Do I need to enter the world of flats?

[Vega]

I've been taking darks during my photography sessions and using the auto-image set processing in ImagesPlus to process my darks with my lights. I've never entered the realm of flats but not sure if that will solve my problem. I seem to suffer from quite large area's of 'noise' (vignetting ?) on my images. Most epsecially noticeable on images over 60 sec @ ISO 1600. I only notice it when I move the slider left in the digital development section of ImagesPlus. I'm not talking about random hot/dead pixels, it's usually a large area of the image and most often reddish in colour. I can get rid of it by not stretching the slider quite so far (less detail brought out though) or use the method as described below...

-Gaussen blur the image and make a copy of it.

-Edit the blurred image. If it's an image of a galaxy or Nebula, cut and paste a dark area over the brightish areas that I want to keep in the image and re-gaussen blur it (rather crude but kinda works)

-Use arithmatic to subtract the blurred image from the original image (et voila).

Is there a better method or do I need to enter the dreaded realm of flats :?

Vega

[EDIT: I will upload a sample of one of my M31 subs later tonight to show the noise in question. It might be amp glow but it's not on the edge of the frame so not sure. If it is amp glow thats quite worrying cus that indicates that im not processing my darks properly ]

[Vega]

Here is an example of the noise... Are my darks failing ?

Vega

[SteveL]

yourdarks are fine, its flats you need... I`m just getting to the same point with my stuff, and trying to wor out the easiest way to take flats at 3-4-5am

[Astroman]

You need flats for each configuration of your setup. If you move the camera, you need new flats. A filter wheel makes it easier, as the camera itself doesn't move, but you need a flat for each filter. You can take them at twilight by aiming at the sky after sunset, but before twilight ends, (sky flats). There are also dome flats, box flats, t-shirt flats and use-your-imagination flats. The key to flats is a uniform light source that is divided into the image to even things out and take visual flaws out.

[Vega]

You probably saw this coming but... a few questions...

1. So is outside temperature important in relation to taking flats compared to lights? If so, an awkward one by any stretch of the imagination. :shock:.

2. I saw somewhere someone who had a white screen installed in their obsy shed for taking flats with. It got me thinking though, should flats be taken with camera attached to scope as it would be for the lights or is that not important ? :?

3. How many flats should be taken in relation to lights/darks etc ?

4. What settings should be used ? (equal to lights ? no hang on wouldn't that overexpose big time ?)

Sorry for all the frantic questioning.... Can of worms moment ahead.....

Vega

[SteveL]

flats need to be taken with the camera attached EXACTLY how it was when you were imaging (which can be tricky to do at a later time).... they are used to eliminate any uneven illumination (including vignetting) in the imaging path. As such, they need to be taken with the camera attached, and any other imaging kit that you used at the time (same scope, focal reducer, extension tubes, coma correctors, etc).

[Vega]

Having read up on it here I've been mightily scared off :shock: for these reasons..

-You have to use exactly the same focus and scope positioning for your flats as your target :shock:

-If you take flats before your imaging sesssion, the camera should NOT be removed even to find a difficult target otherwise your flats are useless :shock:

Might just stick to my trusty gaussen blur / subtract arithmatic method!

Vega

[MartinB]

1) flats need very short exposures, the temperature isn't important

2) As per Steve and AM

3) Flats can introduce noise so you need a few to avoid this. Seems reasonable to aim for 30 flats. the recommended combine method is median. I have read why but I can't remember now!

4) You should be aiming for around half the saturation point of your camera pixels. This varies from camera to camera but is invariably less than the half way point of the brightness levels i.e 128 for 8 bit 32500 for 16 bit. I aim for around 25000 reading in Maxim. This deterimines the exposure time.

The hardest part of flats is getting even illumniation. Sky flats and t-shirt flats are easy to do but not normally at the same time as an imaging session when it's dark - ok if you image until dawn though. A light box is a good solution.

[SteveL]

If I was looking at getting a light box, am I correct in assuming that it must be at LEAST as big (wide or tall) as the aperture of the scope you intend to use for the flats? i.e NS8GPS would need something at least 203mm across at its narrowest, or preferably larger...

[MartinB]

Yep! I'm trying to cobble something together that works with all my scopes but polystyrene engineering isn't my forte :?

[Astroman]

MartinB has a good post. I have a 12" dome flat mounted on the dome for my C8, and I use a nice hot metal halide lamp for when it's dark. I mostly do sky flats with my filter wheel. The position of the scope isn't as important as the focus with an SCT. I'm thinking of building a light box, but one of ~7,000 projects to do.

If you take the time to learn flats and their application, your photography results will be very, very much worth it. Plus, you'll be ready to jump right into real science as well!

[Vega]

Anyone got a pic of these light boxes ? They sound cool

Vega

[Robd]

This months Sky at Night mag has an article by Martin Moberly on how to make a light box.

Rob