Posts Made By: Dan Ritts

If the glue didn't work or was messy, you might try an industrial VHB double-stick tape, such as the ones made by 3M. It's quite expensive and hard to find if I remember correctly. VHB stands for very high bond, and this stuff is amazingly strong. It will NOT come apart, is easy to work with and it's easy to find uses for this stuff around the house. I work in the woodworking industry and have also used a product sold for a totally different purpose which I think is similar in performance and available in single rolls. It's called SpeedTape, made by FastCap... link below...

It's 10 bucks for a 50' roll 3/4" wide and I bet would stick kydex to itself very well. Clean both surfaces with acetone prior to bonding.

http://www.fastcap.com/speedtape.aspx

I recently got a 130 GT too, but I haven't yet tried to fly with mine. But I've been flying a lot lately, thinking about my GT, and I'm confident the 30# would not be a problem except on very small regional flights in light aircraft. A greater likelihood is that in a very full flight, if you're late to board the plane, they could say there's no more overhead bin space and force you to check the GT. For that scenario I would (I will, eventually) make a padded nylon sleeve for just the lens cell to slip into. I'd hold this on my lap for the flight and feel okay about checking the remainder of the OTA.

Anyhow, there is no official TSA weight limit, though some of the big airlines say 40 lbs max for the sum of all carry-on and personal items (purse, laptop case, etc). This is not enforced however, nor have I ever seen any capacity for the airlines to weigh carry-on bags at the gate. My carry-ons are every bit of 30lbs and I've never been challenged. The max. overall bag/case dimensions acceptable for carry-on is 22 x 14 x 9, which presumably are the dims of the ScopeGuard case made for the disassembled GT. When I get around to making a case for mine (I'm a cabinetmaker, so I may as well) I'll shoot for these numbers or slightly below. Hopefully I can engineer the foam in such a way to fit four or five eyepieces too!

Thanks for all the replies thus far... I am just arriving home late tonight but will measure the additional parameters in the morning. As I recall, I made my upper cage inner diameter 16" or a touch over, with a 3/8 thick focuser board, on top of which sits a JMI-NGF? focuser from about 1994 vintage. My first question is determining the precise location of the focal plane relative to the optical axis. Someone once suggested pointing the scope at the moon with the eyepiece out and finding where it focuses on a piece of paper? That should do it, correct? In any case, with a Tectron sight tube pushed all the way into the focuser and the focuser racked all the way in, I still cannot see the entire primary in the secondary's reflection. I don't have to rack my Naglers way out to get them to focus either, which would seem to suggest that I have not positioned the secondary unnecessarily close to the primary. Is that right?
More tomorrow... thanks ya'll.
-Dan

Alrighty, I got the measurements:

Inside diameter of upper cage: 16 1/8"
Focuser board: 7/16"
Height of Focuser racked all the way in: 1 5/8"
So the center of the OTA to the top of the focuser is 10 1/8"
I have not net determined how much further out the focal plane is from the top of the racked-in focuser, but from memory I would say my Naglers come to focus if I rack out the focuser 3/4" to 1" from the all-the-way in position. Perhaps that will tell you more experienced where the focal plane is exactly on my scope? If not, if it stops raining and I can get a look at the moon tonight I'll push the scope out and get the moon to focus on a piece of paper and report back.
...and to repeat the other specs...
Primary: 14.5" f/5.2 (75.4" focal length) according to mfg. specs (Galaxy Optics)
Secondary: 2.14" minor axis
Eyepieces used: 5, 7, 9, 13, 20 Naglers, 24 Panoptic
Also have Tectron sight tube, cheshire and autocollimator

And to repeat, I cannot see the entire primary in the secondary reflection even with the sight tube pushed all the way in and the focuser racked all the way in. So the pinhole of the sighttube would then be 10 1/4" from the OTA centerline.

Drawing the light cone in AutoCAD I see that if a 2.14" diagonal intersects the on-axis rays from the primary 11.128" from the focus of the primary, then you have a single point of 100% illumination. So the top of my focuser being 10 1/8" from the center of the light cone, and my eyepieces coming to focus just fine would seem to mean that if I'm not seeing the whole image of the primary through the sight tube, my secondary and focuser might be too close to the primary and consequently the focal point of the primary too far out beyond the top of the racked in focuser? As I said, I can't remember exactly how far out I have to rack the focuser to achieve focus with my eyepieces, but it's more than just a little bit and probably about 1". When I plug in all these numbers to NEWT for windows, it says I should have 100% illumination spot diameter of about .22" with a 75% illuminated spot dia. of 1.21". This would seem to be just fine in theory, wouldn't it (for visual only use)?

I took another round of measurements by two approaches. First, by Neil's method with a little light bulb at twice the parabola's focal length. Second, by projecting the moons image on the back of a business card and by very carefully measuring the distance from the mirror surface to the centerline of the focuser, and then from there to the observed focal plane. To the best of my ability, I concluded that my ACTUAL best estimate of focal length is 74-1/8", which makes this mirror f/5.11 and NOT f/5.2 as advertised.

As an aside, the two methods disagreed slightly, though I suppose barely within the estimated uncertainty of each measurement. The light-bulb-at-twice-the-focal-length method gives me f.l.= 73-7/8" (+/- 1/8 I'll assume due to measurement and focus error combined). I measured with both a tape measure, but also a Leica laser distance meter which is very accurate and doesn't suffer from floppy tape syndrome. The moon projection method and the direct measurement of the telescope part separations yields 74-3/16" +/- 3/16". So I suppose for now I can conclude that the focal length of the primary is almost certainly between 73-7/8" and 74-3/16". I'm going with 74-1/8 as my "final answer" based on my confidence in how I performed each method and oh-so-scientific gut feeling.

More interestingly, I put a mic on my secondary, or I should say the VISIBLE portion of the secondary's mirror surface, and it actually measures 2.05", NOT 2.14" This is because the secondary holder rolls over the surface of the mirror just slightly.

So my entries into NEWT are as follows:

Primary Dia: 14.5"
focal ratio: 5.11
diagonal minor axis: 2.05
tube inside dia: 16.0625
tube thickness: 0.47
focuser to front of tube: 6.875
focuser height: 2.4375 (exact height of focal plane for obj. at infinity measured above the surface of the focuser board)
focuser in-travel: .8125 (top of focuser racked all the way in)
add'l height for camera: 0
focuser inside dia: 2

All of which gives me a 100% illuminated circle at the focal plane of... drum roll please...

-0.296"

Time for a bigger secondary. Anybody need a 2.14 diagonal and secondary holder?

I would GUESS it's not visible, maybe not even under ideal seeing conditions. Someone like Roland Christen would probably be able to give you a very reliable answer. I remember him once commenting that he doubted anyone would be able to detect the difference between the best of one of his batches and the worst. If memory serves me he places .95 as the lower limit on his finished triplet strehls. So if that's "the worst", and he occasionally achieves .99x (the best), and even he can't see the difference...

My guess is that differences in tube baffling, thermal mass, coatings and other effects would drown out the very small deficiencies of the lower strehl scope.