Eyepiece woes....

[shaneaust]

Just bought a 7mm Televue Nagler from a fellow IIS member, for my 10" dob.

My scope handles low powered ep's really well, and I've got some good, sharp views out of some new 25mm and 32mm Plossls I got recently.

But - the 7mm ep is not going well - "far" images are blurry, and this has been the case with every high powered ep that I've ever used in this scope. (However, viewing the moon with the 7 mm is great and gives me a lot of awesome detail that I hadnt experienced before).

Any ideas as to what the prob with trying to view "distant" images might be? I built my scope a few months ago - it's an F5, 10" dob.

Any/all help would be appreciated! :help:

[janoskiss]

Assuming the optics is good and not pinched, it all comes down one or more of: collimation, thermal equilibrium (i.e. cooling) and seeing.

[chunkylad]

HI Shane

Er.....what was it you were trying to see with your 7mm that's 'further away' than the moon?

To my mind, if the moon is sharp, then anything else should also be in focus. Of course, as you pump up the power, you show up any alignment errors with your optics. Perhaps you have collimation issues? Have you collimated and star tested your new 'scope?

Or, maybe the seeing is simply not good enough for the 7mm (although 178x for a 10" newt doesn't sound too much mag to me, even on a poor seeing night).

I'm sure you'll get the advice you need from other, more experienced members of the forum.

Good luck with it!
Dave

[RB]

Is it possible for you to try the EP in another scope?
Just for a comparison.

[iceman]

I also wonder what you mean by "far"? If images of the moon are ok, what are you viewing? Stars? Galaxies? Nebula?

The 7mm will give you 178x magnification in your scope, which isn't excessive, but it does require fairly steady seeing to view sharp images at that magnification. It's worth noting that the seeing in NSW and VIC has bordered on horrible for the last 2-3 weeks thanks to the jetstream.

I'd suggest the same as Steve - it has to be collimation and/or thermal equilibrium, in combination with seeing.

With lower power eyepieces you won't see the effect of bad seeing as readily. The higher magnification you go, the more seeing comes into consideration and if the seeing is bad (or your mirror is not ambient), the image won't come to snap focus - it appears blurry and no matter how hard you try you can't focus it properly.

Is that what you're experiencing?

I'd also suggest trying it during the day on a tree on a far off mountain or something.

[GrampianStars]

a 7mm Televue Nagler will only be usefull on a few days YEARLY :sadeyes:
it will only give you a sharp immage when the apmosphere is exceptually steady


also colimation, colimation, colimation :D

[iceman]

Hi Robert.

In an SCT that may be true given your longer focal length, but in a 10" dob at 1250mm FL, it's only 178x and I'm able to use that magnification on almost every viewing session, depending on the object(s) i'm looking at.

[shaneaust]

Hi Andrew:
I think yours is probably the best suggestion for my particular situation - thanks, I will certainly give it a try in a borrowed 'scope!

Much obliged, as they say...

[shaneaust]

Thanks, all - what I'm meaning by "far"/"distant" is mainly stars - trying to get them into focus, I'm finding, is pretty nigh impossible with **any** high powered ep, not just this 7 mm I recently acquired. A 15 mm is about as far as I can go with any sort of clarity, anything higher than that is just a blur.
I will try this one out in another scope, though, as Rocket Boy suggests.

[iceman]

Make sure you try it in another similar focal length scope, not an SCT!

[ving]

let us know how you go :)

[rickericn]

I did run into a similar situation with my 12" and a 9.5mm Epic (not even in the same league as a 7mm Nagler). The solution in my case was to adjust the tilt of the secondary mirror.
I'd sure check the collimation.

Best of Luck,

-Rick

[ving]

oh hang about... is it a case of not having enough travel to focus? :confuse3:

[shaneaust]

er...as Manuel in "Fawlty Towers" would say - Que??

[ving]

focuser travel...

I once used an IIS members nagler. i couldnt bring it to focus cause the focuser didnt have enough outward travel. I have in my own collection a 10.5mm televue plossl. one again it doesnt focus as there isnt enough outward travel in the focuser.
i fixed this by having the EP 1-2mm out of the focuser and clamping it there. it gave me the right amount of outward focus to use the EP.
:)

er... is this you problem?

[casstony]

A moon image will always look better than a star image if the atmosphere is not steady enough to support a sharp image at the magnification you're using. The moon is craters within craters within craters and you may not realise that you're missing out on finer details if the image is blurred just a little from unsteady air. With the star you only have fine detail to deal with so the image will look worse in comparison to the moon. There may be nothing wrong with the eyepiece.
-Tony