View Full Version here: : Advice on Magnification limit

25-06-2007, 09:37 PM
I have a Skywatcher MAK 150mm OTA. Recently I read that it would max out at a magnification of 375. With the focal length of 1800mm I figured that a 5mm eypiece would do the job giving a magnification of 360. Since then though I have been advised not to go beyond 300, I was hoping too buy an Orion Stratus 5mm as I have been very happy with the 17mm I bought last year. The closest I can get in Orion is 8mm, yielding only 225. However if I go for the max magnification of 300 then the nearest quality eyepiece I can find is a Meade 6.7MM eyepiece. As a relative newbie I need advice.... Thanks.

25-06-2007, 09:48 PM
the 300 figure comes from the common rule of "50x per inch of aperture" so with your scope that comes out at 300x.

However, the only time you will ever really get to use this is when you have perfect seeing, meaning the eyepiece will rarely get used. You may be better off getting an eyepiece around the 8-10mm mark anyway if you want an eyepiece for higher power viewing, or alternatively getting a 2x barlow which will be the same as an 8.5mm eyepiece when used with your 17mm eyepiece that you already have.

25-06-2007, 09:49 PM
the 'seeing' will determine the highest magnification you can use at any given time, with my 12" i rarely go over 250x unless the seeing is very good.
you would get a lot more use out of the 8mm than a 5mm IMO, alternatively you could get a 2x barlow which would turn your 17mm into a 8.5mm

26-06-2007, 10:55 AM
As Kal metions the 50X per inch rule is a theoretical limit only, it rarely applies in practice because it does not take account of the prevailing seeing conditions . Some scopes because of poor optics will never get to this, others because of excellent optics will easily exceed it, in good seeing. Always remember that a large scope is more severely affected by seeing conditions and thermal stabilisation issues than a small scope. A 6" scope is small/medium but a Maksutov can take a long time to temperature stabilise because of the thick meniscus corrector lens combined with the closed tube design.

I think an eyepiece of either 7mm or 8mm would be ideal for your needs. This will give you 257X and 225 respectively which are good powers for lunar/planetary observation. You would find that because of local poor seeing, you would get a lot more use out of an 8mm eyepiece than you would out of a 7mm. I would consider the 8mm Stratus/Hyperion for 225X or the 8.5mm Pentax XF for 210X.

CS-John B

26-06-2007, 11:25 AM
In my 6" = 150mm Mak the highest useful magnification is not much over 200x for planets, but often more pleasing at around 170x. Any higher and the image starts getting too dim.

26-06-2007, 12:06 PM

Thats a personal call. It's a fine line when using a small scope to have enough magnification to give an adequate image scale, without dimming the target image too much and losing that additional detail.

CS-John B

26-06-2007, 12:18 PM
John, yes, good point. it is indeed usually a personal preference of mine for smaller but sharper and brighter image. Sometimes I do like to go nuts with power also though and it is fun for a change.

26-06-2007, 12:19 PM
As Kal, John and TJ advise, the 50 times per inch of aperture is only a theoretical maximum. In practice I work with only 30 times per inch for most seeing and have only gone as far as 150 x.

There will only be 2 or 3 days of the year where the sky and atmosphere might allow the seeing conditions to approach the theoretical limit the scope. As Steve suggests, the best view often isn't with the highest magnification, as there often isn't enough light to blow up the image, a bit like pixellation when zooming in on JPEGs.

I suspect that you share the common expectation to see planets in their full pin sharp, image enhanced, punchy colour glory of Hubble type images we see in magazines. There is no way you will be able to get postcard views of planets just using your eyes. :(

One factor often overlooked when determining the limits of seeing conditions is the limitations within the human eye itself. The Fovea part of the retina contains mainly Cones in the central part and Rods in the outer part. Cones give you colour vision during daytime, whereas Rods allow you to see black and white at night. That's why you often have to use averted vision at night to see dim faint objects in your peripheral vision, to get the light from the faint object to hit your Rods. At night your cones might as well have gone down the beach, since they need lots of light to determine colours, so in effect you are almost blind to colours and rely on your rods for black and white vision instead. So now you know why colours look faint at night, as we are mainly daytime creatures, well some of us are. :rofl:

26-06-2007, 10:30 PM
Thanks to all of you for your excellent replies. I have heard the message loud and clear and in fact ordered my Orion 8mm eyepiece today.