460EX Noise.

[mickhyde]

Here is a 1:1 crop of the Pelican Nebula. Shot with Sky-90, 460EX, Baader Ha 35nm, 600s, 1x1 bin, @-20c.
Converted in FITS Liberator 3, lightly levelled and cropped in CS5.

The thing I'd like to know is, is this noise normal? One of the reasons I bought the 460EX was for it's low noise, but it actually seems fairly noisy.

Any ideas?

[topboxman]

I've never heard of a 35nm Ha filter. The largest I've heard is 12nm. I have 5nm Ha and I get the same result as yours. If you do proper calibration like calibrate the lights with darks, bias and flats, then the image will look cleaner. I don't do dark subtraction, I use Nebulosity's Bad Pixel Mapping to remove bad/hot pixels. Is your image calibrated?

Taking images with narrow band filters generally require heavy stretching to reveal DSO. Heavy stretching can make the images noisy but these noise generally have very low ADU values. You can measure the ADU values with your capture or processing software. To remove these low ADU noise or bad pixels is done by calibration.

10 minutes sub-expsoures is generally too short for narrow band imaging. I typically use 30 minute sub-exposures.

Peter

[mickhyde]

Hi Peter,

Many thanks for the reply.

The filter is here:- http://www.altairastro.com/product.php?productid=16357
I'm still waiting for my narrow band filter set to arrive.

This image is not calibrated, just one of the fits from the session. I have tried darks and bias on sets before now, but I've seen no difference in the output. Only flats seem to work. I'm using DSS by the way.

Still experimenting with things, as long as this looks fairly normal I'm happy. Didn't want to do a load of work and find out there was a problem with the camera. I'll research ADU values.

Thanks again.

[NickK]

I've never heard of a 35nm Ha filter. 

I'm also a 35nm Ha owner, so they exist


Hmm what is the power voltage at? The 383L is normally run at a higher voltage than straight 12V - such as 13V which seems to resolve a large chunk of it's noise. Maybe the support guys can say if you can use ~13V too..

[mickhyde]

I'm using one of those Bench PSU's from Maplin, that actually measures 5A 13V. Same one I use for my CGE mount.

[MikeM]

My guess is that there is nothing wrong, just a too short of an exposure for the use of an Ha filter, regardless of the bandwidth.
I believe that if you shoot longer, i.e., 20mins or combine a few more of the 600s images the noise will dissipate.

[mickhyde]

Thanks very much Mike.

[topboxman]

Not only calibrating your Ha images with darks, bias and flats will help, so will dithering Ha images during capture. Stacking dithered and calibrated Ha images will further clean up the Ha image.

Peter

[mickhyde]

Ah yes, forgot all about dithering. Thanks