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Rate my back focus


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Apologies for another thread, just hoping for confirmation :)

WItches broom in veila nebula. 25x60sec

NO sign of back focus problem I can see. 

M101 20x60sec  bottom left corner has eggy stars, other corners seem ok.  Taken right after above image, fairly different position in sky, so suggests focus tube slop? It's not a huge amount of eggyness....

 

ngc696015x60sectest.thumb.jpg.277c3ff7c2d390df6271e59cadbbc9f2.jpg

m10120x60sectest.thumb.jpg.572348149a1d320b6425d81e0036cfdf.jpg

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Posted (edited)

I'd be more concerned about that bright spot on the edge, what is it a metallic shard or something on one of the glasses?

Filters can cause vignettes, that's the whole point of applying flats (as well are removing dust motes) but with a 11 x 11mm sensor or thereabouts your 1.25 inch filter should be fine.

 

Edited by Elp
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1 minute ago, Elp said:

I'd be more concerned about that bright spot on the edge, what is it a metallic shard or something on one of the glasses?

Filters can cause vignettes, that's the whole point of applying flats (as well are removing dust motes) but with a 11 x 11mm sensor or thereabouts your 1.25 inch filter should be fine.

 

Not sure, it move before I took my flats I think. I was hoping to keep my flats for a while, but new gear, new dust I guess :(

I'll try take some new flats tonight, see if it fixes above.

will do fresh flats every session for a while I guess see if it settles down after a while.

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I don't think you can reuse flats.because your focus position will be different every time, unless you use an EAF and use the same focus point every time with it.

I've just learned to accept it's part of the routine.

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10 minutes ago, Elp said:

I don't think you can reuse flats.because your focus position will be different every time, unless you use an EAF and use the same focus point every time with it.

I've just learned to accept it's part of the routine.

Meh. They're easy to do but still annoying. I was lazy with my test shots. Only tests doesn't matter, but I had to clone in gimp like an Imperial Scientist to fix my m31 mosaic to be even vaguely useable.

Flats every night it is. At least my darks and biases will last. -5c should be doable even in the heat of English summer nights ;)

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Posted (edited)

If you really want to get into it I think dark flats (or darks for your flats an easy way to remember it) are better than bias as they remove the residual sensor signature generated during the flats from your flats prior to applying them to your lights (well that's how I understand how you should do it, I subtract the master dark flat from the flats before making a master flat, so you're not removing an additional amount which your master dark already corrects for).

Edited by Elp
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Posted (edited)
4 minutes ago, Elp said:

If you really want to get into it I think dark flats (or darks for your flats an easy way to remember it) are better than bias as they remove the residual sensor signature generated during the flats from your flats prior to applying them to your lights (well that's how I understand how you should do it, I subtract the master dark flat from the flats before making a master flat).

I wasn't planning on making masters,  just stacking with matching temperature darks and biases and flats (but flats now taken every session).

Tbh I assumed dark flats were another term for darks. It was a confusing YouTube search for dark flats, so I just decided they must be interchangeable terms.

It's also possible my flats aren't great. I read some people matching 35k adu for theirs, but Nina flat wizard seemed fine after I turned tablet brightness up a touch.

Edited by TiffsAndAstro
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Your software usually makes masters prior to calibrating the lights. As I use Siril manually I do the masters myself so I can check them prior to applying them. Doesn't take long at all.

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Just now, Elp said:

Your software usually makes masters prior to calibrating the lights. As I use Siril manually I do the masters myself so I can check them prior to applying them. Doesn't take long at all.

I'm still semi manual stacking. Once I get a lot more hours on a target I can try harder with the stacking.

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I think your backfocus is fine but what on earth is causing that blue supernova at the edge with its black counter-shadow? Ouch!

4 hours ago, Elp said:

I don't think you can reuse flats.because your focus position will be different every time,

Noooo....

None of the excellent imagers who have worked here shoot flats per filter and per focus.  Some shoot flats per filter and some use L flats for all, but I've never known anyone try to replicate individual filter focus when shooting that filter's flats. Enough is enough in this game.

