Flat trouble!

[BenDavis]

Hi all, sorry more questions!

I would dig more for an answer on-line but this maybe unusual?

So I have been trying to make flats recently and I keep getting a large shadow in one corner of the image. I have tried changing light positions etc. to remove but seems difficult to achieve (but I am going to try again today).

This flat I took here was by pointing into the morning sky before sunrise but the shadow is still there so if that doesn't work it seems odd to me!?

The image of M45 shows the flat shadow light area to the lower left corner (which obviously only appears from the flat being stacked to the luminance image).

Thanks again

[Thalestris24]

Hi

What camera and scope do you have and how are they connected? You'd probably be better off using a laptop or lightbox for taking flats - more reproducible and less likely to introduce spurious gradients. Make sure you don't overexpose - you want to have your histogram peak at about 1/3 - 2/3 or about 25000 - 35000 adu. A lot depends on your camera so you may need to try several exposure times to see what works best for you.

Hth

Louise

[BenDavis]

Hi

What camera and scope do you have and how are they connected? You'd probably be better off using a laptop or lightbox for taking flats - more reproducible and less likely to introduce spurious gradients. Make sure you don't overexpose - you want to have your histogram peak at about 1/3 - 2/3 or about 25000 - 35000 adu. A lot depends on your camera so you may need to try several exposure times to see what works best for you.

Hth

Louise

MN190 with the atik 383L. Thanks for reply, yes I think a light box would be good, I will try laptop screen today and see.

Its strange because even with 3 separate adjustable halogens I still get the shadow!

[BenDavis]

Hi

What camera and scope do you have and how are they connected? You'd probably be better off using a laptop or lightbox for taking flats - more reproducible and less likely to introduce spurious gradients. Make sure you don't overexpose - you want to have your histogram peak at about 1/3 - 2/3 or about 25000 - 35000 adu. A lot depends on your camera so you may need to try several exposure times to see what works best for you.

Hth

Louise

No joy with laptop screen, I'm thinking its the atik OAG prism?

[Thalestris24]

Ah, that's possible... I've no experience of oag's so can't advise, I'm afraid. I'm sure it should be easy enough to check

[BenDavis]

Ah, that's possible... I've no experience of oag's so can't advise, I'm afraid. I'm sure it should be easy enough to check

Yeah it must be that there is nothing else in the way. Not sure what to do about it though!

[Thalestris24]

Another possibility might be your focuser?

[BenDavis]

Another possibility might be your focuser?

Turns out it was the camera shutter. Flats coming out ok now!

[ollypenrice]

With a shutter your flats need to have exposures long enough to eliminate the wipe of the shutter. I think we see this in the top left?

That said, there is nothing unusual about having an assymetrical vignetting effect in flats. Focusers and optics are not perfect - which is why we take flats.

The shadow of an OAG appears along one of the sides of the flat if it is going to appear at all. That certainly isn't the case here.

Olly

[Uranium235]

Ive seen this dark corner pattern before (I mean the really dark parts, top and bottom left).

Are you using 1.25" filters with the 383? If so, that is your cause right there - and no amount of flats will fix it.

In regard to the slight shadow to the top left (which causes the bright patch in the processed image, bottom left), that is the shutter. To be ultra safe, make sure your flats for any filter are at least 10 seconds long. In fact, the flat lengths I did with my 383 the other night were 28 seconds (Ha).

For a replacement lightbox, use a large monitor running either notepad or my preferred choice (Al's Virtual Lightbox).

[martin_h]

You could try one of these ........................... http://ledlights4you.co.uk/index.php?route=product/product&product_id=193&search=DAYLIGHT#.VKUe9tKsWSo

[BenDavis]

With a shutter your flats need to have exposures long enough to eliminate the wipe of the shutter. I think we see this in the top left?

That said, there is nothing unusual about having an assymetrical vignetting effect in flats. Focusers and optics are not perfect - which is why we take flats.

The shadow of an OAG appears along one of the sides of the flat if it is going to appear at all. That certainly isn't the case here.

Olly

Thank you Olly, that makes sense.

[BenDavis]

Ive seen this dark corner pattern before (I mean the really dark parts, top and bottom left).

Are you using 1.25" filters with the 383? If so, that is your cause right there - and no amount of flats will fix it.

In regard to the slight shadow to the top left (which causes the bright patch in the processed image, bottom left), that is the shutter. To be ultra safe, make sure your flats for any filter are at least 10 seconds long. In fact, the flat lengths I did with my 383 the other night were 28 seconds (Ha).

For a replacement lightbox, use a large monitor running either notepad or my preferred choice (Al's Virtual Lightbox).

Thanks Uranium, yes they are the 1.25 filters. I have been contemplating upgrading to the 2" but think I can live with it for the time being.

[Uranium235]

Ahhh, thought so. I found its just about tolerable at f6, but at f5 and below those 1.25" filters start cutting into the light cone - and that was with the FW butted directly against the camera.

The only (easy) way out of it I found, was to upgrade to 2". Worked out to be a bit on the expensive side though!

[BenDavis]

Ahhh, thought so. I found its just about tolerable at f6, but at f5 and below those 1.25" filters start cutting into the light cone - and that was with the FW butted directly against the camera.

The only (easy) way out of it I found, was to upgrade to 2". Worked out to be a bit on the expensive side though!AH

Ah yes that's what puts me off at the min, although if I ever go narrow band I will go for the 2" ones but will have to save up even more!