Magnification and Seeing

[Bart]

Hello All,

From a number of threads it appears to be readily accepted that magnifications above 200x are too much for UK (and probably Irish) seeing. Certainly when I've tried to go above that the viewing is awful. However, if 200x is about the max, what would the norm be? When I observe Saturn in my 6" reflector which i think is reasonably well collimated, I get a lovely crisp image at 100x, and then at 166x there is a marked deterioration in the image. I can determine the gap between the rings and the planet but the detail is not great.

I wondered at 166x is this the seeing, or is it a fault of the kit (collimation, cheap celestron eyepiece kit bought in eagerness at astronomy onset)??

Thanks.

Bart

[Mark Evelkin]

It depends also on the size of the mirror.

10 inch=500x

8 inch=400x

6 inch=300x

2x per every mm of the mirror. But its limits of the telescope with ideal weather.

[Carbon Brush]

Sounds like your kit is performing reasonably well and the weather is to blame. I use some electricity pylons arond 2Km away for scope/eyepiece checking. Amazing how different they look on different days.

David.

[John]

It depends also on the size of the mirror.

10 inch=500x

8 inch=400x

6 inch=300x

2x per every mm of the mirror. But its limits of the telescope with ideal weather.

Those are theoretical figures. In practice, as you suggest, the seeing conditions will limit the max useful magnification to somewhere between 180x - 240x even with 10" scopes.

Occasionally it's possible to use more, but not often I've found.

[Moonshane]

there are general rules and then occasions when they can be broken.

I would agree that it's more likely to be conditions other than your scope that are affecting views of Saturn. e.g if you view over rooftops after a warm day, the heat rising will create turbulence which reduces the possible magnification. atmospheric seeing does the same and generally for planets restricts mags to perhaps 150-200x on average.

BUT theoretically, my 6" f11 newtonian scope should not be able to give a good image at 533x (almost 90x aperture in inches or 3.5x in mm). OK I use excellent quality eyepieces, but I routinely use this magnification and certainly between 350-400x to good effect on double stars.

on planets it's about the same magnifications you were mentioning (sometimes more) and on the moon 250x is usually perfectly fine.

do be aware of the 'issues' with planetary observing. i.e. you need to leave the focus alone once you find the position of sharpest focus. the seeing will bring the image in and out of sharpness but it will eventually come back and perhaps 10 seconds in every minute (spread in little split second bits) is all you will see in sharpness unless you reduce the power right down to maybe 100x.

personally, I push up the magnification until the image breaks down and then back off a step or two. sometimes though a nice wide view will e.g. reveal more moons around Saturn.

play it by ear and more time spent with the scope will hone your skills in these areas.

[Bart]

Thanks all for comments/reassurances

[RikM]

With my 127 and 150mm scopes about 150x mag gives me the best views of the planets. I found with 200mm and now 254mm scopes that 200x and 240x respecively are possible. (same object, same garden, same night, same eyepiece, same eye). I think the extra apperture gives a brighter and sharper image. So for a typical night I think of 1x per mm.

[acey]

A study of optimal magnifying power was done by T Lewis in 1913. He looked at the favoured magnifications of a large number of double-star observers and obtained the following empirical formula:

magnifying power = 140 x square root of aperture in inches

His paper is here:

Double star astronomy

The formula gives the following suggested optimal magnifications:

4" x280

6" x343

8" x396

10" x443

12" x485

14" x524

16" x560

Obviously this is limited by the seeing, and also by the type of mount (hand tracking gets hard at very high power). But it should be born in mind that Herschel regularly used powers above x600 - and his eyepieces were of a very simple type, with small fields of view. In practice you just keep raising the power until you can see no further benefit.

As for the seeing, the best that can be said is that it varies, and you just want to catch the moments when the air is least turbulent. There have been lots of studies of seeing by professional astronomers, especially in connection with adaptive optics: the scientific measaure of seeing is the Fried parameter, which for typical UK skies is around 4" - implying that any aperture larger than that is not going to gain anything, and you might as well stick to the optimal 4" magnification of x280. In practice, though, we know that we can do better than that in good moments.

Astronomical seeing - Wikipedia, the free encyclopedia

Fried parameter - Wikipedia, the free encyclopedia

[Moonshane]

that's a fascinating read Acey. I'll look at the links shortly too.

[RikM]

So do I understand it correctly that you can use higher powers on luna because of the brighter surface and on double stars because point sources don't diminish in brightness as you increase magnification. Planets, being less bright than the moon and not point sources, are more susceptable to seeing / scope limitations hence a lower magnification must be used before the view degrades?

Quite a few times I have barlowed my 8.8mm to give 270x on Saturn and it has always been too much. 10mm+2x for 240x has always been much brighter and sharper. Because of this, I have never tried using higher than 240x on the moon and doubles.

I am in a housing estate, so there are plenty of heat plooms to mess with my local seeing though

[Moonshane]

I think that's spot on Rik and certainly supports what I see through the scope. Planetary nebs can often taker higher mags than planets etc too for the same reasons.

give high magnification a try on some doubles. obviously the ones at the zenith are more stable but the other night I looked at Saturn and it was a little wobbly at 175x whereas I managed a good clean split of Porrima at 260x (and probably even more) despite them being over a house roof after a hot day.

apart from anything else, it's a great way to check your collimation

[bish]

On Saturn I tend to use x300 - x400 unless the seeing is terrible. I would much rather look at something which is a bit bigger but not quite as sharp and see more detail that way. I'm probably in the minority but tiny bright images are a waste of time for me no matter how sharp they are. The main thing is to find what works best for you, some people can see loads of detail at x150.

[Moonshane]

good philosophy Bish. there's so many different views on this sort of thing and the only one that really matters (to yourself) is your own!