Olly

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I wouldn’t  class myself as an excellent imager, but I also don’t take new flats with a focus change. I don’t see the dust motes on the image change from filter to filter either, I think it is dust on the sensor window alone that creates the visible motes so I’m happy to just use the Lum flats with the RGB data.

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Posted (edited)

Just as a precaution I always take flats every session, I do not have a fixed setup and setup and break down every time. The focus position is not always exactly the same as the previous session, I don't use an EAF so I don't know how you can reuse flats in this case.

Edited by Elp
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My plan is to go back to flats at end of each session. I was just lazy on this session as I was only intending to test back focus, maybe gain & offset.

Thanks all for confirming my back focus seems ok.

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29 minutes ago, Elp said:

Just as a precaution I always take flats every session, I do not have a fixed setup and setup and break down every time. The focus position is not always exactly the same as the previous session, I don't use an EAF so I don't know how you can reuse flats in this case.

But focus position is defined by your objective's focal length. How much can that vary? You may have a change in the position of the draw tube because the main tube has expanded or contracted a little but anything causing dust bunnies is downstream of that, surely, and at an effectively fixed distance from the sensor. The distance between the bunny-producing surfaces and the objective may be made up of more main tube and less draw tube or more draw tube and less main tube, but how does this affect the bunnies, which are defined by the distance between dust and sensor?

How do you replicate focus position for flat shooting at the end of a session?  Have you tried not bothering? I am certain you will see no difference.

I'm happy to accept, though, that a rig torn down and re-built will be more prone to variable dust bunnies. But flats varying per focus tweak? In amateur imaging, I simply don't believe it. What would you do if you re-focused automatically every 'x degree temperature change' etc. Shoot separate flats per temperature, calibrate all subs separately before stacking and then stack individually calibrated subs in a second pass? Does anybody do that? Nobody I know.

Olly

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2 minutes ago, ollypenrice said:

What would you do if you re-focused automatically every 'x degree temperature change' etc. Shoot separate flats per temperature, calibrate all subs separately before stacking and then stack individually calibrated subs in a second pass? Does anybody do that? Nobody I know.

From experience early on from having spent 30 hours on one target only for 10 hours of it to be ruined by flats (for some other reason) did I decide it's best practice to take flats at the end of each session without changing anything. If I'm going to go out to bring the setup back in it doesn't take much time at all.

I do calibrate each session with their respective calibration files, then stack each master stack for the final image, no issue in doing that.

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3 minutes ago, Elp said:

From experience early on from having spent 30 hours on one target only for 10 hours of it to be ruined by flats (for some other reason) did I decide it's best practice to take flats at the end of each session without changing anything. If I'm going to go out to bring the setup back in it doesn't take much time at all.

I do calibrate each session with their respective calibration files, then stack each master stack for the final image, no issue in doing that.

But in this method you are changing your filters to shoot flats per filter, but are you replicating the focus position used for that filter when capturing the lights?

In a mobile rig I can see the case for flats per session, certainly, but not for focus-specific flats per filter. Perhaps I misread your earlier post.

Olly

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Posted (edited)
13 minutes ago, ollypenrice said:

are you replicating the focus position used for that filter when capturing the lights?

Once my focus is set prior to running the lights I do not change focus at all, not for ambient temperature change, not after meridian flips (relevant for SCTs that might have mirror flop). End of session at same focus point I point scope up, add flat panel on top of dew shield at dimmest setting with respective perspex to dim the panel as much as possible, run auto flats (via asiair which works out the exposure). Maybe also take darks if I'm running a non cooled camera.

It might be a force of habit, I very rarely image with the same optics night after night in which case flats are needed for each hardware change.

But I do note your point, in sessions where I've left the rig assembled and running the focus routine again at the start on the next night (an incredibly rare occurrence), focus is usually the same so I can see why you might not want to do flats again. But when imaging opportunities are few and far I personally don't risk not taking flats again, it would be different if I had it permanently mounted in an obsy type setup, but the OP doesn't have that, though I think they'll likely keep their optical train unchanged once they've decided on their backfocus which seems okay from the tests above.

