Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User
SpaceRooster
member
Reged: 12/23/12
Loc: Georgia
|
Eyepeice focal length descision
#5642997 - 01/25/13 05:16 PM
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
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  ). 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.
|
DHurst
super member
Reged: 03/10/06
|
Re: Eyepeice focal length descision
[Re: SpaceRooster]
#5643044 - 01/25/13 05:44 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
I can use a 3mm or 4mm once in a while during excellent seeing, but that is not often. Get the 7mm IMOHO.
|
rflinn68
scholastic sledgehammer
Reged: 03/09/12
Loc: Arkansas
|
Re: Eyepeice focal length descision
[Re: DHurst]
#5643092 - 01/25/13 06:20 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
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.
|
acochran
sage
Reged: 06/19/08
Loc: So. CA
|
Re: Eyepeice focal length descision
[Re: SpaceRooster]
#5643136 - 01/25/13 06:44 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
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
|
SpaceRooster
member
Reged: 12/23/12
Loc: Georgia
|
Re: Eyepeice focal length descision
[Re: acochran]
#5643166 - 01/25/13 06:59 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
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.  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.
Edited by SpaceRooster (01/25/13 07:00 PM)
|
rflinn68
scholastic sledgehammer
Reged: 03/09/12
Loc: Arkansas
|
Re: Eyepeice focal length descision
[Re: acochran]
#5643168 - 01/25/13 07:00 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
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
|
SpaceRooster
member
Reged: 12/23/12
Loc: Georgia
|
Re: Eyepeice focal length descision
[Re: rflinn68]
#5643173 - 01/25/13 07:04 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
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.
|
|
20 registered and 21 anonymous users are browsing this forum.
Moderator: ausastronomer, Scott in NC, iceblaze
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 220
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
