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Burgess Optical 91mm Triplet Eyepiece Selection

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With time on my hands waiting for Burgess Optical to deliver their next shipment of 91mm triplets I thought I'd do a bit of research into eyepiece selection for this scope.

Vic Maris at Stellarvue recommends 20, 13, 7 and 3-6zoom Naglers for the SV90T, so I used this as my starting point. Forum member Phil M however (who already owns a BO91) made the valid point that a 2x barlow would negate the need for the 7mm Nagler (and we've the 6mm end of the zoom), so we're down to the 20, 13 and 3-6mm.

Then I came across this Eyepiece Primer by none other than Al Nagler himself http://tinyurl.com/y5klan

Essentially what Uncle Al is saying (as I understand it) is that eyepiece selection should be made on the basis of individual exit pupil size when mated with a particular telescope.

The formula used to calculate exit pupil in the article is: exit_pupil = objective_dia / magnification

I've been using a different formula which is: exit_pupil = eyepiece_focal_length / telescope_focal_ratio

If we test these formulae for the Burgess 91mm Triplet which has a 600mm focal length and a f6.6 focal ratio we can see they return the same values.

For example, using the Paragon 40mm eyepiece we see that:

1. exit_pupil = 91/(600/40) = 6.07mm

2. exit_pupil = 40/6.6 = 6.06mm

In the final paragraph Al recommends a selection of eyepieces (for any scope) that yield exit pupils of 0.5, 1, 2, 4 and 7mm.

Using the above formulae this would equate to eyepiece focal lengths of 3.3mm, 6.6mm, 13.2mm, 26.4mm and 46.2mm for the Burgess 91.

So...working to these values my ideal eyepieces for this scope would be:

1. The included 40mm Paragon (close enough to the widest exit pupil in the above list)

2. 24mm Panoptic (the 26mmT5 Nagler is over 400 quid!)

3. 13mm Ethos (well we've saved some money buying the Panoptic :()

4. 3-6mm Nagler zoom

Not a very imaginative choice I know... and expensive. But then you've got yourself 4 elite eyepieces that'll give you years of service.

What do you think... would you do different?


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Sounds great Col. I would go ahead and buy them and lend them to an existing BO 91 owner to "test them out" for you while you are waiting on your scope :D

I do like the idea of the Nagler Zoom. It's a gap area in my eyepieces for this scope. Last night I was using a barlowed 8mm Radian on Saturn and it would have took a bit more. Given I'm sticking a lot of glass in the way then the 3-6 could be a nice planetary eyepiece for this scope.

The 24 Pan is a bit tighter on eye relief than the discontinued 22mm, but then it's also a lighter eyepiece.

As for the ethos - where's a drool icon when I need one? Envy will have to do :(


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Choosing an eyepiece range by exit pupil is fine but will obviously only 'work' with one scope/ eyepiece range combination at a time. Another thing to note is that you still tend to end up with the largely the same eyepieces that you would of if you'd have just gone for a even spread of magnifications....

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As for the ethos - where's a drool icon when I need one? Envy will have to do :(


:D Yeah Phil. I may be going to Canada and the US later in the year so I might try to pick one up then. At current exchange rates they're over £100 cheaper.


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