130PDS secondary mirror and Eos 500d gradients

[Uranium235]

Mines about halfway to saturation (and I seen other suggest a slightly higher value). My flats are taken with a laptop screen with Al's lightbox and three sheets of A4 to diffuse the harsh light it gives off. I also use the computers power settings to dim the display to a better level.

Typically, my flats are in the reigon of about 10s long (to avoid the shutter)

[Welrod50]

Your image is overexposed with just white in it so not usable as a flat.

Can you explain more about how you took your flats?

Camera mounted onto OTA in position,  2" LP fitted - basically just as if I was about to use it for astro.

Then white pillow case (thin) stretched over the end of the OTA and click away.  I was always under the impression a flat frame should be evenly lit and pure white (or as near to), to allow software to 'realise' how evenly rendered the final stacked output should be.  Obviously not

Could someone please post a jpg flat frame example of what yours look like?  I take it if I expose correctly, the histogram should be peaking to the left, a third of the way from shadow to highlight then?  Maybe even look a little grey due to the exposure?

Thanks all    

[Xplode]

Flat frames should be grey so there is no chance of overexposure.

Your process seems ok so i don't understand how they can be overexposed?

You should try to experiment with manual settings to get the flats to be usable.

Flat frame for my EF 200 2.8L at f/4.2 (with aperture mask), this has around 10% vignetting in the corners

[ollypenrice]

My full frame flats are worse than that.

It calibrates out just fine, though. The vignetting is extreme because I use 2 inch mounted filters with a full frame CCD at F5. It would have been better to use 2 inch unmounted but the final result is the same. (We have an identical setup running parallel on a tandem rig, except that the other one has unmounted filters. There is no difference in the final images. At F7 the vignetting is far less.

I suspect you may have been misunderstanding the way flats work. The idea is to image the irregularities in your illumination so that the dark pixels on the flat will be brightened on the real image and vice versa. If I calibrate the flat above with a flat (in this case itself) what I get is this.

So the defects seen in the first image (the flat from the rig) would be present in any image taken with that rig. The second image shows how the division by a flat field has corrected them superbly. The same corrections go into any images taken with that rig.

People do make flats from a third to two thirds saturation. I've gone lower than that because on some setups I find they work better. On most it doesn't matter, though. I've used the low exposures when flattening reflector images, interestingly enough. (Don't ask me what's going on though!I haven't a clue.)

Olly

[Welrod50]

I'm so glad I've raised this here.  It seems I've been doing myself a serious disservice before now!

Had a play earlier with manual settings of 1/30th, 1/60th and 1/200th sec exposures all mounted up at iso 400 (same as my subs as 800 iso is pretty noisy on the Eos 500d).

To get flats which look like Ole's the 1/60th sec is the one which appears best. 

1/200th is way too dark and 1/30th again looks a bit too bright. 

Olly, I haven't been able to replicate what yours come out like, although I suspect this is just down to different imaging rigs?

I will have another go at my recent IC 434 session later with new flats I've shot and see how they turn out.

Thank you very much guys

[ollypenrice]

I'm so glad I've raised this here.  It seems I've been doing myself a serious disservice before now!

Had a play earlier with manual settings of 1/30th, 1/60th and 1/200th sec exposures all mounted up at iso 400 (same as my subs as 800 iso is pretty noisy on the Eos 500d).

To get flats which look like Ole's the 1/60th sec is the one which appears best. 

1/200th is way too dark and 1/30th again looks a bit too bright. 

Olly, I haven't been able to replicate what yours come out like, although I suspect this is just down to different imaging rigs?

I will have another go at my recent IC 434 session later with new flats I've shot and see how they turn out.

Thank you very much guys

Don't worry about replicating mine, but for me a credible flat looks like that in having a slight brightening towards the centre. I have an extreme brightening but that's because I'm using a rather extreme rig with non-optimal filters. If you get a top to bottom gradient or a random illumination be suspicious. 

Olly

[Andy Milner]

Just a quick one. Can anyone link to their best guides online for taking flats. I'm about to start taking my first flats soon. Only tried subs, darks and bias so far. And I get bad graduents And vignetting. I've ordered a LPS-D1 filter and think that will help. Once I'm taking flats I'm good to go I think.

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