Do my flats look ok?

[StuartT]

Last night I got a good session of a couple of hours on a DSO. I left the whole rig as it was and this morning I did my flats by pointing to the zenith (fully overcast sky) and stretching a white t-shirt over the aperture. I then used the flats wizard in NINA. But I seem to have a gradient. Is that right? Shouldn't flats be.. well.. flat?

 

 

[Shimonu]

The point of flats is to capture the vignetting and dust motes, things that affect the distribution of light on the sensor. It's not expected to be flat, it's supposed to make your image more flat as an end result.

How does it look when you use it in your stacking?

Edited October 2, 2021 by Shimonu

[StuartT]

On 02/10/2021 at 10:20, Shimonu said:

How does it look when you use it in your stacking?

I tend to find my images look better without the flats added! (so presumably that doesn't say much for the quality of my flats - as they are supposed to improve the image, right?)

I've also tried using a tracing panel from Amazon, but the light was just too uneven.

I'm reluctant to shell out £250 for a fancy flat generator panel, but maybe that's the only way? Surely this shouldn't be this hard?

[Stuart1971]

13 minutes ago, StuartT said:

I tend to find my images look better without the flats added! (so presumably that doesn't say much for the quality of my flats - as they are supposed to improve the image, right?)

I've also tried using a tracing panel from Amazon, but the light was just too uneven.

I'm reluctant to shell out £250 for a fancy flat generator panel, but maybe that's the only way? Surely this shouldn't be this hard?

Get one of these, they are superb, also sold on amazon, with 5 easy Interest free payments…

https://www.rothervalleyoptics.co.uk/geoptik-flat-field-generator-210mm.html

[Stuart1971]

Or this one if your scope is under under 160mm

https://www.rothervalleyoptics.co.uk/geoptik-flat-field-generator-164mm.html

Edited October 3, 2021 by Stuart1971

[Stuart1971]

Here on amazon

https://www.amazon.co.uk/Telescope-30b303-Generator-Field-Plan-Illuminated/dp/B002TR1WX6/ref=sr_1_6?dchild=1&keywords=Geoptik&qid=1633257116&qsid=261-9505794-0091520&sr=8-6&sres=B074DKHY75%2CB002P4FSWY%2CB002TQZRRY%2CB002TR1WX6%2CB0017GXHQA%2CB0153EZXWW%2CB01DL8MRMG%2CB0153EZFHU%2CB0153EZGZ6%2CB0153EYRTC%2CB00Z97WLK6%2CB077QFM69R%2CB0017GZB4Q%2CB002P4RYJO%2CB005JW2G06%2CB00342USVC&srpt=TELESCOPE

[vlaiv]

22 minutes ago, StuartT said:

'm reluctant to shell out £250 for a fancy flat generator panel, but maybe that's the only way? Surely this shouldn't be this hard?

Look up one of these:

https://www.amazon.com/Facon-Surface-Interior-Indicator-Motorhome/dp/B0751K8Y6F/ref=psdc_11439431011_t2_B08S2T5WCW?th=1

or similar.

You just need to fashion some way of putting them on telescope.

[Clarkey]

I use one of these with a few sheets of paper under it.

AGPTEK Magnetic Tracing A4 LED Light Box Drawing Pad Dimmable Brightness, Physical Buttons Control, USB Power Cable for Animation, Designing, Stencilling X-ray Viewing/Sketching Diamond Painting : Amazon.co.uk: Home & Kitchen

For my smaller scopes I use a tablet with a 'white screen' app.

[ollypenrice]

The flat you post is suspicious in just one respect: there is no vignetting in the lower left corner. It's not impossible that this represents the truth about the way your optics are illuminating the chip but I have never seen optics vignette clearly on three corners and not at all on a fourth. Apart from that, the flat looks perfectly credible. Personally I expose for slightly less time to bring the histo peak down to about a third of full brightness but this isn't all that critical.

So my question would be, why no vignetting in the lower left hand corner? Does that corner come out too dark in the final image when you apply this flat?

