Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Is this sub-frame out of focus?

  • Please log in to reply
15 replies to this topic

#1 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 21 September 2020 - 03:35 AM

Okay, I know this is probably a stupid question, but I'm a beginner here, so please bear with me...  I set up to take some narrowband shots of Melotte 15 last night. I started with a good, crisp Batinov mask focus and kept a nicely tight PHD2 guiding graph, then got the target framed. So far, so good, and a step forward for me. 

 

I took a 300 second Ha sub frame which is attached here, roughly stretched and hugely compressed. Just checking, but the issue with this shot is that it is out of focus, right? Somehow, although I started with a perfect focus, I think my telescope got defocused during the workflow of star alignment and PHD2 calibration. I was about to refocus when the clouds moved in. Will be back out tonight, refocusing just before taking any subs, but please can you confirm that's the issue with this shot?

 

Melotte 15 out of focus.jpg


Edited by jezphil, 21 September 2020 - 03:37 AM.


#2 Tapio

Tapio

    Gemini

  • -----
  • Posts: 3,146
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 21 September 2020 - 03:38 AM

Yes, clearly out of focus.

Make sure your focuser is tight after focusing. Also you may have to refocus if temperature changes (few degrees) during imaging run.


  • dswtan likes this

#3 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 21 September 2020 - 03:42 AM

Yes, clearly out of focus.

Make sure your focuser is tight after focusing. Also you may have to refocus if temperature changes (few degrees) during imaging run.

Thank you! Will get this nailed in the end.



#4 drd715

drd715

    Viking 1

  • *****
  • Posts: 609
  • Joined: 07 Jan 2015
  • Loc: Fort Lauderdale

Posted 21 September 2020 - 09:10 AM

You did focus with the Ha filter in place? The focus point of white light (or just green) is different from the Ha focus point in a refractor. Also the filter glass path will slightly move the focus point. Best to focus just before starting the imaging sequence. The shorter your effective focal ratio the more difficult it is to hit the focus sweet spot.

Sent from my SM-N950U using Tapatalk
  • dswtan likes this

#5 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 21 September 2020 - 09:16 AM

You did focus with the Ha filter in place? The focus point of white light (or just green) is different from the Ha focus point in a refractor. Also the filter glass path will slightly move the focus point. Best to focus just before starting the imaging sequence. The shorter your effective focal ratio the more difficult it is to hit the focus sweet spot.

Sent from my SM-N950U using Tapatalk

Ah - okay. No, I focused without the filter. That's really helpful - thanks.



#6 ESzczesniak

ESzczesniak

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 30 Apr 2020

Posted 21 September 2020 - 12:54 PM

Ah - okay. No, I focused without the filter. That's really helpful - thanks.

That's probably your main issue.  As mentioned, temperature can change focus and if you don't have an electronic focuser, gravity if not locked in place. But the difference I see in focus through an L or UV/IR cut filter and NB filters can be significant.  I'm only just getting started in NB myself, but it's a pain because focusing through NB also requires longer exposures to check and adjust your focus.  But it is what it is.



#7 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 22 September 2020 - 08:47 AM

You did focus with the Ha filter in place? The focus point of white light (or just green) is different from the Ha focus point in a refractor. Also the filter glass path will slightly move the focus point. Best to focus just before starting the imaging sequence. The shorter your effective focal ratio the more difficult it is to hit the focus sweet spot.

Sent from my SM-N950U using Tapatalk

Thanks for your help, and others above. It made a big difference taking your advice focusing with the Ha filter. I got this Ha stack of subs last night and am yet to gather OIII and SIII to add to it. This is my first stack of CCD narrowband images. A beginner's shot, but a big step forward for me as this only began three months ago when I stuck an iPhone over the eyepiece of a $100 telescope to shoot the moon and got hooked. This stack needs some more of the bad subs pulled out and restacking. 

 

 

Small Melotte.jpeg


Edited by jezphil, 22 September 2020 - 10:30 AM.

  • dswtan, pedxing and jerahian like this

#8 nimitz69

nimitz69

    Mercury-Atlas

  • *****
  • Posts: 2,684
  • Joined: 21 Apr 2017
  • Loc: A barrier island 18 miles south of Cocoa Beach

Posted 22 September 2020 - 10:44 AM

Look like it may still be slightly out of focus And you also look like you may have some tilt and\or back spacing issues. Check you stars in the corners ... Lower left look more or less round but OOF, stars in lower right are oblong. Upper left and right also look oblong

Edited by nimitz69, 22 September 2020 - 10:45 AM.


#9 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 22 September 2020 - 12:46 PM

Look like it may still be slightly out of focus And you also look like you may have some tilt and\or back spacing issues. Check you stars in the corners ... Lower left look more or less round but OOF, stars in lower right are oblong. Upper left and right also look oblong

Thanks. I think the issue is that I didn't inspect the subs properly and ASTAP didn't pull out all the bad ones. I've taken a more careful look and weeded some out which seems to have helped a lot with the elongation issue.

