Decent quality flats for DSLR

[Psychobilly]

The shape of the histogram will depend on the source used and the amount of vignetting that your system produces...

The important thing us where the hump is...

I used to use Av mode to get the exposure value and then shoot the flats in M mode - only to make sure that each flat in the set had identical exposure times...

You might want to shoot sets of flats a half stop or so either side of the nominal and see what works best for you..

As long as your not seeing horizontal banding with your "source"  your on the right track...

Peter...

[sharkmelley]

Flats need to be as noise free as possible hence I always use ISO 100 for flats even though I usually use ISO 800 for lights.  Just set your panel brightness and exposure time so the histogram is 1/3 from the right.  A narrow histogram peak is actually an excellent result.

Mark

[julian489289]

Thanks Mark, 1/3 from the right. I will have to have a play to see what settings achieve that. 

Really 100 ISO,  I always thought that your flat should be the same as the light?? 

[Psychobilly]

I shoot my flats at the same ISO as my lights ... 

Avoid "paralysis by analysis" and experiment...

Peter...

[sharkmelley]

The purpose of flats is to quantify the relative sensitivity of pixels as well as vignetting and dust.  None of these are affected by the ISO setting, so I go for the ISO setting that gives the least noise i.e. ISO 100.  On the other hand, darks and bias should always be done at the same ISO as the lights.  To be honest it is only nit-picking - the important thing is get some good flats, whatever ISO you feel happy with.

Mark

[wxsatuser]

My best quality DSLR flats come from using a light diffuser panel over the end of the scope in daylight - a "white cloud" day rather then a sunny one.

Search for "3mm OPAL ACRYLIC 500 x 500 mm Light Diffuser Sign Box Plastic Sheet Panel" on eBay - less than a tenner.

http://www.ebay.co.uk/itm/400143709684

Taking manual exposures, increase exposure length until part of the image just begins to saturate and then back it off by 1 or 2 stops.

Mark

Going to try some, for the price a no brainer.

[Psychobilly]

Skyflats are great if you have a permanent setup.. But if you don't then you need to take them before or after the session thats where the light panels come into it... 

If you can find a local sign making company they will sometimes have off-cuts of the material and you can pick it up cheap...

The histogram will broaden if you have significant vignetting or dirty optics...

Peter...

[wxsatuser]

Mine is permanent so should be ok.

I try to keep the front element of what ever I'm using clean.

After about 3 or 4 sessions there is normally some dust and some fine other residue, not sure what it is.

Most likely polution in the air, right under Gatwick flight paths, and there are at least 3 woodburners nearby as

I can smell the stuff all the time.

Sensor should be clean, keep the clip filter on all the time.

Must start doing some calibration frames......been rather lazy on that score.

[Psychobilly]

I had an A3 EL panel in a diffuser box mounted on the wall of the obs and used to slew the scopes around to it to take my flats...

Peter...

[julian489289]

Thanks for the further additions guys.

Firstly, do you really need an acrylic sheet in front of the EL panel in order to get the best quality flat?

I have redone my flats for the other night's session.  This time however, I have set my light panel screen and exposures in order to get a histogram in the 65-75% area.

I have found that setting my screen to 75 for brightness (range 0-255) and 1/30 exp, I achieve this.  However, I have noted that when I take these flats looking at the histogram in live view on the back of the camera looking at the light panel, the histogram shows about midway 50%.  However, if I then view the picture on my camera, the histogram shows 75%.  Why is the live view and actual sub's histogram different?? Which one shows my histogram more accurately?

https://www.dropbox.com/sc/39yeeqf0ee6js5k/AAA1Y1NKnQdUg6pY5TseWahda

https://www.dropbox.com/sc/xjd1enwvxt5yo09/AACXPeFrNyq5bEmudc0itFc9a

https://www.dropbox.com/sc/ucwyiwdpjdszy5n/AAA7jasaCVeddg7YZ6AablpTa

[wxsatuser]

A guess.

May be the histograms are different because two different metering methods were used.

I believe the Liveview image is evaluative metered but your exposure is spot metered.

[Psychobilly]

If you have light flats and bias all at the same ISO just chuck them into DSS and let it sort it out... It won't moan too much that way....

EL panels and LED panels like  the tracing pads are driven differently...

I always had better results by running the panel flat out and using a diffuser... Diffusers are only really necessary if  you get banding  and/or need a shorter shutter speed ...

As I have said before provided your getting flats with no noticeable banding straight from the camera or when aggressively stretched you should be ok...

Peter...

[julian489289]

Thanks Peter

No, I do not get any banding so hopefully I am there now.  Thanks for your help along the way

Thanks to everyone else as well.