Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_dslr_mirrorlesss_winners.thumb.jpg.9deb4a8db27e7485a7bb99d98667c94e.jpg

jezhughes

Focus slip or thermal?

Recommended Posts

Hi all

I was pre processing a bunch of data from last week when I noticed a shift through each session (see image which i've overstretched to illustrate - left hand side is the first image of the session, right hand side is the last over 2 hours).

When I went through the first batch I thought it was just the seeing degrading throughout the session, then I looked at an additional three sets of data from the same night and noticed the same thing happening - a gradual shift throughout the exposures.

Could this be my gear slipping the focus or thermal? Or something else I've not considered?

I'm a bit stumped because if it was slippage, i'd assume it would yield a more visible and drastic change in focus after 2 hours. I'm not sure it's anything thermal as the same thing happened in my last series of the night, which was 14 hours after I set up, and the temperature shift through the night was only maybe 2 degrees? Almost certain it wasn't dew as I had the heaters on and check the lenses every hour, and as I mentioned I doubt it's anything to do with seeing, as there was a good 6 hours of stable seeing which this happened within. I don't think it was shifting because of falling altitude of targets in to LP during sessions either as it affected my M42 data set too, which as you know in the UK spends a couple of hours within +/-2 degrees

Anyone seen this before? It's the first time I've noticed it.

:huh:

hmmm.jpg

Share this post


Link to post
Share on other sites

Hard to say as the two images are not processed in the same way (right hand much lighter background) and the two images are of different star fields. It would be easier to compare the exact same FOV from the start and end of your session so you can see the difference on the exact same star. Like you say, seeing could have an impact (though you doubt this) and maybe guiding was not so good towards the end of the session. You would be surprised what a difference 2 deg can make to focus though at least it does on my setup.

Share this post


Link to post
Share on other sites

I see it all the time - or at least I would if I didn't refocus continually throughout the night :)  Changes in ambient temperature are only a proxy indication, you really need to know the temperature of your optics and mechanicals.. Changes in seeing can also dramatically affect FWHM even within a single session, not to mention height of the object above the horizon.

ChrisH

  • Like 1

Share this post


Link to post
Share on other sites
9 hours ago, Freddie said:

Hard to say as the two images are not processed in the same way (right hand much lighter background) and the two images are of different star fields. It would be easier to compare the exact same FOV from the start and end of your session so you can see the difference on the exact same star. Like you say, seeing could have an impact (though you doubt this) and maybe guiding was not so good towards the end of the session. You would be surprised what a difference 2 deg can make to focus though at least it does on my setup.

 

Hi Freddie

Sorry I should have explained better - they're not processed differently, this is the effect happening over time causing a shift. The two images below demonstrate it better. The darker one being the image from the start of the session, the lighter one being the end image (2 hours later)

1.jpg

2.jpg

Share this post


Link to post
Share on other sites
8 hours ago, ChrisLX200 said:

I see it all the time - or at least I would if I didn't refocus continually throughout the night :)  Changes in ambient temperature are only a proxy indication, you really need to know the temperature of your optics and mechanicals.. Changes in seeing can also dramatically affect FWHM even within a single session, not to mention height of the object above the horizon.

ChrisH

 

Hmm yeah, I would assume after 5 or 6 hours all my gear would have 'cooled down'.

Either way I've tightened my focus right up so it shouldn't slip at all, and until I get automated focus set up next year I'll have to babysit the focus I guess!

Share this post


Link to post
Share on other sites
31 minutes ago, Freddie said:

Why would focus drift make the background lighter?

Focus slipping reduces the contrast in the image

Share this post


Link to post
Share on other sites

So to achieve the same brightness in the stars the background is made brighter in processing as an out of focus background would still be the same brighness as an in focus background.

What are you using to check focus?

Share this post


Link to post
Share on other sites
23 minutes ago, Freddie said:

So to achieve the same brightness in the stars the background is made brighter in processing as an out of focus background would still be the same brighness as an in focus background.

What are you using to check focus?

 

Not sure what you're referring to - In those two images I did an identical levels layer on top, so its the raw data which has the lighter background, it's not a result of any processing data. The average pixel value increases by almost 20% throughout the imaging run.

I use a B mask and tweak it with FWHM

Share this post


Link to post
Share on other sites
On 07/12/2016 at 20:22, jezhughes said:

Hmm yeah, I would assume after 5 or 6 hours all my gear would have 'cooled down'.

Either way I've tightened my focus right up so it shouldn't slip at all, and until I get automated focus set up next year I'll have to babysit the focus I guess!

It's very rare, I find, for focus to settle down completely. It needs to be monitored pretty constantly. How constantly depends on your pixel scale and optics. The fewer arcsecs per pixel your system is running, the more crtical is the focus. The steeper the light cone, ditto. (Fast optics need more checking.)

I can be sure that none of my focusers slip because they are good R and Ps and one of them uses a tangent arm which makes slippage impossible. They all need regular checks, though.

Like you I use B mask tweaked in FWHM.

Olly

PS I don't believe a small drift in focus will have a very significant effect on background brightness.

 

Edited by ollypenrice

Share this post


Link to post
Share on other sites

I was always surprised at how much temperature shifted focus ...... Now I am running auto focus at every 1 degree change, so I really don't know how often it changes and re focuses. When I was doing it manually, and I was using an 8" RC scope, in the winter I would focus at say 19:00, then again before I went to bed at 23:00. I'd get up at 02:00 and refocus and then generally by the end of the run at sat 06:00 the subs were far enough out of focus to be unusable.

Share this post


Link to post
Share on other sites
On 12/7/2016 at 02:42, jezhughes said:

Or something else I've not considered?

Perhaps it's the moon. To explain: 

I image from a low light pollution site and continuously refocus throughout the night. When I look at the calibrated subframes with CCDInspector I always see some variation in the average background level. If I have have good seeing and I limit the analysis to one night then the item that normally creates the largest variation in the background level is the moon's position relative to the object I'm imaging. Since I run an automatic image capture setup, I decided to use ACP's "moon up" facility which takes into account the location of the object, the phase of the moon and the camera filter type. It then works out the minimum angular distance between the moon and the object for an acceptable image. If it's OK the image is taken, if not then ACP will go onto another object or select a different filter type that allows a greater angular distance.  I've found that the default setting work very well and allows me to maximize my imaging time without a noticeable reduction in quality. 

Alan  

Share this post


Link to post
Share on other sites
4 hours ago, alan4908 said:

Perhaps it's the moon. To explain: 

I image from a low light pollution site and continuously refocus throughout the night. When I look at the calibrated subframes with CCDInspector I always see some variation in the average background level. If I have have good seeing and I limit the analysis to one night then the item that normally creates the largest variation in the background level is the moon's position relative to the object I'm imaging. Since I run an automatic image capture setup, I decided to use ACP's "moon up" facility which takes into account the location of the object, the phase of the moon and the camera filter type. It then works out the minimum angular distance between the moon and the object for an acceptable image. If it's OK the image is taken, if not then ACP will go onto another object or select a different filter type that allows a greater angular distance.  I've found that the default setting work very well and allows me to maximize my imaging time without a noticeable reduction in quality. 

Alan  

 

Good point Alan. In my scenario we were on a new moon, so not really applicable, but I agree with your example for sure.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.