Subs and Flats

[enigmacell]

Hi,

weather here is going to be terrible for the week so I thought I'd learn how to do a bit of DSO imaging. Could someone please explain to me what subs and flats are? Also when taking an image of jupiter does everyones image start of about the size of a smiley as there is no eyepiece?

[hemihaggis]

how are the clouds in irland today ?? , okay ...

a sub is just the name of individual pics ..ie aec sub is 1 30sec exposure pic .

a dark is a pic taken with the main lens cover on the scope

a flat is a picture taken with a white cloth over the the end of scope (these both are taken after your "subs" have been done so the temperature and other factors are similar )

the image of jupiter will be very small , i take it you are using a dslr to take this image ?

a really good way (and cheap !) is to get hold of a spc900webcam from morgans this includes the ir cut filter and the adapter , use a barlow and bingo ...

cheers

[studdedsole]

Hi, Are you really sure you want to get into this, if/when you get hooked you'll need to dig ever deeper into your pockets.

A sub is simply a single image frame taken with an either a CCD or DSLR. We take many subs and combine them by stacking to improve the signal to noise ratio so that we can stretch the stacked frame to reveal the low level dim signal without exposing excess noise.

A flat field image is one of a number of calibration frames used to remove fixed noise from the signal (light) subs.

The imaging train, CCD+telescope(lens)+filters and other optical attachments, can result in imperfections in the light frames due to dust, vignetting (uneven lighting). A master flat frame can be used to subtract these imperfections from the light frames.

If you have some spare time the attached gives a good short explanation of each of the calibration frames we can use and how they affect the quality of the final image.

Happy reading

Tony

Jim Solomon's Astrophotography Cookbook

[enigmacell]

Thanx,

I got a cheap dslr body (manual is in polish though! but found it in english online) and will be getting a 5x barlow soon. The t ring adapter should be with me by monday hopefully.

Do the darks and flats have the same exposure time as the sub or are they much shorter? How are they different from just adding black images and white images to the files to be stacked?

[enigmacell]

Just noticed how long it takes me to write a message lol! Thanx I'll give that a read now.

[studdedsole]

Darks have the same exposure as the light frames but with a lens cap on or telescope covered. They contain electronic noise from the sensor related to the ambient temperature and a fixed pattern of hot and cold pixels due to defects in the sensor. Dark frames also show up any amplifier glow if this is evident in the exposure time.

Flat frames are a little more difficult but the rough aim is to take an image of an evenly lit background and control the exposure time to a percentage of the full well depth of each pixel site, (roughly 50% from memory as I don't use these) - exposure is typically less than 5 seconds.

[studdedsole]

Left hand side is an image with dark spots due to dust the right hand side is the flat frame. Subtracting the flat from the image will remove the dark spots.

The images are copied from this site

Flat Frames

[enigmacell]

Ok starting to get the hang of this I think. So when I have my subs darks and flats do I just put them all into registax and start processing or do I have to do much more first? Also do I take lots of subs and only a few darks and flats?

[ollypenrice]

Flats are applied by division and darks by subtraction but the point is well made above. Webcams do give better results on planets than DSLRs and the inherent tracking errors mean that the same pixels don't sample the same bit of the object so the noise drastically diminshes when 1000 to 2000 video stills are stacked and averaged.

Olly

[studdedsole]

Flats are applied by division and darks by subtraction but the point is well made above.

An unfortunate choice of words on my part using "subtract" - Olly thanks for clarifying that important distinction

[Astronut1982]

Hi,

Once you have your subs, flats & darks, use a free program call Deep Sky Stacker. You can download it here :-

DeepSkyStacker - Free

It takes a bit of getting used to at first but its worth it. My advice would be to read a few tutorials & speak to people on here & you'll be taking awesome snaps of the sky in no time at all. Thats how I got started a few weeks ago & I'm now taking some decent pics of DSO.

Also, I would recommend taking about 10 - 20 flats & darks each using the same exposure length, ISO setting & temperature as your subs. It is entirely up to you how many subs you take but bear in mind the more you take, the more detail you'll be able to pull from them.

Jeff

[enigmacell]

Thanx for that. I've just installed that software. Wish all these rain clouds would disappear now so I can get started.

[ollypenrice]

You could do an awful lot worse than buy Steve Richards' book Making Every Photon Count. It goes through a shedload of stuff that really do need to know about AP.

Olly