 

 

Melotte15 post LR.jpg


Edited by jezphil, 22 September 2020 - 01:25 PM.


#10 nimitz69

nimitz69

    Mercury-Atlas

  • *****
  • Posts: 2,684
  • Joined: 21 Apr 2017
  • Loc: A barrier island 18 miles south of Cocoa Beach

Posted 22 September 2020 - 05:06 PM

Yeah that looks better. I’ve found that even filters which are parafocal I’ve seen slight shifts as the night progresses so I just “always refocus upon filter change” and that takes care of it

#11 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 23 September 2020 - 02:28 PM

Yes, my filters are described by the manufacturers as parfocal. Does anyone set the telescope in focus and make a mark physically on the slider part to note the best focus for that filter? Then use that to easily focus next time. Too inaccurate?

Edited by jezphil, 23 September 2020 - 02:31 PM.


#12 Scott1244

Scott1244

    Sputnik

  • -----
  • Posts: 46
  • Joined: 30 Dec 2017

Posted 23 September 2020 - 07:22 PM

Looks like its time for you to start to learn the joys of autofocusing. Once configured and mastered, it really makes life much easier in this difficult hobby.



#13 idclimber

idclimber

    Viking 1

  • -----
  • Posts: 565
  • Joined: 08 Apr 2016
  • Loc: McCall Idaho

Posted 23 September 2020 - 11:43 PM

Yes, my filters are described by the manufacturers as parfocal. Does anyone set the telescope in focus and make a mark physically on the slider part to note the best focus for that filter? Then use that to easily focus next time. Too inaccurate?

Focus will also change with temperature so the short answer is no. I will second that a focus motor and software greatly simplifies this with much greater precision than I could do with a focus mask. My current automated routine only takes about 30 seconds. It repeats this throughout the night anytime the temperature changes. 



#14 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 24 September 2020 - 06:43 AM

Focus will also change with temperature so the short answer is no. I will second that a focus motor and software greatly simplifies this with much greater precision than I could do with a focus mask. My current automated routine only takes about 30 seconds. It repeats this throughout the night anytime the temperature changes. 

That's quick. What are you using?



#15 idclimber

idclimber

    Viking 1

  • -----
  • Posts: 565
  • Joined: 08 Apr 2016
  • Loc: McCall Idaho

Posted 25 September 2020 - 10:43 PM

The routine I am using is called @focus3 and is part of TheSkyX with the imaging modules. It takes as many samples at each focus position as I program it too. I currently have it set to do four.  This increases accuracy especially if the seeing is not great. It usually can get a focus solution with five focus positions. So that is minimum of 20 images. I focus with the Lum filter and use focus offsets for each filter. Duration of the exposure depends on the target and what is available in the overall image. Most of the time it is about 1/4 sec, but it is automatic and will adjust as needed. 

 

This focus routine does not use FWHM but instead uses contrast. As such it will also focus on things like the moon. It does not need to move the scope to find a suitable star. This also speeds things up. 

 

TheSkyX is not really recommend or discussed much here at CN. The very first version came out back in the 80's with some of the very first PCs. They use an all sky telescope pointing system called T-point that is used by larger observatories. This gives extremely accurate GoTo and tracking. Although the software is tied to the Paramount mounts, it also works with most other mounts. The big issue is price, as the current imaging edition is  $495. But this gives you integrated plate solving, polar alignment, guiding, autofocusing, and pretty much anything else you need. I run it fully automated with a script. 


Edited by idclimber, 25 September 2020 - 10:43 PM.


#16 jezphil

jezphil

    Sputnik

  • -----
  • topic starter
  • Posts: 26
  • Joined: 17 Aug 2020

Posted 03 October 2020 - 02:45 PM

The routine I am using is called @focus3 and is part of TheSkyX with the imaging modules. It takes as many samples at each focus position as I program it too. I currently have it set to do four.  This increases accuracy especially if the seeing is not great. It usually can get a focus solution with five focus positions. So that is minimum of 20 images. I focus with the Lum filter and use focus offsets for each filter. Duration of the exposure depends on the target and what is available in the overall image. Most of the time it is about 1/4 sec, but it is automatic and will adjust as needed. 

 

This focus routine does not use FWHM but instead uses contrast. As such it will also focus on things like the moon. It does not need to move the scope to find a suitable star. This also speeds things up. 

 

TheSkyX is not really recommend or discussed much here at CN. The very first version came out back in the 80's with some of the very first PCs. They use an all sky telescope pointing system called T-point that is used by larger observatories. This gives extremely accurate GoTo and tracking. Although the software is tied to the Paramount mounts, it also works with most other mounts. The big issue is price, as the current imaging edition is  $495. But this gives you integrated plate solving, polar alignment, guiding, autofocusing, and pretty much anything else you need. I run it fully automated with a script. 

Hi

 

Thanks so much for the information and apologies for the late response. Your system looks great. I've now been trying out a Batinov mask and this has been pretty good, even with filters in place. Probably the other end of the scale from the above technically, but seems to work well for a cheap bit of plastic. 


  • idclimber likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics