9 replies to this topic

[Francy]

Hi, my name is Francesco and I'm writing from Italy. I have a question for you that I can't find an answer to. Two telescopes, one 60mm and the other 120mm, with a PR of 2" for the first and a PR of 1" for the second. If we point them at the Moon and consider a seeing of 2", will the 120mm one, despite the seeing, still have more resolution than the 60mm or will it also stop at the limits imposed by the seeing? Thanks.

[TOMDEY]

Submit your topic just one time; otherwise redundant posts appear on site.    Tom

[PEterW]

Is this a night vision or a refractor forum question, the moon would nicely fry an image on to your phosphor tube!

[dawnpatrol]

Submit your topic just one time; otherwise redundant posts appear on site.    Tom

The duplicate posting was likely caused by a glitch and not deliberate.

 

Is this a night vision or a refractor forum question, the moon would nicely fry an image on to your phosphor tube!

Moving to Refractors for a better fit.

[Astroman007]

Yes and no. It depends on how bad, or good, the seeing is. Under terrible seeing conditions the larger scope will simply magnify and increase the blurriness except at very low power while reasonable views for the conditions may be obtained with the smaller scope leading to a loss of advantage of the larger aperture until conditions improve enough to reap the benefits. That's the case for visual and I know nothing of night vision etc. but I'd imagine seeing would also limit the effectiveness of the technology.

[Sean Cunneen]

A small scope in good seeing is better than a large scope in bad seeing.

[Francy]

many thanks for the answers

[Jay_Reynolds_Freeman]

I would say that there is no definitive answer -- I have seen conditions when the larger telescope is favored and others when the smaller one performs better. Part of the problem is that '2" seeing' is a little vague -- seeing is complex.

 

Clear sky ...

[hyia]

Hi, my name is Francesco and I'm writing from Italy. I have a question for you that I can't find an answer to. Two telescopes, one 60mm and the other 120mm, with a PR of 2" for the first and a PR of 1" for the second. If we point them at the Moon and consider a seeing of 2", will the 120mm one, despite the seeing, still have more resolution than the 60mm or will it also stop at the limits imposed by the seeing? Thanks.

I am not going to disagree with any of the above.  For simplicity, however, I would view the telescope and the target separately.  We can say, yes, the scope with the higher resolution ability still has higher resolution ability.  The part which is difficult to answer is whether this matters with regards to what you are viewing.  So, if the details which you want to see are smaller than the seeing allows, you will not see the details in either.  If the details are larger, then you will benefit from having a scope with higher resolution ability.  The note that many would add is that even in poor seeing, there may be moments of good seeing.  Best Regards.

[Jon Isaacs]

My 2 cents;

 

So exactly what does 2" seeing mean?   

 

As someone who enjoys viewing close doubles, I would say this:  The Dawes limit for a 60mm scope is 1.93", the Rayleigh Criterion is 2.31", these are difficult splits and require rather extreme magnifications because the Airy disks are overlapping.  It actually takes considerably better seeing than 2" for a 60mm to split a 2" double because of overlapping Airy Disks, its a very low contrast split so any smearing by the seeing makes it impossible. 

 

If the 60mm is able to split the 2" equal magnitude double, if that is definition of 2" seeing, then the 120mm will be able to split significantly closer doubles because it is not fighting with the large diameter Airy Disks and the smearing effect of the seeing.

 

From the other side, if the 120mm were barely able to split the 2" double, then I would not expect the 60 mm to be able to make the split. 

 

Some years ago, Vladimir Sacek (Telescope-Optics.net) reported some simulations of seeing.  My recollection is that an 8 inch scope was optimal for 2" seeing.  

 

Jon

Edited by Jon Isaacs, 14 April 2025 - 04:23 PM.