Donegal astro landscape with blobs

Hi all,

I'm new to astrophotgraphy.

I took this landscape shot in Donegal. It's taken with a Nikon D70, 18mm lens, f4.5, 235 seconds exposure. (Note Vega in the top right of picture). The building was illuminated by a combination of moon light and rear flash.

I have a problem with the purple blobs also on the right of the picture, I found that the the longer the exposure the worse they are.

Any ideas why or can it be prevented. I was able to remove it more or less in Photoshop......bit of a drag if I have to do that every time.


Sorrry when I previewed the picture it seemed a bit big on my screen!

Can anyone tell me how to make it fit the screen?!!

Regards

Aidan

Resize it in your photo manipulating software and upload it then..

I think

added:- can't help on the question sorry.

Aidan - good job I've got a 19" monitor

I find an image width of 600 pixels usually fits quite well on most folks screens. Open the image, then choose 'image' ad 'image size' from the top menu of PS. The adjust the width to 600 pixels. Set the resolution box to 72 or 96 pixels/inch. That should do it. I find those settings keep enough detail whilst keeping the file size down, which the Webmaster (Bart) will love you for... :lol:

The purple blob to the right (I only see one) might be a dust mote. Typically that's the effect you see on a image... You can check by shooting an image of the sky at twilight or dusk, a nice neutral background and not too bright (focus on infinity). They should then fairly apparent.

There are three ways to get rid of them:
1. Clean your imaging system
2. Shoot a flat frame (as described above) and subtract it from whatever image(s) you are taking
3. Edit them out in Photoshop!

Hope this helps....

By the way, nice shot. And welcome to the wealth limiting hobby of astrophotography!

Cheers

Dave McD

Aidan,

I recognise the purple blobs, They are in fact thermal effects in the image sensor, and will occur on all long exposures From what I've read, they appear in most DSLRS. I get these too with my Canon DSLR.

The way to remove them is to take a dark frame. That is an exposure of the same lenght as the actual exposure itself but with the lens cap on (or the lens removed and the main dust cap on). You then subtract the dark frame from the original, hence removing the purple blobs and other thermal noise which is inherent on all CCD/Cmos based sensors. (Thermal noise shows up as red and blue pixels randomly scattered around the image - look closely you will see them. If your not sure how to subtract the dark frame just tell me what image processing package you have? Photoshop, Paint shop pro, the gimp (which is free) all allow you to do this. Just let me know what you use and I can tell you how to do it.

Oh and BTW welcome to the boards and to astrophotography:)

Sorry I see you have photoshop. Heres how to do it.

Open the original image and the dark frame. Copy the dark frame to the clipboard and paste it into the original image as a NEW LAYER. Now go to the layer options and set the layer which contains the dark frame to "difference" the purple blobs and repeatable noise will vanish leaving just the original image.

I noticed those red and blues dots on the full size versions of my pics I posted a while back. Found them very peculiar! Cheers Dave.