Focus on celestron c90

I just picked up a vintage Celestron C90. It came with a number of
accesories. The focus terestrially with the included 18mm kellner eyepiece
is perfect. But with that eyepiece and a 2.5x barlow the focus is less than
crisp. Is this typcal for a barlow or is the 1000/18 times 2.5 magnification
just too much for the 3.5inch scope? I am interested in purchasing an
assortment of eyepieces Should I consider a .965 to 1.25 adapter and buying
1.25 inch eyepieces? Are any of the zoom eyepieces worth while? Any help is
appreciated.
Thanks
David

Like you, I recently purchased a C90 of not-so-recent vintage. Mine
was the "telephoto lens" type. Google this group and you'll find all
sorts of info about accessorizing your new acquisition. The
conventional wisdom, at least for astronomical pursuits, is to get set
up for 1.25" eyepieces. To do that you might need the following:

1) Large Accessory Ring (often referred to as LAR)
2) 1.25" visual back
3) 1.25" diagonal
4) 1.25" eyepieces. Plossl's are plenty good; fancier EP's for this
scope would probably be a waste of money. Besides, I don't feel too
comfortable balancing my 14mm UWA Meade on a C90!
5) Celestron 5x24 finder with bracket. I had to drill and tap holes
for the bracket. If you attempt this DON'T DRILL INTO THE MIRROR!

Other items include:
6) Mount. I got an EQ1, which works reasonably well.
7) .63x focal reducer.
8) Baader filter (have yet to try this)

Some folks have gone as far as lining the barrel with flocking paper,
but I haven't done this. But I did disassemble mine and regreased the
helical with lithium grease. Now it's much smoother but still lacks
all the convenience and feel of a true SCT focuser.

Now to answer your original question, you can't expect to go high mag
on this puppy, at least from my experience. With an 18mm EP, the
magnification is 1000/18 = 56X. With a 2.5X Barlow, your at 139X,
which can be okay but nothing to write home about. I like it for low
power viewing and prefer to use my Tele Vue 32mm Plossl.

However, because it is a small scope, the fun-factor is pretty high.
I have seen the Ring Nebula with the C90 under mag 4.5 skies.

S&B


"david johanson" wrote in message ...
I just picked up a vintage Celestron C90. It came with a number of
accesories. The focus terestrially with the included 18mm kellner eyepiece
is perfect. But with that eyepiece and a 2.5x barlow the focus is less than
crisp. Is this typcal for a barlow or is the 1000/18 times 2.5 magnification
just too much for the 3.5inch scope? I am interested in purchasing an
assortment of eyepieces Should I consider a .965 to 1.25 adapter and buying
1.25 inch eyepieces? Are any of the zoom eyepieces worth while? Any help is
appreciated.
Thanks
David

"david johanson" wrote in message
...
I just picked up a vintage Celestron C90. It came with a number of
accesories. The focus terestrially with the included 18mm kellner
eyepiece
is perfect. But with that eyepiece and a 2.5x barlow the focus is less
than
crisp. Is this typcal for a barlow or is the 1000/18 times 2.5
magnification
just too much for the 3.5inch scope? I am interested in purchasing an
assortment of eyepieces Should I consider a .965 to 1.25 adapter and
buying
1.25 inch eyepieces? Are any of the zoom eyepieces worth while? Any help
is
appreciated.
Thanks
David
When you increase the magnification, the 'faults', whether from the scope
themselves, or from the atmosphere, are also increased. 140*, is pushing
magnification levels for the scope, and even more so in the daytime, when
the atmospheric blurring is worse. A lower magnification will allways
appear 'crisper', on any size scope, and even a much larger scop would
show similar degradation for terrestrial viewing. You are more likely to
find 'better' eyepieces in 1.25", and also the eyepieces will then be
useable latter if you buy a bigger scope. In the past, you'd have to say
that 'zoom' was questionable in terms of quality, but in recent years a
number of zoom eyepieces have appeared that are very useable.

