Posts Made By: Jeff Young

pierre francois said:

Does anyone know what the difference is between the AP1200 version 3.09 and the most current version?
Thanks!
Francois

Francois --

The current version is 4.12. For a version history, see:

http://www.astro-physics.com/tech_support/mounts/keypad_update/keypadversions.htm

There's also a Yahoo group on which you can get lots of specific answers: "AP-GTO". Roland Christen himself participates.

-- Jeff.

Anyone?

Scott --

Yes, that's my observatory under construction. There's a few threads on it starting here:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=355146

Cheers,
-- Jeff.

Jeff Young said:
....

One couterpoint is that when observing planets I usually spend 20 min or more at the eyepiece, during which I'll often get several seconds of much better than average seeing. These several seconds are likely to be better than 1 arc-second FWHM, and so if the same is true of deep sky targets I still might be missing out on the few instants that the bar, or the arms, or the tidal tail pops into view.

I was having this conversation with Joe Haberman and he had a fascinating partial refutation of my counterpoint. Evidently, the dark adapted eye has a slower transfer rate to the brain, and so I'm less likely to get the advantage of very brief moments of improved seeing when viewing DSOs.

-- Jeff.

I took another run at this from the CTF angle, rather than from the Airy disk angle. All the CTF charts (Suiter, Rutten and van Venrooij, etc.) show the central obstruction curves with a significant dip between 20% and 50% of the maximum spatial frequency of the instrument.

So what exactly does that mean for a 16"-ish f/10-ish instrument on deepsky targets?

I got out the calculator and did a little math. (It wasn't too painful.) At f/10, a 40% CO shows:

* a contrast loss around 60% at 50 lp/mm (1 arc-second with a 16" aperture)
* around 10% at 10 lp/mm (5 arc-seconds)
* negligable contrast loss at 5 lp/mm (10 arc-seconds)

So I think we can say that the CO plays little role in features between 5 arc-seconds and 10 arc-seconds, and almost no role in features larger than that.

So, how big are features? Here's a sampling:

* M51's features are large. Even the bridge is probably 40" wide or wider.
* M95's bar is about 30" wide, and its inner, nearly circular ring is about 15" wide.
* NGC3631 (a bright but small face-on spiral) has arms also about 15" wide.
* The visible granulation in the Sunflower galaxy is on the order of 10" clumps.
* The Eskimo nebula is about 40" in diameter, with some visible features in the 4" to 5" range.

No issues, with the possible exception of the Eskimo. Here we're starting to see some degradation from a 40% CO, but it may not yet be visible to the human eye.

Planets? Yes, big CO bad. DSOs? Not so much, at least at 16"+ of aperture looking at Messier/Herschel-type objects.

Comments?
-- Jeff.

Larry --

Now that you've had it a few months, any further thoughts on your Mak? (I'd be particularly interested in how it performs on DSOs if you've used it in that capacity.)

Thanks,
-- Jeff.

So here's a hybrid thought:

6 sets of vent holes in the back, 3 with fans. Vent holes around the corrector, with some way to close them.

Corrector holes open produces a laminar flow up the tube; closed cools the back of the mirror. Not quite scrubbing, but it gives run-time options.

I just can't figure out a neat and tidy way to close the vent holes around the corrector. Perhaps a simple cover ring with a couple of magnets in it?

-- Jeff.

James --

"DIY" usually stands for do-it-yourself, so I expect those two components were home-built by the poster.

The "DIY rotator" looks to be about the same size as Tak's rotator (otherwise known as a camera angle adjuster or CAA). There are several different CAAs to suit different size focusers -- you'd want TFP1100.

Similarly, the CA-35 is probably the poster's home-made equivalent to Takahashi's CA35 (camera adapter -- TCA1030). I expect you can also get the Canon T-ring from Takahashi (perhaps TWM0004 -- is the Rebel the same as the EOS?).

But the easiest way might be to email/call Art at TNR. I'm sure he'd be able to tell you exactly what you need.

-- Jeff.

Shawn --

I used a 12.5mm reticle at 2000mm and found it worked fine. Tried the same reticle at 4000mm and it was too hard to get (and keep) the stars in the field. I now use it with a 0.5 filter-threaded focal reducer, but I think it's operating at more like 0.7 or so because I still find the field a bit more cramped than I'd like.

In the 3000mm-ish range, I'd be looking for something around 18mm. A 12.5mm reticle with your 6.3 reducer ought to work well.

-- Jeff.

Floyd --

For visual, I like the simplicity of alt/az. But I can't stand the lack of thrust or conical bearings in the mass-market fork mounts, and their associated poor dampening times.

At the professional end, all the forks are equatorially mounted, which I don't find that much more intuitive than a GEM. At that point I'd rather have the stability (ie: short overhangs) of a GEM.

So... one mount. As nice a GEM as you can afford. Mount a toilet paper roll with some cellophane over it if you have to.

-- Jeff.