Telescope limits

[marshy123]

Hi all

Quick question about the maximum magnification I've read about; apparently 50x per inch of aperture is the maximum mag that is going to be useable for a scope on a perfect night, less for an average night (25 or 30x). I have a 130 (5.1") so on an average I am looking at getting 5.1 x 25=127x ish. BTW I don't have the paraboloid mirror version.

I have a 900mm focal length and an spc900 which I believe is effectively a 6mm eyepiece. 900/6=150x. Oops. I am over my maximum already. Also, with a 2x barlow, that's 300 x which is way over!

Can anyone please explain all this lark and also what actually happens over the maximum. I managed to image Saturn and, while the detail wasn't great, it was good enough for me!

Cheers

[Scott]

Telescope Science: All About Telescope Aperture

you might find this link helpful.

scroll down to the bottom and use the aperture link. make sure you put your telescope info at the top of calculator

[Cornelius Varley]

With imaging you can disregard the visual limitation of 50x aperture. The optimum focal ratio for lunar or planetary imaging usually falls between f20 and f30ish depending on the telescope and camera. I take photos of the moon with a Skymax 150 and Dmk 21 plus a 2x powermate which would give me an apparent magnification of 600x with no ill effects. If I was to use a DSLR at the same f number the apparent magnification is much less. Only the field of view has been increased.

Peter

[Mr Spock]

For me, I always find no more detail becomes available from about x1 per mm. Beyond that the image just gets larger. At about x1.5 per mm the image starts to loose contrast and degrades, though it may hold up beyond this for double stars.

Seeing conditions can also limit the amount of magnification. On most nights even the largest scope will stop somewhere between x200 and x300.

On a few rare nights, I've pushed my C9.25 to x392 on the moon. The image was suprisingly crisp. Recent views of Jupiter have been at not more than x235 however.

[RikM]

For me, I always find no more detail becomes available from about x1 per mm. Beyond that the image just gets larger. At about x1.5 per mm the image starts to loose contrast and degrades, though it may hold up beyond this for double stars.

Seeing conditions can also limit the amount of magnification. On most nights even the largest scope will stop somewhere between x200 and x300.

On a few rare nights, I've pushed my C9.25 to x392 on the moon. The image was suprisingly crisp. Recent views of Jupiter have been at not more than x235 however.

That's exactly my feeling too for visual observing.

With imaging things are different because cameras are more sensitive than your eyes and using frame stacking software can compensate for the seeing by selecting only the sharpest bits from each frame. This way you can end up with a sharp final image even though none of the individual frames look very good. (If your focussing is good...which mine is normally not )

[decomo50]

related to the above topic, seeing conditions are as important as any other factor. I once read somewhere that the amount of naked eye stars that can be seen in the square of pegasus is a good indication of seeing conditions? can anyone rememebr what the scale was?

[Naemeth]

Agreed on the seeing conditions, light pollution obviously makes a difference too.

[Mr Spock]

related to the above topic, seeing conditions are as important as any other factor. I once read somewhere that the amount of naked eye stars that can be seen in the square of pegasus is a good indication of seeing conditions? can anyone rememebr what the scale was?

That would be transparency rather than seeing. 'Seeing' is to do with how steady the atmosphere is. The steadier the atmosphere, the more detail you can see. When seeing is poor, the image can be turbulent or blurred, requiring a much lower magnification to get things sharp.

[decomo50]

I stand corrected Mr Spock

[umadog]

It goes like this: the atmosphere generally limits you to about 200x (perhaps 300x) so that's your ultimate limiting factor. So if you have a 3" then you can in theory push it to 50x per inch on most nights, since 50x per inch amounts to only 150x, which the atmosphere can easily handle. On the other hand, if you have a 10", you will never get to 50x per inch since there's no way the skies will ever support 500x. So in practice it's not really true that you can only do 50x per inch in perfect conditions: it depends on the aperture of the scope.

So what happens if you pass 50x per inch? The image gets dimmer, but that's not the important bit. Telescopes don't focus light down to a point, they focus down to pattern of finite size known as an Airy disk (Airy disk - Wikipedia, the free encyclopedia). This is also known as a point spread function (PSF). At low power, stars appear as points. At high powers (under perfect seeing) they will appear as Airy disks. If you massively over-magnify then the stars will look like ping pong balls. So basically, you reach a situation where you're getting no more resolution and are just blurring the image. This will hold for anything you see through a telescope (planets, moon, DSOs) but it's easiest to visualise what is going on if you think about stars.

So how much can you magnify before you see this effect? Well, you'll need to figure it out for your scope but here's how: The angle (in arc seconds) from the center of the Airy disk to the first minumum is 2.76/[mirror radius in inches]. So for a 2" telescope that's 1.38 arc seconds. The diameter is twice that: 2.76 arc seconds. 50x per inch isn't really a hard limit, but you'll need very good optics to reach it. With great optics you may be able to go beyond it if seeing will allow.

[marshy123]

Fantastic, thanks guys. Umadog, that was just the summary I was after. Cheers.