Streaky image

[Marine Bio]

Hi,

I've just produced my first image of M51 which admittedly contains very little data. It is made up of 50x30sec light and 3x30sec darks, stacked in DSS. I know it needs a lot more data but I wondered if someone could tell me what causes the streaky effect in the image?

many thanks!

M51.tif

[Daniel-K]

i use to get that if my darks were taken at a diff temp

[rfdesigner]

Hi,

I've just produced my first image of M51 which admittedly contains very little data. It is made up of 50x30sec light and 3x30sec darks, stacked in DSS. I know it needs a lot more data but I wondered if someone could tell me what causes the streaky effect in the image?

many thanks!

M51.tif

I've had this on my DSLR. No matter what I tried I couldn't get rid of the effect, only reduce it with more data and more darks. (3 isn't enough really. ideally you need at least 8, preferably more)

The biggest problem with DSLRs is that you don't have control over the sensor temperature and to make darks work you need to be quite close.

A program I use (IRIS) has a clever routine that correlates the noise in a dark frame with the noise in your image and then tries to scale it to match, but it still isn't perfect.

Derek

[Marine Bio]

Thanks guys, I guess it probably is a temperature thing. I took the darks at the end of the session and it was definetely a lot colder by then!

Just as an extra is there a correct ratio of dark to light frames?

[rfdesigner]

Just as an extra is there a correct ratio of dark to light frames?

Ah... cue big argument.

There are some that insist on taking several times more dark frames than lights. In my very concidered opinion this is not necessary, it all boils down to image registration. If each light frame needs to be aligned, then any errors in the dark frame are shared out between a series of different locations in the final image, so you only need to concider the required number of dark frames for a single light. Generally I would want to see 8 or more.

Process order is very important.

1. Dark removal

2. Flat field

3. Align

4. Stack

Under no circumstances should you ever align before either flat fielding or dark removal. The idea of darks and flats is to remove both pixel to pixel variations in the sensor and gradual field of view variations in the scope. Don't smear flat fields to make them smoother.. take more if you want them smooth, then you capture variations in pixel sensitivity.

But don't just take my word for it. Take a look at this: http://www.astrosurf.com/~buil/iris/tutorial2/doc8_us.htm

hope this helps

Derek

[Psychobilly]

My solution to streaky DLSR images was to stop using darks.... This worked for me with a 1000D

I use Lights, Flats and Bias and dithering instead...

Peter...