aengusastro

Stephan's Quintet:

This is a combined image of 6, 300 sec images
With no processing other than automatically adjusting the background and levels with CCDOPS. It was imaged using the F3.3 Reducer and in High Res. The image has been cropped.

M109
A track and accumulate image 12 x 60 secs at F10. Autodark and Flatfielded.
Unsharp mask in Photoshop.

This is one of the first images I have taken with the permanent mount.

M104 The Sombero Galaxy

In Virgo at a distance of 65 Million Light years....

M51 The Whirlpool Galaxy

In Canes Venatici....

Only 15 Million Light years away!
16 x 60 sec Track and Accumulate at F10 in Medium resolution

M94 The Croc's Eye Galaxy

15 x 60 secs Track and Accumulate at F10 in Medium Resoultion

There is a lot of detail in that bright glow and when I ever figure out how to process images I might get to see it!

Dark Frames and Flat Fields:

Here is a raw, uncorrected iamge of M105 in Leo......

It contains dots which have nothing to do with the stars in the sky....the dots are caused by CCD.
While the image is being taken some pixels receive unwanted electrons from the electronics in the chip itself...the higher the chip temperature the more electons the electrics generate...hence one of the reasons we cool CCD Cameras...

Here we have a dark frame...an image taken for the same length of time, but with the shutter closed......this can be an automated process with each image that you take...but then you spend half the time imaging the sky, and the other imaging the back of the shutter....but you can take a series of dark frames once the camera has reached it's operating temp...and save the dark frames to subtract from you raw images later...when you are finished......

Can you see some of the electronically generated dots here and then above........in the original

So we subtract one from the other....very easily done by the computer and now we have an image free of electonic noise......

But that is not our only problem with CCD images...

Over the period the image is taken variation in intensity can effect the image due to dirt/dust on the optics....or vingnetting due to focal reducers....or in my case some light reflections due to nieghbours lights below where I was imaging.....

It might not be obvious but it's there in the image..

So before the evening starts I take an image (or series of images...which I later add together and take a mean from the lot)..Called a Flat Frame, and this is what this image looks like....loads of dust donoughts...and darker at the top than at the bottom...(Actually....the bright band at the bottom is due to the light being reflected off my dew shield....which is still while cardboard...!!!!

The flat field is taken around dusk...or against a white evenly illuminated card....if I don't get started before dusk....(partners are handy for holding up the card....children tend to wave them about)

They are only short images.....sometimes only 0.11 sec....and then they are saved to disk....later to be used to correct the real 'important' images......well......flat field and dark frames are important images in their own right but I think you know what I mean....

Anyhow.....below is the result of dividing the image that has been dark subtracted by the flat field......

You might not think there is much of a difference beween this image and the one above the flat field...but look again....this one is more even where the one above shows the effects of the dark band at the top and the light band near the bottom...

Have a good look and see if you can see what I mean..

Now...there you are, you understand flat fielding and dark frames....

Well there is a lot more to it but thats a start..