

 Focusing with one filter then switching to another might not work as even filters which are stated as parfocal are not.

I use remote focusing as a) i like to control it all from indoors but b ) not touching the scope while focusing makes it easier and more accurate.

I have a Robofocus compatible stepper motor attached to my focuser and use a Lightphse control box for it. (http://www.cheadleastro.co.uk/)

I use the focus function in Nebulosity and aim for the lowest consistant number for my focus point, I use 5 second subs with all filters to do this.

Its not a cheap option, but once you have the equipment you can use it on any scope with the right mounting plate for your focuser.

There is a fair bit of potential for even greater control, once you go into setting up the focuser, I use SGP for imaging mostly now and it will automatically check focus triggered by various parameters, ie, every x subs, ever x drop in tempreture or every x miniutes. You can even have measure focus shifts for filter changes.

[Thalestris24]

Have you tried binning x 3 while you are still trying to focus as this should speed up the download.  I use Artemis on my Atik cameras binned x 3 to frame and focus and this works a treat.  I did try APT once but found the "delay" time prolonged the ordeal, but at that time I was not so au fait with CCD imaging in APT.  I think there might be a frame and focus option you can use, hopefully you might be able to bin in this too.

Make sure you remember to take off the binning option before you start imaging though   

I tend to these days use Artemis to frame and focus as it's so easy in Artemis and then switch to APT for capture so I can dither, so I am using a bit of each (I know you won;t have the Artemis option).  Thanks for the FWHM tip in APT, I must take a look at that.  

I also think you can also stretch the image in APT to show the detail better when you are trying to frame and focus:

Open the Histogram panel and right-click to move the white point in left to see more of the faint detail.  this should also help with the stars.  

However not sure if all these features are available for DSLR imaging.  Ask Ivo the author he is very helpful. 

Carole 

Hi Again

Yes, can bin 2x2 (default in liveview) or 4x4, but then the image gets a bit small for me to see with my old eyes. Mind you, I have to admit that I didn't actually use the APT focus aid when I tried the 7nm. I should have really! Yes, I always make use of the histogram pane. I'm not so concerned with the dslr side as I'll only be using it with the Canon lens so easier to manage, if you know what I mean. When I get the fw I'll try again with the bin 4x4 and focus aid

Thanks

Louise

[Thalestris24]

 Focusing with one filter then switching to another might not work as even filters which are stated as parfocal are not.

I use remote focusing as a) i like to control it all from indoors but b ) not touching the scope while focusing makes it easier and more accurate.

I have a Robofocus compatible stepper motor attached to my focuser and use a Lightphse control box for it. (http://www.cheadleastro.co.uk/)

I use the focus function in Nebulosity and aim for the lowest consistant number for my focus point, I use 5 second subs with all filters to do this.

Its not a cheap option, but once you have the equipment you can use it on any scope with the right mounting plate for your focuser.

There is a fair bit of potential for even greater control, once you go into setting up the focuser, I use SGP for imaging mostly now and it will automatically check focus triggered by various parameters, ie, every x subs, ever x drop in tempreture or every x miniutes. You can even have measure focus shifts for filter changes.

Hi Earl

Well, I'm indoors anyway! The Robofocus looks very nice but very expensive! I think maybe just a basic motorised focuser would do me if I can get one for my scope. I'll see how I get on with things once I actually get the fw - and the skies to try it! Weather's not looking good for the next 5 days at least APT appears to support focusers but it's all an unknown for me at the moment.

Thanks

Louise

[carastro]

Yes, can bin 2x2 (default in liveview) or 4x4, but then the image gets a bit small for me to see with my old eyes.

Ah, the image gets smaller in APT.  You can increase the size in artemis.  

Anyway, worth knowing for later at least.

Carole 

[Thalestris24]

Hi Carole

Maybe Yoddha/Ivo will improve on the displayed binned image size one day! In fact, I've just posted a request  He's also implemented half flux diameter (hfd) focussing which is supposed to be more robust than fwhm and will aid autofocusser support (but only in the full version, I think)   I don't think I've actually used the hfd yet! (I don't refocus very often...) He has an autofocusser beta, I think. However, I think it's aimed at dslr lenses with focus motors. There's a v2.60 on the way

There are already options for boosting the liveview for dim targets so I must try that out too. I forget things when I'm busy imaging!

Louise

[carastro]

HI Louise, 

I just took a look at APT with the DSLR which I haven't used for well over a year and it doesn't look like you can bin with DSLR which is a pity.  I'd have a word with Ivo though as I am sure there will be some way of improving the live view, there is certainly a tool which affects how many frames per second on the connect page.

I'll follow this thread to see whether you find a better solution.

Carole 

[Thalestris24]

Hi Carole

No you can't bin a dslr owing to the camera's internal processing. I'm really only concerned with using the qhy8l with the Ha filter and getting it focussed without too much trouble. It's not a problem with the Canon as I'll only be using that with the Canon zoom lens and adding single filters to the front of the lens. As I say, easier to manage.

Cheers

Louise

[RobH]

Just focus on a bright star then slew to your target.....easy as that. At the moment, my robofocus isn't working so I have to use a Bahtinov mask, but in all honesty it's hardly any effort, and a lot less effort than getting the darned robofocus program working again!

[Thalestris24]

Hi Rob

Ok - I'll give it a go when I can. Is Robofocus software problematic then?

Thanks

Louise

[carastro]

Hi Louise, well if it's the QHY8L you might be able to bin in APT with that.  There should also be a fast download function.  I used to own a QHY8L and used APT with it a few times, but not for over a year.  

Carole 

[Thalestris24]

Hi Carole

Yes I do bin the qhy8l and yes it does have a fast download option which is very handy

Louise

[RobH]

Hi Louise....no, robofocus is usually fine but I think I have a problem with my usb to serial interface, which will probably take a couple of hours to fix....sometimes I simply can't be bothered sitting with my head stuck in the computer when I could be out on the beach walking my dog though, hence not fixing it yet !!

[russellhq]

Hi Louise, I use a Bahtinov mask and the EXCAP software that came with the QHY8L to set my focus. I've found though that I have to use BIN4 and a bright star (currently Deneb, Vega & Altair make good candidates) with exposures of 10-20s to get a good image to inspect for focus.

As for autofocuser, I've been watching this thread with interest!

http://stargazerslounge.com/topic/218975-arduino-ascom-focuser-mark2/

[Thalestris24]

Hi Louise, I use a Bahtinov mask and the EXCAP software that came with the QHY8L to set my focus. I've found though that I have to use BIN4 and a bright star (currently Deneb, Vega & Altair make good candidates) with exposures of 10-20s to get a good image to inspect for focus.

As for autofocuser, I've been watching this thread with interest!

http://stargazerslounge.com/topic/218975-arduino-ascom-focuser-mark2/

Hi Russell

Unfortunately, I have a very limited view of the sky (I image through a window) and it's often the case that a low mag star just isn't visible. But I'll manage somehow!

I'd quite like an autofocuser - a precision stepper motor one would be nice . I'm thinking I might have a go at making one - when that would be, who knows!

Cheers

Louise

[Thalestris24]

Hi Louise....no, robofocus is usually fine but I think I have a problem with my usb to serial interface, which will probably take a couple of hours to fix....sometimes I simply can't be bothered sitting with my head stuck in the computer when I could be out on the beach walking my dog though, hence not fixing it yet !!

Hi Rob

Well walking the dog on a beach does sound very pleasant

Louise