Chris,

I did not mean to imply that it makes sense to use the 23m focal
length. Two years ago when Jupiter and Saturn were nearing opposition
in October I had taken eyepiece projection photos of them using a
range of eyepieces. The exposures where 1/4s and 2s, respectively,
for the exposures using the extreme focal length of 23m so they
are underexposed, but given the other factors that compromised these
shots (unsteady surface-my NYC roof, inexperience focussing on my
Minolta's screen, uncollimated scope, etc.) I am still sort of proud
of these photos despite all their flaws. These links are to scans
of the prints which adds even another generation of blurring.

Jupiter- http://members.bellatlantic.net/~tarashna/0009-04.html
Saturn- http://members.bellatlantic.net/~tarashna/0009-08.html

I was just testing the limits of my system at the time, and I do not
believe I have revisited using that extreme focal length since. I
prefer the ~f/37 and 7.5m focal length provided by the 15mm Super
Plössl eyepiece for my rare excursions into planetary photography.
And I think I remember even trying the 12.4mm for some lunar shots.

I wouldn't say that f/115 is useless, but it is extremely difficult
to tame all the elements involved to get any sort of result, much
less a usable result, at that extreme. Then again, my first 60mm
scope was supposed to give me 455x magnification. It did with the
supplied barlow and 4mm eyepiece, but only in broad daylight of
rocks on an outcropping a mile away. I could tell they were rocks,
because they weren't a green fuzz or a blue fuzz...

Taras

On Tuesday, July 31, 2001, at 06:54 PM, mapug-digest wrote:

> Date: Tue, 31 Jul 2001 18:38:49 -0400
> Subject: [M]: Re: Meade Promotional Eyepieces
>
> Taras R. Hnatyshyn wrote:
>
>> I bought the 6.4mm and 9.7mm when I bought my 8" f/6.3 LX200. When the
>> seeing is good enough and the scope is well collimated, the 9.7mm gives
>
>> great views of Jupiter and Saturn. I haven't had good enough skies to
>> do the 6.4mm justice visually, but I have used it for eyepiece =
>> projection
>> photography. I was able to get an effective focal length of about 23m
>> using that eyepiece.
>
>> Taras
>
> My response:
>
> A 23 m focal length on a .2 m diameter scope implies an f-stop of 115.
> The "strain" point on most scopes is about f/50.
> If you want the quantitavive analysis of why this is so I will work it
> out
> and e-mail it to you. Suffice it to say that f/115 is very rarely
> usefull.
> It will also only give you about 20 lines/mm at the focus, thus you
> could
> only enlarge your image about 4 times (5 lines/mm) and retain a
> reasonably
> sharp image. In addition, if you are interested in doing
> astrophotography,
> f/115 will require 5.3 times the exposure time of f/50 and 132 times the
>
> exposure time of f/10.
>
> For example a typical Mars exposure, using a 3X Barlow (f/30) will be
> about 6 seconds.
> At f/115 the exposure time will be 88 seconds.
> There is no way that the atmosphere will hold steady that long.
> There is no way that your scope will guide on Mars that long.
>
> Bottom line:
> f/115 is useless.

--
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