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Walking on the Moon

Image size... and how to change

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Firstly apologies if this is a real numb nuts question but I really am taking my first baby steps in AP.

I have a roughly 600mm focal length  WO frac. Great for widefield shots etc. Now, speaking in more general photography terms, if I wanted to 'zoom in' on say Jupiter, how could I achieve a larger subject image ( obviously within the limits of the objective diameter, camera sensor etc )

Can I simply insert a Barlow to achieve this, and if so, will this need anything else to allow proper focussing?

Hope this makes sense and apologies if this is below even the most basic level of understanding. Like I say, baby steps.....?


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Yes indeed, the only parameters you have to play with for image size are focal length and chip size.  For a given optical train you can, and many do, use a Barlow lens (or a focal reducer to go the other way) to change the focal length of the system.  For Jupiter, people commonly use x2 or x3 Barlows, dependent upon telescope type, camera, and, of course, seeing conditions.


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In planetary imaging the trick is to use Barlows or tele-centric lenses like TV PowerMates to achieve a focal ratio that matches the pixel size of the sensor. For cameras with pixel sizes around 5-6 micron a focal ratio around 25-30 is roughly optimal,  for the 3.75 micron pixels of the ASI224 I have a value around F/18-F/22 seems to work well, whereas the ASI178MM I have (2.4 micron pixels) fares best at F/12, or thereabouts. Any faster an detail will be lost, as you go longer, little or no extra detail is gained, but exposure times go up

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This is great advice on the planets, whch are bright. Obtaining a 'closer' view in deep sky imaging is more problematic because any kind of focal extender slows down the focal ratio at a catastrophic rate. Double the focal ratio and you quadruple the exposure time. Not good!

It is a common mistake to think that a small chip will get you 'closer' to an object. It won't. It just limits your field of view. However, if you change to a camera with smaller pixels you will put more pixels under the object's image on the chip and, when viewed on your PC screen, each of these camera pixels will be given one screen pixel so the object's image on screen will be bigger.


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