helpwith flat fields

[Rigel003]

Extending my first attempts in DSLR photography and tried taking a set of flats for the first time. White T shirt over the end of the telescope. I ended up taking them the next day with the same optical train and temperatures may have been different. In my processed shots the dark smudgy spots (presumably dust on the sensor) have come out as bright coloured spots.

Can anyone give me advice about exposure length for these? Also are they meant to be white or gray? Mine looked pale pinky orange on the laptop screen and the dust marks and vignetting were only just noticeable.

Processed horsead photo attached. Spurious light blue splodge at bottom left, also blue streaks at top left and is that amp glow in the bottom right? Is flat fielding supposed to take that out too?

Grateful for any help.

[JohnG]

Hi Graeme

I will try and answer your questions for you.

Without knowing the combination of telescope, exposure times etc I will base my answer on my own experience with the Tak FS-102 and Caon 350D.

First off, that is Amp Glow in the lower right of the shot, the blue smudge is an internal refection.

As far a Flat Field shots are concerned, I use the white T-shirt method, I point my scope as far away from the sun as possible and use a blue sky (without clouds) as the backdrop. My exposures are usually from about 1/400 to around 1/650, when you take the shot, check using the View button by looking at the Historgram, the peak should be about 1/3 to the right. I have made a Master Flat (see attachement) by combining some 30 shots.

With the Amp Glow, you need to take Darks, the same length and, if posible, same temperature as your lights. Once again, I have a master dark for various exposure lengths, the attachement is a master dark, 600 seconds, used as the dark subtraction for the HH I have also attached.

There is some conjecture over whether you should take a Bias shot or not, dont if your Darks are taken at the same time as your Lights, it appears that the taking of a Bias 'could' introduce some artifacts which detract from the overall image.

I have included for your info a shot of the same area taken with the Tak FS-102 and 350D combination. Prime Focus, Lights and Darks @ 600 seconds, very little proccessing, just a quick run through the High Pass Filter in PS 7.01 and NoiseWare.

I am sure some of the other users will chime in here and maybe give better advice.

Cheers

JohnG

[Rigel003]

Thanks, John. It's good to hear from an experienced user with exactly the same telescope/camera combination. That info is just what I needed. I was looking at the histogram but didn't know what was desirable. I did the flats indoors the next morning with the t-shirted OTA facing a lit wall. The exposures were almost 1 sec. How crucial is it that the temp is the same for flats and lights? Presumably even at twilight it's warmer than when you're imaging later.

My exposures were 360 secs at ISO 800. I did 20 lights, 15 darks and 15 flats and also bias shots at 1/4000. Not sure about the internal reflection - it seems to correspond in position to one of the black spots in the master flat.

Finally, does the master flat need to be changed to grayscale? As I mentioned, mine had a colour cast when viewed on the laptop in Images Plus.

[JohnG]

Hi Graeme

If your tracking and guiding are good, I would go for a longer exposure, mine was a total of 50 minutes, 5 x 600 secs @ ISO 800. That blue smudge has all the hallmarks of an internal refection so it is interesting that it corresponds to another mark. I make a habit of using a blower to remove any residual dust from things prior to taking any shots, may be a worthwhile step to take.

As far as flats are concerned, it dosn't make any difference when they are taken so long as the telescope and camera are in the same orientation, the shots are so short it makes no difference that I can see.

I don't know if you do it or not but, all my shots are RAWs, Lights, Darks, Flats and Bias, I then use ImagesPlus to combine them in the Automatic Proccessing routine, I don't convert to Greyscale.

Cheers

JohnG

[Rigel003]

Hmm... I had the same optical setup and focus position but the camera may have rotated relative to the telescope when I took the flats. Wouldn't any vignetting be symmetrical anyway?

In Images Plus, I've been setting it to convert the RAW frames to TIFFs at capture, as each is downloaded to the laptop. Are you suggesting leaving this conversion until you do the automatic processing routine? - I suppose it would save a bit of time in the field.

[JohnG]

I use the Takahashi wide T-Ring, not a standard one and get very little if any vignetting.

I know my camera orientation when I take my flat field shots, I use the centre line of the telescope and camera and have marks on the focuser tube for my various combinations (Tak 1.6x extender or Tak reducer).

I use DSLRFocus for my focus and download control and only download straight to my Laptop, RAW shots, ImagesPlus will then convert your RAW's to TIF's.

The only jpg shots I do are for focusing.

A typical imaging run for me would be, focus with DSLRFocus, automate maybe 20 - 25 Lights, depending on the length of exposure, 30 - 60 seconds between shots, then darks with identical times, all downloaded straight to the Laptop as RAW's.

I then download to my main computer and Auto Process in ImagesPlus, all conversions etc are down within that process.

Your end files within ImagesPlus should end up with a Master Dark, Master Flat and a Combined Average, these will be TIF's.

Hope that helps a little.

Cheers

JohnG