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Flat fields Any need to retake if focus postion varies night to night

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Eric
Flat fields Any need to retake if focus postion varies night to night

The dust and other matter that causes the need for the taking of flats lies within the optical train, mostly on the glass surfaces of the focal reducers, filter, and ccd chip.

I usually take flats for each filter at the end of an observing session and if the focus position does not change much I will use them again on another night. If there is  a big change I will retake them

But I have been thinking is there a need to retake  take them if there is a reltaively large change in focus position.

In other words are the flat fields appearance and form changed by a change in focus.  Would one set of flat fields for each filter last several sessions irrespective of the focus postion.

 

Eric

Eric Dose
Eric Dose's picture
Prob not, but you can verify

I've never found focus-specific flats to be needed. But it's easy enough to be sure for your own system, by comparing flats taken at different focus positions. [The easy way: use one flat as image, and calibrate it using another flat as a flat.]

Eric
Focus specific flats

Eric,

thanks for your comment and suggestion I'll give that a go.  I suppose I could also subtract one flat from another.

Eric

 

hambsch
hambsch's picture
flats

Hi,

I take flats and use them for weeks even months.

I also never focus except there was a change in the optical trian. I have a carbon tube and even though temperature changes all night it keeps focus very well.

Josch 

Eric
focus specific flats

Hi Josh,

my second telescope about to "come online" is a carbon fibre truss for that very reason, but my C14 Edge unfortunatly is not and there is focus change with temperture, particularly nigh to night.

Eric

Eric Dose
Eric Dose's picture
This is prob not a concern

My scope is a C14 Edge with focal reducer; you'll be happy to hear that it's not extraordinarily temperature-sensitive. My entire annual focus range is only about 5.5 mm from mid-summer dusks to mid-winter dawns (New Mexico). Even with my extremely shallow Atlas focuser, I never need to adjust the primary mirror position, all year.

More relevant: focus just doesn't matter for defects on the CCD window or filters, as their distance to the CCD chip doesn't change with focus. If your f/ratio is independent of focus (and my C14 is), all those shadows will be constant too. And while defects that are skyward of the focuser will theoretically change with focus, they are already so diffuse that they are very faint, and any focus effects would probably be very hard to see experimentally.

But again, you can verify this experimentally by taking flats at the focus limits.

Eric
flats and focusing

Hi again Eric,

My C14 Edge is housed within a dome here in the UK and over a year my temperature variation is about 20C. During a cold winter's night it can drop from ~ 10C to 0.  The focuser is a moonlite and is fixed-in-line during focusing so  there is no primary mirror movement.

I will certainly verify experimentally as you suggest.

regards,

Eric

hambsch
hambsch's picture
temp variation

My nightly variation of temperature is 20 deg C and yearly it varies much more.

I am remotely observing from the Atacama desert at 2500 m elevation. Still I do not focus and can only  recommend a scope with a carbon truss or tube.

Josch

Eric
Flat field variable focusing

Hi Joch,

which all my future telescopes will be and I have had  16" carbon truss ready for this year once the observatory is completed which incidentally is a trial set up for a planned 0,.65m telescope.  But for the time being I can only play the cards I'm dealt :)

Best wishes,

Eric

Eric Dose
Eric Dose's picture
Right. Most scopes are and

Right. Most scopes are and will continue to be non-carbon-fiber. We should have guidance for that majority.

HQA
HQA's picture
flat-fielding

Good and bad focus cause some subtle effects that require ray-tracing to fully understand with most optical systems.  I usually prefer dawn twilight flats because they occur after you've been carefully focussing through the night.  That said, there are many times when you must take dusk flats (or lightbox flats, or EL panel flats, or whatever), before you've had a chance to look at a star and determine whether your system is in focus.  As Eric suggests, you can experimentally test focus changes by taking flats on one night at large excursions from proper focus, along with focused flats, and then dividing the two flats to see any residual pattern.  Dust is usually the thing that you see most often with change, depending on what surface it resides and where in the optical path - before or after a filter or corrector for example.  My general rule of thumb is to take flats close to the correct focus, so that you don't have to make assumptions.

Likewise, how often to take flats depends on your particular circumstances, and how much of a gambler you may be.  For some closed systems - say, an SCT with a focal reducer, filter wheel and camera that are never opened - new dust is rare and you can go long periods without new flats.  For others - say, an R/C system where the filter wheel sees raw sky, sited in a desert climate - new dust (or fly wings, or cat hair...) is common.  The amazing thing is that flats are free.  You take them during periods that you would not otherwise be doing imaging - dusk and dawn, cloudy nights with a light box, whatever.  Again, a good rule of thumb is to take flats as often as possible.  Then you don't have to make assumptions, and possibly avoid multiple nights of compromised data.

Guidelines are just that, and you can take any approach that you feel comfortable with.  However, calibration images are important, so monitor them regularly.

Arne

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