Olly

[StuartT]

1 hour ago, ollypenrice said:

The flat you post is suspicious in just one respect: there is no vignetting in the lower left corner. It's not impossible that this represents the truth about the way your optics are illuminating the chip but I have never seen optics vignette clearly on three corners and not at all on a fourth. Apart from that, the flat looks perfectly credible. Personally I expose for slightly less time to bring the histo peak down to about a third of full brightness but this isn't all that critical.

So my question would be, why no vignetting in the lower left hand corner? Does that corner come out too dark in the final image when you apply this flat?

Olly

Hmm.. interesting. I am not aware of one corner of the resulting image being dark. By the way, do you know what is causing the two or three circular artefacts of slightly darker green?

But I am learning a lot about flats, so I hope to get better at this. I have in fact just made myself a gizmo to make daylight flats with the scope pointed to the zenith. Two sheets of perspex with cloth between.

[tooth_dr]

1 hour ago, StuartT said:

By the way, do you know what is causing the two or three circular artefacts of slightly darker green?

That would be dust somewhere in the optical path.  Perfectly normal and in essence part of the purpose of flats - to flatten out the image brightness.

Here is a flat I took the other day, with the same camera, on a 7" newtonian.  The darkness is even around the corners.  Annoyingly the small dark mark in the top left corner is some dust on the sensor that was their from day 1 when I received the camera.  It's under the glass so I cant blow it off.

[StuartT]

ok, so just point up at sky (4pm, well away from any sun) and with my new perspex gizmo on top. Here is what I get. (rotating the gizmo does not affect the pattern of vignetting)

I've adjusted the exposure to be more 1/3 to 1/2 of the histo)

 

[tooth_dr]

1 minute ago, StuartT said:

ok, so just point up at sky (4pm, well away from any sun) and with my new perspex gizmo on top. Here is what I get. (rotating the gizmo does not affect the pattern of vignetting)

I've adjusted the exposure to be more 1/3 to 1/2 of the histo)

 

It's possible light is getting in somewhere.  That would explain the gradient in my eyes.  I've given up taking my flats during the day for that reason.  I use the cheapo flats panel on ebay (A3 one I had to buy) and a piece of opaque acrylic over it (others use tracing paper, but I like the robustness of the acrylic and it's only a couple of quid.  If you can try flats at night and post your image?  I have used my laptop screen a few feet away to good effect too.

 

[Clarkey]

As said above it is probably light getting in. Unless is completely dark I put a lightproof cover the whole scope to do flats. The acrylic sheet arrangement will allow light in from the sides. I try to do my flats during set up now, that way if I have to cut a session short I already have the flats done in the dark.

Edited October 3, 2021 by Clarkey

[StuartT]

this has all been super helpful. Thanks SO much everyone.

So, if I have daytime light leakage --> do flats at night
cheap Amazon tracing panel gives uneven illumination --> bite the bullet and purchase the geoptik flat generator panel

Decision! 👍

Edited October 3, 2021 by StuartT

[tooth_dr]

9 minutes ago, StuartT said:

this has all been super helpful. Thanks SO much everyone.

So, if I have daytime light leakage --> do flats at night
cheap Amazon tracing panel gives uneven illumination --> bite the bullet and purchase the geoptik flat generator panel

Decision! 👍

Me and many many others make do with the cheap panel.  Try with your t shirt on the scope first to diffuse the light from panel and rotate 180 degrees for the second half of the flats.

[Clarkey]

If you tape the white t-shirt (or some A4 white paper) between the flat panel and an acrylic sheet this makes a very even flat panel. This is what I use for up too 200mm scopes.

[Dazzyt66]

+1 for the t-shirt approach. I do this outside in daylight and it works great 😀

[tooth_dr]

10 minutes ago, Dazzyt66 said:

+1 for the t-shirt approach. I do this outside in daylight and it works great 😀

No disputing this works, but when you scope is letting in light somewhere, it's not an option.