Pixel size

[jaygpoo]

Hi , I am a newbe to imaging and this site has been helpful. However there appears to be a mixed view on pixel size. From what I read if you use a lower image size ie 3mp the larger pixels will take in more light and the images will look better. I have a canon 500D with a range of images sizes. I was always of the opinion that if you wanted to blow up an image and look closer the higher resolution was the way to go. With deep space the opposite seems to be the way. Can someone clear this confusion up for me please as I am practising my star shots at present and the forever orange overview caused by LP is just one problem I am trying to overcome. I read that so of you are taking dark shots automatically with your cameras as you shoot images . Does the 500d have that in its menu ?

[riklaunim]

Smaller pixel - higher resolution but also less photons hit each pixel = fainter signal. In DS astrophotography high resolution is something you don't want to get unless the scope is very big and the camera efficient. The objects are just to dim.

[dph1nm]

The pixel size in microns isn't really an issue. What you need to look at is the pixel size in arcseconds when you have the camera on the telescope. General consensus seems to be that optimum size is about half your typical seeing (for DSO work - for planetary photography you may well want something smaller), as it is the seeing which determines the resolution of your shot.

The range of sizes of the Canon images is irrelevant, none of them are real changes in pixel size. This is fixed by the detector, and is what you get in RAW mode.

NigelM

[jaygpoo]

Ok thanks but I am still a little confused (being an old fart) Do I need to be taking images in raw ? and in the lowest megapixel setting? The seeing is what determines the resolution of your shot? Is it possible to break that sentence down for me to understand. Jay

[reddoss]

It is better to shoot in RAW and then convert to 16 bit Tiff files. You lose data in your image if you shoot in JPG mode.

[Shibby]

Ok thanks but I am still a little confused (being an old fart) Do I need to be taking images in raw ? and in the lowest megapixel setting? The seeing is what determines the resolution of your shot? Is it possible to break that sentence down for me to understand. Jay

To clarify a little further, RAW images are full resolution, unmolested images. By saving as a smaller (jpeg) image, the camera just resizes the image (then compresses to jpeg) before saving it to the SD card. Not what you want at all.

"Binning" might be confusing matters, a feature available in dedicated CCD cameras (but not DSLRs) to reduce noise by reading from several sensor pixels at the same time.

[dph1nm]

The seeing is what determines the resolution of your shot? Is it possible to break that sentence down for me to understand

The maximum resolution you can get out of an astronomical image is determined mostly by the turbulence in the atmosphere. This effectively blurs the shot a little (or alot on a bad night!). Another way of looking at this is to say that there is a minimum angular size any astronomical object will have. This varies with the weather conditions, but it is usually a few seconds of arc - this is the 'seeing'. No matter how small the pixels on your camera are, objects are never going to look sharper than this.

Now if you stick your camera on a telescope, you can work out how many seconds of arc each camera pixel will cover (there are calculators on the www which will do this). The ideal to aim for is (supposedly) to have two pixels per this miniumum 'seeing' width. In practice I doubt many people achieve this, if for no other reason that the 'seeing' varies from night to night and people's cameras general do not!

If your pixels are too small you are wasting disk space and processing power, if they are too large, stars will come out looking rather square.

NIgelM