Best Wishes

I just picked up a vintage Celestron C90. It came with a number of
accesories. The focus terestrially with the included 18mm kellner
eyepiece is perfect. But with that eyepiece and a 2.5x barlow the focus
is less than crisp. Is this typcal for a barlow or is the 1000/18 times
2.5 magnification just too much for the 3.5inch scope? I am interested
in purchasing an assortment of eyepieces Should I consider a .965 to
1.25 adapter and buying 1.25 inch eyepieces? Are any of the zoom
eyepieces worth while? Any help is appreciated.
Thanks
David
---------------------------------------------------------------
Hi David,
As others have mentioned, Celestron does make an Adapter called the LAR
(Large Acessory Ring) That will enable you being able to apply a 1-1/4"
SCT Visual Back, and then fitting a 1-1/4" Diagonal, preferably a mirror
type, not a prism.

There have been a few very high quality prism diagonals made over the
years, but would no doubt exceed the cost of the C-90.

Yes, a Zoom eyepiece might be a very nice choice for this scope, and
Televue has made a fairly decent one that shouldn't cost more than $100.
Astro-Mart would be a good place to look for all these items, to save
you money versus buying brand spanking new.

The C-90 is not a bad little scope at all (I had one years ago myself),
but understand that clear aperture on this scope is only something like
76mm, so in all actuality, it will not/cannot perform any better than a
short tube 80mm Refractor.

For terrestrial use, they are nice, as they are compact, easy to use,
and can be permanently left on a small lightweight camera tripod (If you
have the spotter that is) for quick peeks around the neighborhood. Also
once the LAR is aquired, a T-Ring/Camera Adapter could be utilized, and
this C-90 could be used as a 1000mm Telephoto lens.

Many times, as years go by, the focusing goes cruddy on these, as the
grease gets old, and stiff. A couple of allen retaining screws can be
removed on the side of the OTA, unscrew the front of the tube from the
rear, clean threads carefully with some solvent like naptha, and a clean
shop towel making sure to get none on the optics, and apply a very light
coat of fresh grease to the threads, and re-assemble.

The corrector Plate (Both sides) at this time could be carefully cleaned
also, but I'd refrain from touching any of the mirrors (Primary, and
secondary spot on the corrector) with anything. You'll do more damage
than good. Maybe just lightly blow any loose dust from mirrors with a
rubber blower bulb, and that's it.

With the fresh grease, the focusing will be lots smoother, enabling you
to better achieve precise focus. Hope this helps. Mark

Hi David, One thing I did fail to mention though, and I'll mention it
now, is that if you did switch to a mirror type diagonal, you will wind
up with a reversed image when using the scope terrestrially. Not
important when using for astro, but can be a bit of a pain when veiwing
objects on land.

For terrestrial use, the prism diagonal should work just fine.
Sorry for overlooking this, Mark

Thanks everyone for the advice. The scope I purchased works
beautifully. I'm going to buy a .965 to 1.25 diagonal and a couple
1.25 eyepieces. I have already purchased a small equitorial mount very
similar to the Orion EQ-1 ($15 on EBAY) and have figured out how to
mount the scope on it (I built an adapter) and how to align it. Now
all I need is a bit of clear sky. I'll head out tomorrow morning
around 4 if the sky cooperates and give it a try on Jupiter and venus.
My wife is very concerned about this whole enterprise. She thinks that
this little scope may lead to a slightly larger, and more expensive
one...
Thanks Again
Dave

(Mark D) wrote in message ...
Hi David, One thing I did fail to mention though, and I'll mention it
now, is that if you did switch to a mirror type diagonal, you will wind
up with a reversed image when using the scope terrestrially. Not
important when using for astro, but can be a bit of a pain when veiwing
objects on land.

For terrestrial use, the prism diagonal should work just fine.
Sorry for overlooking this, Mark

My wife is very concerned about this whole enterprise. She thinks that
this little scope may lead to a slightly larger, and more expensive
one...

She's right. But don't tell her that.

S&B

Shes a smart one Dave!
Better get rid of her Now ! :O

A EX-girlfriend of mine Loves Astronomy...
And I sure couldn’t afford to keep her!

Clear Skies, Roger ?;^)


My wife is very concerned about this whole enterprise. She thinks that
this little scope may lead to a slightly larger, and more expensive
one...
Thanks Again
Dave