Re: First Light: 14.5" f/5.4 Newtonian
From: Axel (ritesh_laud_at_hotmail.com)
Date: 07/06/04
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Date: 5 Jul 2004 22:30:13 -0700
Hi Jason.
> I was interested to learn that Jim made you a structure with that slow a
> focal ratio. His website indicates that he will make them only F/4.5 or
> faster.
That is correct. Two months after I ordered the scope in December
2002, NSS stopped offering custom focal ratios. I'm not sure what
prompted Jim to do it; but executing the design for my scope may have
been the straw that broke the camel's back. ;-) He did charge an
extra $250 for the custom f/ratio. At any rate, the scope seems to
have turned out beautifully and the construction looks to be up to par
with his standard scopes.
> The focuser is at 72" or so when at zenith, which is EXACTLY where my eyeball
> is flat-footed, so it's nice to not need even a step with a 16" scope.
I would have certainly desired being able to stand on the ground (or
even better, to sit on a chair), but the benefits of the longer
f/ratio were more important to me.
To me, the main benefit is with respect to collimation. Not only is
the required precision of collimation more relaxed, but the chances
are much better that re-collimation will not be required later in the
night. At faster f/ratios, a trivial movement of the primary or
secondary can result in a tangible loss of image quality. NSS's
mirror cell and spider designs are very robust and the chances are
that collimation will hold well into the evening. But my logic was,
why take chances? If I'm laying down several thousand dollars for a
telescope, I want something that's forgiving and not hypersensitive to
a bit of mechanical slop somewhere. This becomes even more important
as the components age and the scope gets banged about a bit;
mechanical slop will eventually be introduced.
Another big benefit is field illumination for a given secondary mirror
size. With a longer f/ratio, one can either reduce the secondary size
to improve contrast or keep the same size and thereby improve field
illumination (important for nebulae). I went for the 2.6" secondary,
which at f/5.4 provides a 100% illuminated zone of 0.47° and a 75%
zone of 1.23°, according to the Newt software. I strongly considered
the 3.1" secondary for even better illumination, but decided that I
already had plenty for the uses I intended it for. I probably won't
be looking at huge nebulae too often.
Coma is another issue, but even at f/5.4 the coma is quite obvious.
While using the Nagler 31 I easily noticed coma in stars in fully the
outer third of the FOV. I was in fact quite surprised at how much
more noticeable it was at f/5.4 than in my 8" f/6. It didn't bother
me, but it was distracting. I might consider a Paracorr.
Another benefit is the ease of focusing. The shallower light cone
makes focusing more forgiving. Of course, with the superb two-speed
focusers available today, this is pretty much a moot point.
Now if it had been possible to *sit down* if I went for the shorter
f/ratio and gave up these benefits, I would probably have done it in a
heartbeat. But since I have to stand either way, I don't mind
stepping up 6" in order to gain these benefits. I'm sure other people
feel differently about standing on any potentially unstable structure
to look through the eyepiece, but this StarStep Observing Chair is
really quite a nice little tool.
> Also, I bet you paid big bucks for your Royce mirror. I've seen on their
> website the prices.
His mirrors certainly aren't inexpensive. ;-)
> My Pegasus mirror by John Hall (Carl Zambuto's mentor) is very primo - 1/19.5
> P-V, and 1/52 RMS, with a .986 Strehl. Does Royce provide comparable spec
> info?
He only states that mirrors up to 16" are guaranteed to have an RMS of
0.036 or better and a Strehl Ratio of 0.95 or better. Also, primary
surface ripple shall be below a discernable amount in a star test.
> BTW, there's no need to remove them from your AN, in that the unit will use
> the DC cable if connected, and then you can use your AA batteries as back up
> in case of what happened to you. Just a thought.
Good to know!
> The 2005 date will likely be Feb 9-13. Your new scope would love those dark
> skies! We had some guys from that area who are part of JSC Astronomical
> Society and they thought they were in heaven. Watch our website this fall
> www.geocities.com/kingsville_astronomical_society/dsts.htm.
I might actually join you guys there. The TSP is too far for me, but
Kingsville is doable.
> PS - What other eyepieces do you use other than the 31 Nagler?
Nagler 22, 12, 7 and the Zoom. The big gap here that I should fill is
between the 12 (166x) and the 7 (284x). Maybe a 9mm would be perfect.
Just what I needed, yet more money into this hobby. ;-)
Cheers
P.S. I really think these NSS scopes are amazing values. I don't know
how Jim does it.
- Next message: Axel: "Re: First Light: 14.5" f/5.4 Newtonian"
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