Black Flats

[Johns22]

This morning I attempted my first flat calibration frames on my Skywatcher 200P, using a white tshirt and the morning sky at 7.15am. Setting my Canon 600D SLR to AV mode, I took 20 photos but all were completely black. I repeated the process with just one layer of the tshirt over the scope but still they were completely black.

At this point, the sunlight was across the tshirt but still the images were completely black. I did check all lens covers were removed!

Please can anyone advise what am I doing wrong?

 

[Merlin66]

Can only assume your exposure time was too short - longer exposures needed.

 

[michael8554]

AV mode should have calculated the correct exposure ?

What exposure is showing in the image info?

Michael

[Johns22]

File info shows 1/2000 sec. 50mm @ ISO800 with 2 layers of t-shirt, then 1/2500 sec. 50mm @ ISO800 with 1 layer of t-shirt. This suggests the AV mode has detected the change in light?

Files attached as above, in case there's any clues...

IMG_1499.CR2 IMG_1539.CR2

[michael8554]

Here is your Flat, and one of mine.

The only difference is you had a 50mm lens attached, mine was attached to a scope, both with Aperture Priority.

Could you see a grey image in the viewfinder?

I can only suggest you take Flats in Manual, with Shutter Speed and Aperture preset.

(Do you have Auto Rotate set on your camera ?)

Michael

 

  

Edited March 27, 2020 by michael8554

[Johns22]

Hi Michael,

The camera was directly connected to the scope when the flats were taken so not sure why the photo's say a 50mm lens was attached. Could this be a setting in the camera?

I haven't changed anything in the settings in relation to Auto rotate - should that be set?

Cheers

John.

Edited March 27, 2020 by Johns22

[michael8554]

That's very puzzling then.

A daytime test is hold the camera body about 12" away from a plain wall in your house, take an AV shot, and look at the result.

If that looks okay try again on the scope, if no joy  try a range of shutter speeds until you get one that gives a histogram on the camera screen that's in the middle of the range.

Auto Rotate ON could be why your image is displaying portrait instead of landscape.

Michael

[rickwayne]

You've no exposure adjustment dialed in for Av mode, I assume.

[Tommohawk]

If its not a silly question, why are you using AV mode? Why not just go to manual and adjust shutter speed as required - that always worked for me with Canon 550D. Maybe with no lens connected AV mode doesnt function normally.

[Tommohawk]

Another consideration. I think you maybe using sRGB colour space which can do odd things. Try shooting as RAW and see what happens cos colour space doesnt matter then, or experiment with AdobeRGB colour space..... 

Edit... just checked my 550D is set for sRGB and never gave issues so thats a red herrring. Try Manual and RAW

Edited March 29, 2020 by Tommohawk

[Johns22]

Thanks Tommohawk, I had seen the tip online to take flats in AV mode. I will try again on the next imaging session to just adjust the exposure to see if I can resolve this.

[tooth_dr]

1 hour ago, Johns22 said:

Thanks Tommohawk, I had seen the tip online to take flats in AV mode. I will try again on the next imaging session to just adjust the exposure to see if I can resolve this.

I also used AV mode for flats. It’s works 100%.  You could use the +/- dial to fine tune if you wish. But I 1/2000 is too short. Try a few more layers of T-shirt to get it down to 1/50 or there abouts. 

[tooth_dr]

21 hours ago, Tommohawk said:

If its not a silly question, why are you using AV mode? Why not just go to manual and adjust shutter speed as required - that always worked for me with Canon 550D. Maybe with no lens connected AV mode doesnt function normally.

Ive never had any issues with AV mode with telescope attached.

[Tommohawk]

As you say it ought to work, but in AV mode you can't set the histo to your liking. Also, with no lens attached, how does the camera calculate the correct exposure? I guess it just assumes maximum aperture. In which case why doesn't it work for OP? 

I would still try manual... at least you can fiddle with it to see what's going on. 

[michael8554]

Just to round up some of the comments. 

AV mode should work on all Canon cameras, I've used it on everything from a 300D to a 6D.

AV mode exposes, with or without a lens attached, to give a mid-grey exposure, which is ideal for flats. 

I imagine the camera's exposure system calculates the required shutter duration. 

The Op's image is too dark, so adding more layers of T shirt will only make it darker. 

Johns22, did you try taking an AV exposure pointing the body at a plain wall, how did that look? 

Beginning to sound like the camera's exposure system is faulty, as even with the lens cap on   😞   it should have produced a long exposure mid-grey. 

Colour Space is for geeky Colour Technicians in isolation ..... 

Michael 

 

 

 

[alacant]

Hi

I'm with @Tommohawk on this. Trying M and adjusting the shutter speed takes the camera's Av exposure metering out of the equation.

Cheers

Edited March 30, 2020 by alacant

[tooth_dr]

1 hour ago, Tommohawk said:

but in AV mode you can't set the histo to your liking

You can using the wheel to adjust the exposure stops.  You don’t have this set to -5? @Johns22

Edited March 30, 2020 by tooth_dr

[Tommohawk]

By a bizarre coincidence I have my SW200 set up for doing flats with my ASI1600MM as we speak. Indoors, very bright daylight, 2 layers T shirt.

It took me all of 1 minute to switch to  the EOS 550D and I can confirm that in AV mode I get exposures of 1/13th second with mid grey exposure, so agree that method seems fine. If I set it manually to 1/13th second it looks the same. So the 1/2000 exposure you are getting on AV seems way out.

I would suggest going to manual to  test, but does look like AV mode isnt working as it should

[michael8554]

You don't need any Exposure Comp with AV for your DSLR flats, it takes the perfect mid-range  exposure.

Michael

[Tommohawk]

4 minutes ago, michael8554 said:

You don't need any Exposure Comp with AV for your DSLR flats, it takes the perfect mid-range  exposure.

Michael

Certainly my test just now confirms this. I've just gone through all the settings and the only way I can make AV come up with significantly faster exposures is if the compensation is set to -5. Then I got 1/400th sec, with  ISO at 800. So OP def should check that exposure compensation is set to zero.