Help - My new Dobsonian

[Steve]

... the same negative could produce a print at say 6x4" or 5x7" etc...

Yes, but the viewing distance changes. You normally hold a 6"x4" print closer to your eye, a poster sized print is typically viewed from a few feet and an advertising billboard from the other side of the road so the theory still applies .... more or less.

[Psychobilly]

Whatever

A shameful cut and paste ...

"magnification is a meaningless concept with imaging. It could be calculated however if you REALLY wanted to. You would need to know the focal length of your scope plus the size of your detector plus the number of pixels plus finally how large the print you ultimately make and to make things even more complex, how far you are standing from the final print!

Have fun with that!

I think dividing the focal length by 50 comes from the idea that in 35mm cameras the 50mm lens is considered "normal" or 1x. If your detector is smaller, than divide by the diagonal of your chip size.

However in the end it will still be rather meaningless as it only take into account half of what you need to know to calculate magnification."

Billy...

[Steve]

"You would need to know the focal length of your scope plus the size of your detector plus the number of pixels plus finally how large the print you ultimately make and to make things even more complex, how far you are standing from the final print!"

I think that is what I said

Not sure I agree with needing to know the number of pixels, unless there aren't enough to make your preferred print size at a sensible resolution.

[Psychobilly]

But because of the variables it makes the concept of magnification pretty pointless...

[Steve]

It is a common question so it helps to have a reference, even if it is a bit rough and ready

[SniffTheGlove]

The webcam in question is the Philips SPC880NC. I have looked all over the Philips site but can not find any details on this product however as it is supossed to be the same as the SPC900NC apart from the side button to take a still image then these are the details for the webcam.

Sensor VGA CMOS

Video resolution (Max.) VGA (640 x 480)

Image resolution (Max.) SVGA (800 x 600)

Illumination < 5 lux

Does this help to identify the sensor size?

[SniffTheGlove]

But because of the variables it makes the concept of magnification pretty pointless...

I am just interested in if I will be able to capture say Jupiter the same size as I would be looking through an EP of some dimension.

It would be nice to see the cloud bands and moons in as much detail as I would with using an EP as with the webcam. I know I can put barlows in but I am trying to gauge what the plain base magnification would be just with the primary optics if you know what I mean.

[barkis]

With film, I remember Covingtons formula was

Image size on film = focal length/110. So with your focal length of

1778mm the size of the moon on film at prime focus would be 16mm. I'm afraid I need educating when it comes to digital work though.

Ron.

[barkis]

F/7 is a relatively slow instrument for AP. Imaging Jupiter using eyepiece or Barlow projection, will fall foul of movement I would suspect. The exposures would need to be quite fast to be free of blurring.

[SniffTheGlove]

F/7 is a relatively slow instrument for AP. Imaging Jupiter using eyepiece or Barlow projection, will fall foul of movement I would suspect. The exposures would need to be quite fast to be free of blurring.

Until I get a EQ Dob Platform for tracking then long exposures are out and everything will be done by AVI and stacking until then.