Edited by Elp
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2 minutes ago, Elp said:

Once my focus is set prior to running the lights I do not change focus at all, not for ambient temperature change, not after meridian flips (relevant for SCTs that might have mirror flop).

Now this is a mistake. Focus does change on every instrument I've ever used, refractors, reflectors, etc. Temperature is the main cause. Oddly enough, the RASA 8 probably changes the least and usually gets to the flip without a change. My Tak FSQ changed fairly quickly, though it was the old fluorite which changed less than the newer ED models. The TEC 140, at a relaxed F7, still changes and needs a refocus 2 or 3 times a night.

Olly

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Posted (edited)

I've never felt the need to but I rarely image for longer than 4 hours even during winter, my images haven't suffered much as a result (but then again I'm not chasing perfection). I can see the star FWHM change after registration via the graph plot and decide if indeed some of the data is junk, I have noticed on these shorter nights this changes very significantly as it gets brighter. 

I find camera lenses perform very well, the refractors might change a bit (which could also be attributed to the focuser+load), oddly the C6 changes the least.

Edited by Elp
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Posted (edited)

I set NINA to refocus with an HFR change of >5% and with my F7 refractors I typically get a refocus once an hour over a 6 hour session. Like Olly, I have found the RASA8 is a lot less frequent. 
When I used to set up and take down each session (seems a long time ago now)  I used to take flats at the end of the run, just as a precaution. It did require a lot of dedication when you are cold, tired and still have to dismantle and put away, with work in the morning…

Thank goodness for retirement and a permanent observatory.

Edited by tomato
Typo corrected
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19 hours ago, TiffsAndAstro said:

Apologies for another thread, just hoping for confirmation :)

 

WItches broom in veila nebula. 25x60sec

NO sign of back focus problem I can see. 

M101 20x60sec  bottom left corner has eggy stars, other corners seem ok.  Taken right after above image, fairly different position in sky, so suggests focus tube slop? It's not a huge amount of eggyness....

 

ngc696015x60sectest.thumb.jpg.277c3ff7c2d390df6271e59cadbbc9f2.jpg

m10120x60sectest.thumb.jpg.572348149a1d320b6425d81e0036cfdf.jpg

In this first image the stars are misshapen on top right and bottom right of the image, bottom right being the worst, also they are all slightly out of focus, so maybe some tilt in the camera of focuser sag…

Are these take with you new Svbony 533 you purchased,,.?

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13 hours ago, ollypenrice said:

I think your backfocus is fine but what on earth is causing that blue supernova at the edge with its black counter-shadow? Ouch!

Noooo....

None of the excellent imagers who have worked here shoot flats per filter and per focus.  Some shoot flats per filter and some use L flats for all, but I've never known anyone try to replicate individual filter focus when shooting that filter's flats. Enough is enough in this game.

Olly

I really don’t get how just using L flats can work, surely any dust or vignetting from the other filters will still show and ruin the image…🤔

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1 hour ago, Stuart1971 said:

In this first image the stars are misshapen on top right and bottom right of the image, bottom right being the worst, also they are all slightly out of focus, so maybe some tilt in the camera of focuser sag…

Are these take with you new Svbony 533 you purchased,,.?

Yes both with new sv605cc

Just quick stretches no crop

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19 hours ago, Elp said:

I'd be more concerned about that bright spot on the edge, what is it a metallic shard or something on one of the glasses?

Filters can cause vignettes, that's the whole point of applying flats (as well are removing dust motes) but with a 11 x 11mm sensor or thereabouts your 1.25 inch filter should be fine.

 

Took flats last night fixed the super nova.

Big dust spot seemed to disappear on removing filter and returned on putting it back in.

So I decided to clean the filter. That was a 2 hour mistake.

Bigspot still there. But at least it calibrates out

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