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Eyepeice focal length descision

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#1 SpaceRooster

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Posted 25 January 2013 - 05:16 PM

I am thinking about getting one of the TMB Planetary II's that are on sale (was planning on getting a Paradigm, but $40 vs $60 dollars is nice on a budget :cool:). The thing I am trying to decide is which focal length to get. For those of you who have similar scopes, what is the maximum power you are going to get out of a medium sized dob, 10 inch in my case (f/5)?

I am leaning towards the 6 or 7mm versions as the 5mm would fall too close to my 11mm barlowed. Thing is though, if I can squeeze 2.5mm out with decent contrast, I might still go for the 5. Anybody had much experience in this magnification range with this type of dob? For the record, seeing around here is usually 5-7, getting enough contrast out of x218 to see the Cassini division on Saturn.

#2 DHurst

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Posted 25 January 2013 - 05:44 PM

I can use a 3mm or 4mm once in a while during excellent seeing, but that is not often. Get the 7mm IMOHO.

#3 rflinn68

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Posted 25 January 2013 - 06:20 PM

I'd get the 5 or 6mm. I already have the 6 for my 10" dob and just ordered the 5mm today. No way can I even think of barlowing even a 6mm, much less a 5mm and you even have a little more focal length than my f/4.7. Barlowing a 7mm might work on an absolute perfect night. That would give you about 360X mags.

#4 acochran

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Posted 25 January 2013 - 06:44 PM

You mentioned your scope is f/5. Doesn't the ad say recommended for f/6 or longer???
I also was interested, but my scope is f/4.5

#5 SpaceRooster

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Posted 25 January 2013 - 06:59 PM

Actually I just ran the math. Orion posts that the XT10 is f/5 in the manual, but it is actually f/4.7.

And acochran, of course they say that its recommended for f/6 or longer. Fast scopes are known to be less effective for high magnification/planetary viewing. Doesn't mean you CAN'T though. :smirk: Just because a view won't be optimal compared to a slow refactor doesn't mean you shouldn't take a peek anyways. Saturn is beautiful through my scope at 5.5mm (and that is barlowed). Not as nice as an 8" f/10 refractor, but much, much better than staring at it wishing I had a better eyepeice. Don't let a fast f ratio scare you away from getting an eyepiece that will let you get more out of your scope. Especially when it is only $40 buck for a decent EP.

#6 rflinn68

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Posted 25 January 2013 - 07:00 PM

You mentioned your scope is f/5. Doesn't the ad say recommended for f/6 or longer???
I also was interested, but my scope is f/4.5


I think you read something wrong. You can go down to a 4.5mm eyepiece and still be at 1mm exit pupil on your f/4.5 scope. Any of these eyepieces will work fine in your scope. They do excellent in my f/4.7

#7 SpaceRooster

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Posted 25 January 2013 - 07:04 PM

I'd get the 5 or 6mm. I already have the 6 for my 10" dob and just ordered the 5mm today. No way can I even think of barlowing even a 6mm, much less a 5mm and you even have a little more focal length than my f/4.7. Barlowing a 7mm might work on an absolute perfect night. That would give you about 360X mags.


This is exactly what I needed to know. I figured pushing it much past 4mm would be pushing this type of scope to its limit.






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