A try at IC 342, why so noisy

[myk68]

Hi All

As it says on the tin

IC 342.tiff

This is a 20 lumin photo at 300sec, calibrated with corresponding darks, flats and bias. stacked in maxim and stretched.

now initially i thought id over stretched it so started again but I either get hardly any detail or more grain than this.

Have I just underestimated how many images I need or is it I need longer than 300sec on it.

Mike

[myk68]

How do I get the image in the post ???

[lukebl]

How do I get the image in the post ???

Save it as a jpeg, then post it. Done it for you! Not sure of the answer, though.

I have found that if you convert the image to RGB before processing and stretching it, you get a smoother result than processing in greyscale.

[myk68]

ah cool cheers.

[Martin-Devon]

I assume you're defintely cooling the Atik to -20 degrees or thereabouts for taking the subs? If so, then it's a little odd that the calibration frames don't take out more of the noise - I'd look at these to see if they're working properly as a starter. A lot of the grainy data can be handled in Photoshop though, just try Filter --> Noise --> Despeckle and run this a few times, it can make quite a lot of difference, then on top of this there are the usual noise reduction actions available.

[Keiran]

Save it as a jpeg, then post it. Done it for you! Not sure of the answer, though.

I have found that if you convert the image to RGB before processing and stretching it, you get a smoother result than processing in greyscale.

Try saving it as png, you get will get a better image as its ot compressed.

Keiran

[Martin-Devon]

The other obvious thing to eliminate is whether you were getting "dewed" up - even a thin film can cause this sort of grainy image, so check how well your dew heaters are working

[pete_l]

A quick pass through a script I wrote shows some attributes of the image. The black point is down at zero, which makes me think that either the exposure is too short or that the dark frame subtraction took a little too much off the bottom end.

.

The lower image in the attachment is with the histogram clipping indicated and a bit of a GAMMA boost to bring out the faint detail. What do the individual subs look like?

[myk68]

great replies thank you all.

Martin-Devon-

Set point is always -7 as i can get to this then through the day to do darks, flats and what have you

The weather was clear and seeing good. no dew on the imaging scope(its the guide scope im having issues with).

All processing was done in maxim, which only amounts to gamma adjust.

Pete_l

the individual subs when loaded and nothing touched do look a bit more grainy than previous shots. god i hope the cameras ok.

Ive attached an untouched stack tiff(uncompressed) if anyone wants to try and get more out, if you do please let me know the process you use. didnt realise i cant upload fits on here

I think I just dont have enough data/ images/ length of exposure. Ah well out again and try harder lol

IC342 20x300 -7 untouched.tiff

[myk68]

sorry tiff version wasnt untouched

IC342 20x300 -7 untouched2.tiff

[andrewluck]

Took a look at the untouched2 image. My first thought is that the background has a very high value at about 6000 adu. Do you have significant light pollution, either man made or the moon?

The second point about the background is that the standard deviation (measure of noise) is high at about 30. As the signal values from the galaxy are barely above this when you stretch to see the galaxy the noise is a major component.

Brighter stars are close to being saturated so an increase in individual subframe exposure lengths will result in the stars getting bigger, not really what you want.

Go back and measure the background noise in both an uncalibrated and calibrated subframe. Make sure that the calibrated frame noise is lower. Also, compare these values to the stacked image. There should be a significant decrease in noise in the stacked version. If this is correct then you can be confident that the calibration and stacking is doing what it should. In particular, noisy flat frames can seriously degrade images.

I suggest making some test darks, both cooled and uncooled to ensure that the camera coolers are functioning.

Cheers

Andrew

[pete_l]

Hmmm, delving into this a bit further I had a few more thoughts.

IC342 suffers from two problems, it's quite large (20x20 arc-min) AND it's quite dim.

With your 80mm scope and 1.4MPix camera I reckon you're seeing about 12 photons per pixel from the target in each of your 5 minute subs. The question then is: how much LP do you have?

If your skies are about Magnitude 4 visually, (i.e. 18 mag/arc-sec² photographically) then you'll be getting about 300 times as many photons from the sky background. However, if you have Mag 6 skies (21 mag/arc-sec²) then you're "only" getting 20 times more background photons than signal.

An 8 inch reflector with 20 x 10 minute subs in a Mag 6 sky should give you some nice shots of this difficult target, if that's any consolation.

[myk68]

Hi guys, thanks for the replies

I gave this object another go a few night back and even before calibrating it I could see the object and background were nearly the same.

I had started to think the same as Pete, that the LP was to high for this target, I'm not right sure how to tell what it is like around here but i am only about a mile or so from Darlington centre. Ive not had this issue before but then I think this is maybe the most challenging object and I guess I may have found my limit.

With some fogging issues in my SCT I'll have to hold on trying to image this with that for now. Sniff