What does this flat tell me?

[Helen]

Here's a flat from my ATiK383 on the Megrez 90 with 0.8FR, SXfilter wheel with 1.25 filters and this luminance. Any idea what might be causing the gradient?

Thanks

Helen

m90_Luminance_2x2_skyflat30-small.tif

[pete_l]

Any idea what might be causing the gradient

Misalignment?

[Helen]

In what way Pete? given its a refractor, do you mean camera, filter wheel? I've changed the orientation of the camera, but still get a similar effect, so it could only really be sensor or filter wheel I think.

Helen

[RobH]

Are you getting this all the time, on all filters and all exposures?

If so, I wouldn't worry about it, you should see what I have sometimes .

I've no idea why these minor gradients show up, but as long as you have the same thing on your lights, then once the flat is applied the problem will disappear, so it's not worth worrying about.

It actually looks like a pretty good flat to me!

Cheers

Rob

[Helen]

Yes, it seems to be all the time Rob. Its a new camera, so I'm finding its unique traits! Even a moderate stretch on the lights shows it up really badly, but the flats do sort it out and so are critical. I'm doing darks now as well, something I've never bothered with with the H9.

Thanks for the reassurance!

Helen

[RobH]

Hi Helen.

Yes, you'll find the 8300 chips are noisy as heck and you really need very closely temperature matched darks.

Make sure you don't use a very short exposure time with your flats too as this can catch the shutter in motion, and leave a 1 second or so gap between exposures when using the same filter, regardless of whether it's flats or lights. This avoids catching the shutter, and also there can be a hair of vibration from the shutter motion.

Cheers

Rob

[FrankieValley]

I'm doing darks now as well, something I've never bothered with with the H9.

Hi Helen,

I gave up doing 'proper darks' after I read in the Nebulosity instructions that a Bad Pixel Map is probably as good and could even be better as it only removes hot pixels and doesn't inject any noise. I take 2 or 3 darks and use Neb default slider setting to make my Bad Pixel Map. I also re-use bad maps that correspond to my settings, etc. I use them successfully with a Meade DSI II (which is mostly hot pixels) and now a 1000D. Neither are cooled. Something to consider to save you the tedium of taking lots of darks when it's time for bed

[Helen]

Thanks for the tips Rob :-) I'd sussed the shutter problem for flats (my light panel needs 5 extra sheets of paper to get the exposure long enough), but hadn't thought about the filter change stuff. I've started a darks library, but its only got one set in it at the moment, but at least its started!!

Helen

[Helen]

...and thanks for the tips David.

Helen

[pete_l]

In what way Pete? given its a refractor, do you mean camera, filter wheel? I've changed the orientation of the camera, but still get a similar effect, so it could only really be sensor or filter wheel I think.

Helen

Hmm. Originally I thought the centre bright spot was offset, but when you darken & stretch the image, the brightest point is centred. So what's causing the shadow on the left?

I'd check that the filters are properly aligned with the aperture in the wheel and not partially obscured.

Apart from that, all I can suggest is going back to basics: taking out the FW and the FR and see if the shadow persists on those flats, then adding components one-at-a-time to see what causes the shadow to reappear.

[ollypenrice]

Helen, I can't open the image at the moment but I only take flats in the dark. I know many people do take theirs successfully in the light and even manage to get sky flats to work. I have never succeeded and always get gradients unless using the panel in the dark. I thought it was light leakage from my manual filter wheel but I get just the same defects in my sealed OSC or when occasionally using my electric wheel. Yves and I found the same thing in his SXH36/14 inch while setting it up last week as well.

I can offer no logical explanation so I merely pass on my experience for what it's worth.

Olly

[libraryman]

I would be happy if i could get anywhere near this!

Ray

[Starflyer]

Here's a flat analysis from CCD Inspector, it's quite a large variation in brightness, or was the flat stretched Helen?

[Helen]

Update:

I've sorted out the problem guys. It was so obvious I really should have found it before

I removed the filter wheel so that I could install the grub screw (that RobH kindly gave me at SGL7 ) into the OAG to tighten the whole thing up. The grub screw along with a small piece of tin foil shim and a filing of the bottom of the retaining screw to a flat surface has now made the OAG rock solid - result .

While doing this I noticed that the Atik chip was at 90 degrees to where it should be, and so the OAG prism was obscuring the chip . I had to remove the back plate and reattach so that the T thread delivered the camera at the right angle. A quick first light suggests a much cleaner output . I'm relieved.

Helen