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Whistlin Bob

BST Starguider at f4.7?

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Hi all,

I often go for whole sessions without removing my Baader 8-24 zoom from my dob (14", f4.7) but on a recent session I happened to put in my 7mm Celestron X-cel and was stuck by how much better the view of the trapezium was (e and f stars distinct and steady, in the zoom at 8mm they shimmer).

I sometimes use the zoom with a 2.25x Barlow which, when the seeing's ok, works well on globs and planets (it's at about 460x) and immediately started thinking about whether a short FL eyepiece would be even better. After a bit of reading up I was about to push the button on a 3.2mm BST Starguider when I spotted that it's only recommended down to f5. Has anyone experience with a BST on a faster scope? As it will only be occasional use I don't want to go over £100.

Would it still be a good choice? 

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It depends if you are happy keeping away from the edges.  In my experience with 8mm and 25mm. I only had the 5mm for a while years ago,  but gave it away. They are not sharp over all the field at f4.7 , but it has been a while since I used them.  Other than they give pleasant views IMO.

From memory I also recall some  field curvature resulting in out of focus stars the closer you got to the edges , perhaps the 25mm mainly. Sorry I can't help more since I am a bit vague on them at this stage.

Note, I am a bit picky about aberrations,  however at such high magnification ( I have never gone that high ) seeing will be quite blurry anyway. I never found it useful for planets to get out more detail. 

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Hi Bob,

I'm in a similar position to you with my f4.9  12 inch Dob. I'm also after a short focal length eyepiece for high power planetary viewing, and was considering the BST 3.2mm as a possible contender. However, after consulting with our friend rwilkey who has great knowledge of these EP's he considered the 3.2mm to be the weakest of the series, though still very good. 

After a bit more reading I'm now considering the Vixen SLV 4mm as a better alternative, it seems to be very well regarded by many people on the forums., and is still in my price range. Have you had a look at the SLV?

Regards Geoff.

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Thanks Geoff and Alex.

I was put off the SLV a bit as it's only 50 degrees and twice the price- but I guess if the whole view is sharp then it may be worth it.

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The Vixen SLV's that I've used have been very sharp - comparable to good quality orthos (which I did at some length) but with much more observing comfort. Rather like a Pentax XW but with less AFoV.

At 460x it's all going to be a bit of a lottery though :smiley:

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The 3.2-12mm (maybe 15mm) Starguiders will be good enough in terms of correction I think. I've looked through them at f4.4 and don't remember any atrocities. However, the 3.2 needs fine focus control to get a decent image. 

When I had both 7mm Xcel lx and 8mm Starguiders I thought the 7mm was a touch ahead of the 8mm. (at f6) Using a 2x barlow with the 7mm will probably be better than the 3.2mm, if the atmosphere supports that magnification. 

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18 minutes ago, Ricochet said:

The 3.2-12mm (maybe 15mm) Starguiders will be good enough in terms of correction I think. I've looked through them at f4.4 and don't remember any atrocities. However, the 3.2 needs fine focus control to get a decent image. 

When I had both 7mm Xcel lx and 8mm Starguiders I thought the 7mm was a touch ahead of the 8mm. (at f6) Using a 2x barlow with the 7mm will probably be better than the 3.2mm, if the atmosphere supports that magnification. 

What I can say from experience that there is an observable difference with the 8mm BST I had versus a well corrected eyepiece such a TV Radian in my F4.7 scope, but you need good conditions to really find out, which I did few years ago. Back then Jupiter was high up in the sky, I used to do 2 - 4 hour marathons on that object alone. 

I could see the image degrade gradually. My viewing habit was to use around 2/3 of the view, then I would nudge the scope to get the best out of it. 

Also, IMO, as John said, it becomes a lottery anyway at that magnification.

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