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Dom1961

Highest power eyepiece for a 650mm f/5 scope

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Will I be able to drop any lower than a 9mm eyepiece to use on my scope or is 9mm the lowest useful mag I can go?

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What's the aperture of the scope?

It is said maximum useful magnification is somewhere between 30 and 50 x per inch.

So if the scope was 10inches in aperture, the maximum useful magnification would be 300 to 500 times; nearer 300 than 500.

To calculate magnification, divide the focal length of the scope by the focal lenth of the eye piece; so if the focal length of the scope was 1000mm, and the focal length of the eye piece 10mm, the magnification achieved would be 1000/10 = 100x

James

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That would give around 360x which would be difficult to track with a scope which given the size I would presume is a manual dob.

Theoretically the scope could handle more but the atomospheric seeing often limits things around the 300x mark for me unless things are particularly stable.

Any chance of a picture, 26inch f5 is a big scope ! Unless its 650mm long rather than wide of course .... in which case the advice ive given above is wrong.

Cheers

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130 f/5 should take a 5mm if it is a reasonable eyepiece, the BST and X-Cel's are the obvious at not that great a cost. Think both have good reports at f/5 and a little faster.

TV Plossl's are rated to f/4 but stop at 8mm.

There is also the WO 6mm planetary that gets glowing reports, however at one time WO said some (not all) of their eyepieces were not recommended at faster then f/6. The SWANS were one such eyepiece they recommended f/6 of slower, but the wording disappeared. The 6mm appears from the general reports to be good however.

You have it would seem:

9mm X-Cel, 8mm BST, 7mm X-Cel, 6mm WO and either the 5mm in BST or X-Cel.

The problem is that you have to get one and possibly keep getting others until one is no good.

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My guess is that this would be a 130mm aperture newtonian scope ?.

Useful eyepiece focal lengths for an F/5 130mm newtonian would range between around 32mm (20x = lowest power) and around 3mm (217x = highest useful power) I would say.

A barlow lens plus a longer focal length eyepiece might be a sensible way to achive the higher of those two.

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It's not a problem, don't mention it.

------------

Have a look at this website, you can plug your scope and eye pieces in, and it works out the magnification and actual field of view:

http://www.12dstring.me.uk/fov.htm

Someone has kindly suggested above that a 3mm is the highest power eye piece you should go for, so you should be able to use most eye pieces down to 3mm. So yes the 4mm and 6mm ones will be ok.

The problem with barlows, in my experience, is they do degrade the image a bit. So using a 2x barlow on a 10mm eye piece will not give you the same clarity or brightness as using a prime 5mm eye piece. Having said that, a barlow is a wonder addition to a collection for all sorts of reasons (achieving focus when inaging with a camera on a newtonian, making planets bigger for imaging etc).

Your scope is a great entry scope, but unless you live somewhere with a very dark [non-light polluted] sky, i think you'd get most striking views at lower magnifications and therefore wider fields of view, looking at clusters and the like. Others may disagree.

Good luck.

James

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Thanks for linking the site Jambouk, just the sort of thing I was looking for

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you may find Revelation 6mm and especially 4mm, if they are Plossl, hard to use as eye relief is tiny

If you dont have a decent Barlow consider one with removable lens element (like Revelation) so you can have 1.5x and 2x

My 100p takes 9mm with 1.5x ok but not 2x

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I have, somewhat late, come around to considering EPs primarily with respect to exit pupil. There is a train of thought that a 0.7 exit pupil is a useful guide to smallest focal length EP (max mag.) Less than this "visual acuity is significantly degraded" (Illustrated Guide to Astronomical Wonders - Thompson & Thompson). For the 130/650 this give 130/0.7 = 185x. This roughly coincides with how far I felt I could push the magnification on my Celestron 130EQ on a good night. That was looking mainly at the moon which as a bright object will take magnification more readily. In reality I was more comfortable with less and for many objects  a lot less is better anyway.

A 3.5mm EP is needed to get 185x. So an 8mm EP (Starguider) with a Revelation 2.5 x Barlow (very good but in reality nearer 2.25x) would achieve that. That said, I would not get too hung up about chasing the very maximum the scope will give you.

Edited by Alfian

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But just because you have a 3mm or even 4.5mm (9mm with barlow) doesn't mean it week work OK so much depends on the sky clarity that night in that observing moment. A larger blurry image is just not as pleasing as a smaller sharper one. You may find you use your 9mm more.

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