How to work out maximum magnification

[outbacknick]

Hello I would like to know how to work out the maximum magnification for particular scopes I.e 8" dob. Thanks

[madwayne]

Hi Nick

Here is a bit of light reading for you off the Bintel site. Alot of very useful information there. If you go to their site and click on the eye piece link at the top of the home page, as opposed to the retail section on the left hand side you will find more information about eye relief etc.

"HIGH-POWER VIEWING
Once you’ve selected an eyepiece set based on field stop sizes, calculate the magnifications produced with your scope. For planetary or double star observing, you’ll want an eyepiece in at least the 150x range. For determining maximum power, a good rule of thumb is to use no more than 60x per inch of aperture for scopes with apertures up to 6". Higher magnifications may still be pleasing but will not likely reveal any additional detail. Realistically, the atmosphere will usually limit your planetary observing to a maximum magnification of about 300x, no matter how large your telescope aperture.
Basically, you’ll be choosing low and medium power eyepieces by field stop increments to "frame" the subject, and high power eyepieces by magnification increments (based on your scope’s aperture), to reach the optimum contrast and resolution for viewing planets and double stars."

Hope this copy and paste helps answer your question.

Clear skies and wow you must have some dark skies out there too.

Wayne

[torana68]

Google is your friend : http://astronomyonline.org/Science/TelescopeMagnification.asp

or if you mean max per inch of apperture its about 5ox give or take a bit

[Screwdriverone]

Hi Nick,

It depends a lot on the seeing conditions (turbulence and transparency of the atmosphere) but generally its about 2 x your aperture which in your 8" = 200mm case, about 400x magnification.

Now 8" scopes are either 1000mm or 1200mm focal length (on average, if its a newtonian reflector) so the eyepiece you need to get to 400x is going to be a 2.5mm (1000mm FL) or a 3mm (1200mm FL) and at this small EP FL size, the eye relief will be small (need to get REALLY CLOSE to the glass to see anything) so you are better off getting say a 6mm eyepiece and a 2x or 2.5x barlow to get to that magnification level but retain the eye comfort of the larger eyepiece.

Generally, if you have a 1000mm FL scope, a 10mm eyepiece (which gives 100x mag) and a 2.5x barlow (gives total of 250x mag) will be about as far as you can really go in most conditions. To work out the magnification of an eyepiece, divide the eyepiece FL into the scope FL ie. 1000mm / 10mm = 100.

For example, the SMALLEST EP focal length I have (my scopes are 1000mm(5" reflector), 1000mm(8" reflector) and 1500mm (12"dob)) in my collection is an 8.5mm Pentax which gives me a magnification level of 117x or 176x. If I want to bump that up, I slot in my 2.5x barlow to get 292x or 440x (on the big dob). Now on the 12" dob, the theoretical limit is 600x but I have never got that as its too much and the image also gets too dim.

Hope this helps, I tend to ramble on a bit, but I thought I may as well answer as many following questions at the same time.

Cheers

Chris

[astro744]

Using exit pupil as your guide works for any sized telescope to give you maximum magnification. Whether you can utilise that magnification will depend a lot on your seeing conditions and the quality of your optics.

Exit pupil = eyepiece focal length / telescope focal ratio.

Images under 0.5mm exit pupil become too dim and eye floaters are common.

Therefore for a 200mm, f6, (1200mm f.l.) telescope to get a 0.5mm exit pupil you need a 3mm eyepiece. (x/6=0.5, therefore x=3). A 3mm eyepiece will give you 1200/3 = 400x on your telescope. This would be you maximum magnification.

Here's another example for an ED80 refractor, 80mm, f7.5, (600mm f.l.) telescope (ED80 refractor). x/7.5=0.5, therefore x=3.75. 600/3.75=160x. Therefore 160x is your maximum magnification for an 80mm telescope to give you a 0.5mm exit pupil. Note refractors can take a little more power as there is no central obstruction but the image really does begin to dim below 0.5mm.

A third example for a 400mm, f5, (2000mm focal length). x/5=0.5, therefore x=2.5. 2000/2.5=800x. Therefore 800x is you maximum magnification giving 0.5mm exit pupil.

And finally for a 200mm, f10, SCT, (2000mm f.l.). x/10=0.5, therefore x=5. 2000/5=400. Therefore 400x is your maximum magnification.

You can see from the above results that aperture / magnification = exit pupil but I like the first formula as you don't have to work out magnification first. Also works for minimum magnification as generally maximum exit pupil size of 7mm is aimed for. eg. 35mm eyepiece, f5 telescope, exit pupil = 35/5=7mm. This would be an excellent low power eyepiece for any f5 telescope under dark skies.