"Simulated exposure" photo experiments

[cr1]

My camera is a Nikon S10 (with tripod). There was only exposure bias option available (no manual exposure and aperture control) so I took 6 photos continuously and used Fireworks (program similar to Photoshop) to put them together. I used the "Addition" blend mode to blend the 6 photos together.
I pointed my camera to a random point in the night sky (where there seemed to be no bright stars), since I don't really know how to keep track of celestial places...



I was surprised there were so many stars in this final picture. I made sure they were not photographical artifacts by comparing the 6 photos.

Nothing spectacular really, just thought I might share that... for some reason I feel quite excited about my first "real" sky photo

 

[McWgogs]

I suspect that the sharp points of light on this quite noisy image are not in fact stars but hot pixels. You can verify that by taking pictures of total darkness (put the camera against a pillow or something)
The two bright blobs on the lower part of the photo look like real out of focus stars.

 

[TSPenguin]

Too bad the camera is really not suited for astrophotography. Maybe you can optimize your settings with the information on dpreview.

 

[cr1]

I've now taken photos with my camera lens covered.
It seems that what you said, McWgogs, is true; there were colorful dots on the result, dots that I've mistaken for stars.
Even the two blobs near the bottom edge were "fake".

I just had an idea: if I use the "substract" or "divide" blend mode blended to the "covered lens photo" and put them on the top layer, and the night sky photo on the bottom layer, would the "covered lens photo" substract the hot pixels from the night sky photo, so that I get a result that is void of hot pixels ("fake" stars)?

@TSPenguin: Thanks for the link, I'll take a look at it.

 

[TSPenguin]

The ususal method is to take a full white and full black photo and subtract those. You should be able to find some tutorials on this when searching for lightbox and ccd.

 

[cr1]

Found this interesting program (it was mentioned in a thread similar to this one): http://deepskystacker.free.fr/english/index.html

Wonder if that would help with my camera.

 

[TSPenguin]

You need photos with well distinguishable features for stacking software to work.
If you don't see a star in one of your pictures, you won't see one if you stack hundreds of them...

What are the properties of the photos you make? exposure, ISO etc.
Could you post the EXIF information?

 

[Nerull]

I've now taken photos with my camera lens covered.
It seems that what you said, McWgogs, is true; there were colorful dots on the result, dots that I've mistaken for stars.
Even the two blobs near the bottom edge were "fake".

I just had an idea: if I use the "substract" or "divide" blend mode blended to the "covered lens photo" and put them on the top layer, and the night sky photo on the bottom layer, would the "covered lens photo" substract the hot pixels from the night sky photo, so that I get a result that is void of hot pixels ("fake" stars)?

@TSPenguin: Thanks for the link, I'll take a look at it.

That's called a dark frame, and needs to be done with the same exposure time and temperature as the real picture to work well.

 

[gilly_54]

I'd like to add something, your picture seems a bit too "yellow" for a night sky, did you take this picture near a city? Light pollution could make it "noisier" and distort the final result.

 

[cr1]

Hmm yea I took the pictures on my balcony right in the city.

For the photos...
F-stop: f/3.5
Exposure: 1 second
Exposure bias: +2
Sensitivity: 800
Max aperture: 3.6
35mm focal length: 380

I don't have direct/manual control for exposure time, I can only modify the exposure bias.

 

[agentgonzo]

I just had an idea: if I use the "substract" or "divide" blend mode blended to the "covered lens photo" and put them on the top layer, and the night sky photo on the bottom layer, would the "covered lens photo" substract the hot pixels from the night sky photo, so that I get a result that is void of hot pixels ("fake" stars)?

This is very common in low light photography and many programs have the facility to do this. In fact, many digital cameras have settings for this on them. The camera will take a photo in low light with the shutter open, then after the exposure is complete, it will take another photo with the shutter closed (the same as having the lens cap on) and then subtract the second image from the first to give you your photo. If your camera has it, it will be in the manual as something like "black frame subtraction" or something similar.