Object size

[keef_uk]

Hello,

Im still getting my head round all the numbers (maths ) involved with this hobby.

I look around on the board and I see people are taking lovely large moon shots - single shots not mosaics (which I still need to learn) just by connecting their cam to the scope.

However when I do that I just get a small shot - see attached - is this something to do with focal lenght?

I am using a Nexstar102SLT which is a f/6 with my 400D connected. When I connect my Neximage webcam though I get a massive image.

Cheers

PS of someone could point me to a tutorial on how to take mosiac shots with my setup (stitching bit I can do) that would be really great

[brianb]

The difference in scale between the DSLR and the webcam is because the DSLR has a much larger sensor - the size of a large postage stamp, whereas the webcam sensor is the size of a newborn baby's fingernail.

Mosaicing - when taking the individual frames, keep the exposure the same & make sure you have plenty of overlap (1/3 of the frame is good, 1/4 just about tolerable). Resist the temptation to reduce overlap, you'll find you miss bits. Do the terminator first, that's where the light is changing fastest & matching problems are most likely.

[keef_uk]

The difference in scale between the DSLR and the webcam is because the DSLR has a much larger sensor - the size of a large postage stamp, whereas the webcam sensor is the size of a newborn baby's fingernail.

Mosaicing - when taking the individual frames, keep the exposure the same & make sure you have plenty of overlap (1/3 of the frame is good, 1/4 just about tolerable). Resist the temptation to reduce overlap, you'll find you miss bits. Do the terminator first, that's where the light is changing fastest & matching problems are most likely.

Thanks for that but this bit I get - what I dont understand is it something I would be able to do with my setup since the shots are so small with the DSLR

[Ant]

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[ollypenrice]

They are not smaller on the DSLR, they are exactly the same size. But on the DSLR you are also capturing a lot of surrounding sky as well. Your computer software just shrinks your image to fit the screen, but take it to 100% on the screen and it (the Moon) will be the same size as the webcam image. The image scale is determined exclusively by effective focal length, which you can change using barlows, powermates or, going the other way, reducers.

At your present focal length with the DSLR you don't need to mosaic because all the moon fits on your chip. Add a 2 or 3x barlow and it will still fit, by the looks of it.

The reason folks use webcams on the solar system arises from their extreme speed. They beat much of the atmospheric turbulence AKA seeing. By combining a vast number the dismal quality of any one image is transformed entirely. The fact that fast frame cameras have small chips is simply a pain! If you have deep pockets they can be had with bigger ones.

Olly

[StarryEyes]

That makes sense.

Thanks for posting the question keef_uk and thanks for the explanation Olly.

A question that I would probably have been asking when I start imaging.

[StevieDvd]

Is that why my first image of the moon was too big for the webcam? I was having issues with the webcam/software and when I finally sorted it I shoved the webcam in my spotter scope and happily got an image.

I was expecting it to be small due to the low power scope but I could not fit the whole moon in the view on the PC.

The web cam was fitted into the diagonal, should I have removed that or would I need a spacer for that scope?

[Cornelius Varley]

There is a formula to work out how big the moon will be when photographed. Divide the focal length of the telescope (in millemetres) by 109. This will give the diameter of the moon in millimetres.

Peter

[Ant]

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