Re: 4 inch reflector can't resolve Mars at all

On May 6, 6:42 am, pau...@xxxxxxx (Paul Schlyter) wrote:
In article <1178366419.072601.215...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,



<wsnel...@xxxxxxxxxxx> wrote:
On May 5, 4:19 am, David Knisely <KA0...@xxxxxxxxx> wrote:
Chris L. Peterson wrote:
Finally, off axis coma is not a significant issue with an 8" f/6 newt, even
with wide-field eyepieces. In our 10" f/6 dob, we regularly experience
great high-power views while allowing the target to drift completely across
the field. I have never heard anyone else comment on coma when observing
with a newtonian of focal ratio in the range of 6 or higher.

That's a sign of uncritical observing, plain and simple. In an
uncorrected Newt, coma is significant. Its effect on the MTF is much
more important than central obstruction, for instance. Anybody who is a
good enough observer to see the difference in contrast between an
obstructed and unobstructed system will find the effects of coma much
more obvious.

Unfortunately, coma is not usually the main problem in an 8 inch f/6.
The most common problem is usually outer field astigmatism which, in
most of the less-complex eyepiece designs, completely masks any coma
that is present. Many people report the astigmatic appearance of outer
field stars as "coma" when it really isn't. I had to use a
well-corrected wide field eyepiece (Meade "original" 14mm Ultrawide)
before I got to see "pure" coma in an f/6 scope. The Paracorr
noticeably reduced the coma present, but in an f/6, I would still have
to call the amount relatively small (especially when compared to the
coma seen in something like an f/4.5 scope). Clear skies to you.
--

Neither coma nor astigmatism is a "problem" in an 8-inch f/6 Dob
since, as a practical matter, there is no other telescope that can
potentially provide the same light gathering, resolution and contrast
or as stable a mount for such a low price.



In other words, you get what you pay for.... ;-)

In the case of an 8-inch Dob, you get quite a bit more - compared to a
similarly priced 4-inch Go-To scope - when it comes down to what it
can show.


But surely one should be able to see details on Mars with a 4" scope!
I've seen details on Mars with a 2.4" f/15 refractor I got as a kid.

The main "problems" to
solve WRT to scope itself are collimation (not difficult), smooth
motion (easy to remedy), having an appropriate selection of eyepieces/
Barlows, and of course allowing enough time for the scope to "cool
down" after being brought outdoors.

Beyond that, the beginning observer will need to train his eye to see
detail and will need to be patient and persistent enough to be looking
through the eyepiece when the unsteady atmosphere settles down from
time to time.

Very true.... with that 2.4" refractor I got as a kid, I was unable
to see any details on Mars during the 1963 and 1965 oppositions. Not
until the 1967 opposition I was able to see Syrtis Major, Sinus
Meridiani and a few other of the largest albedo features on Mars,
with that instrument.


Think about what you might have been able to see if you had owned an 8-
inch Newtonian on an alt-az mount in '67 (or even '63 or '65)


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