Flats and mechanical shutters

[pmrid]

I decided to run a new set of flats - high time since this is an entirely new scope combo - the only constant factor is the CCD. It's an Apogee Alta U8300 - a good camera without doubt.

But I'm seeing an odd artifact on the flats (and with plenty of pushing, it even comes out in stacked images) - it's a 5-point star-like shadow with somewhat curved and diffuse arms resembling a Japanese shuriken or something similar - occupying most of the image and centered in the middle.

These cameras have a mechanical shutter and it occurs to me that in short exposures, such as you do for flats and the like, there may be an artifact created by the opening (and closing I guess) of the shutter.
Anyone else experienced this?

Peter

[RickS]

Peter,

That's typical for a camera with a mechanical shutter. You will need to make sure your flats are long enough that the artifacts disappear. I never do flats that are shorter than a couple of seconds.

Cheers,
Rick.

[pmrid]

G'day Rick.
Hmmm! That could be tricky. I try to keep the flats at about 25K on the histogram/Info window in Maxim. My light box doesn't have a rheosat to alter the intensity so I may have to fudge a solution. Any ideas.
Peter

[RickS]

Peter,

A couple of sheets of paper or a cheap PWM dimmer from ebay? These days I nearly always do sky flats.

Cheers,
Rick.

[ZeroID]

Cover the light box with a sheet. Stay defocussed.

[pluto]

I was going to look for something to darken my lightbox down a bit too, I was thinking of something like this and using a few layers if nessesary:
http://www.polyfilm.com.au/window-tint-film-35-black-metalized-50cm-x-6m-car-legal-home-office-diy-roll

[pmrid]

Hey Brent, that sounds like a bit of Zen philosophy - stay defocussed, man!
I have found that alcohol defocuses me very nicely.

Peter

[pmrid]

Speaking entirely from a lay perspective, I would say that adding a filter-layer such as that window film would have a negative effect on your flats which would no longer be ambient light as seen through a LRGB etc filter but as seen through that filter and that film so your flat represents only that bandwidth which they both allow to pass. Can that be good?

Peter

[pluto]

The short answer is I don't think it matters, but I'm very open to being corrected on this.

My "lightbox' is made from an electroluminescent panel in a cardboard box. It's kind of blueish anyway and when I do my narrowband flats my Ha and SII exposures are about 5 times longer than my OIII images to get a fairly consistent white level.

Certainly when taking flats through LRGB filters the exposures will be very different lengths to get images with a count around 25k but I'm not using the flats to calibrate colour but just to eliminate gradients and dust etc.

However after seeing Ricks post I think I'll just chuck some sheets of paper over the panel and see how how much light they absorb.

[RickS]

I don't think a bit of a colour cast would make much difference unless you had a scope with bad CA. As Hugh said it's common for flats with different filters to take substantially different exposure times. Even illumination is the important thing.

Cheers,
Rick.

[pmrid]

Thanks Rick and Hugh. I am certainly a little older. I may not be any wiser but, as the saying goes - I am now better informed. I prostrate myself at the altar of your greater knowledge. :lol::lol::lol:

Peter

[Peter Ward]

Yes!

I owned a U47 for time. Apogee use Melles-Griot shutters with an iris closure.

They cause non-linear flat field exposures that has a measurable effect up to about 10 seconds.

I found this made taking sky-flats (read most accurate) nearly impossible.

The only fix was to take dome flats, and adjust the field illumination source to allow 15-20 second exposures with 30% full well capacity.

P.S.
I ended up returning the camera due a plethora of (other) problems.

[RickS]

My experience with the Apogee U16M is that flats are fine so long as they aren't sub-second. That was also my experience with a Starlight Xpress H-18 with a mechanical shutter.

Cheers,
Rick.

[LewisM]

Could you not delay the exposure until after the iris is open and stabilised? MaxIM has this feature in one of the side menu's, and I do this ALL the time with the FLI with a similar iris - I give it a 2 second delay between opening the iris before the exposure begins.

This would seem to circumvent the problem entirely - I also found short exposures BEFORE I used this technique showing the shadow of the iris.

[pmrid]

I employed a more basic solution. A Chesty Bond special across the objective before mounting light box.

P

[Geoff45]

The simple solutions are the best.

[RickS]

A KAF sensor will start counting photo electrons as soon it sees them, i.e. as soon as the shutter opens. I don't see how this could work?

OTOH, a Chesty Bond sounds good.