Posts Made By: Jeff Young

I'm trying to determine the best way to upgrade my 16" Meade SCT OTA. I'm currently visual only -- if and when I get into imaging I would be amenable to purchasing a different scope for that, so let's assume the use will be 95% visual. Let's also assume 80% deep sky and 20% planetary (my seeing is usually Pickering 4 - 7, and very rarely 8 or 9).

I have an observatory and a mount (AP1200GTO), and I'm happy with sitting in a rolling pneumatic chair. So it has to be a Cassegrain of some type.

I've boiled it down to two choices: a 16" f/8.4 RC (optics likely by Star Instruments), or a 16" f/14 classical Cassegrain (Tak C-400). I already find the field of my 16" f/10 SCT somewhat limiting, so I'd probably use the classical Cassegrain with a focal reducer (yeilding f/10.4).

The RC would still have a considerably shorter focal length, which is enticing. But the classical Cassegrain has a smaller central obstruction (29% vs 40%). The RC optics makers have suggested to me that the CO size will not be an issue for my intended application, even visually. I've heard others say the CO size is very important. So where's the truth?

Thanks for any insight you can provide,
-- Jeff.

I'm researching the options for premium optics in a large Cassegrain. The front runners are an f/8.4 RC or an f/14 classical Cass. I'm purely visual, so the smaller CO (and higher contrast) of the classical Cass suggest it should be in the lead.

But I'm worried about the 5600mm focal length, so I looked through my records to see when I used maximum TFoV or minimum magnification with my current (f/11) 4400mm focal length.

Of course some of the obvious targets come up: the Rosette, portions of the Veil, the Leo triplets, M42, etc. Many of these don't even fit in the field at f/11, so doing a bit more panning at f/14 shouldn't be that big a deal.

But there's another class of target where I needed the higher light density of a lower magnification and larger exit pupil. Examples include the Headphones, NGC2403 and Thor's Helmet. At f/14, even a 31T5 gives barely more than a 2mm exit pupil. I can go up to something like a 55mm Plossl for 4mm, but I'm not terribly fond of the soda-straw views it gives.

What do folks with classical Cassegrains do on low-surface-brightness targets?

-- Jeff.

OK, folks, bear with me. I keep going around in circles on this. Just when I was getting serious about minimizing my CO, a friend brought up an argument very similar to one I had made a month or so ago:

At 16" of aperture, the Airy disk is on the order of 0.5 arc-seconds, and the first diffraction ring is on the order of 1.0 arc-seconds. Up to 40% CO, most of the damage is done by throwing light from the Airy disk into the first ring (in fact, a 50% obstructed scope throws nearly an identical amount of light into the Airy disk and first diffraction ring as a 0% scope does into the Airy disk).

So, unless your seeing is better than 1 arc-second (and mine rarely is), you won't often be able to tell the difference between a large obstruction and a smaller one.

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.

The first point would argue for a 20" 40% obstructed scope, while the counterpoint might favor a 16" 30%. Anyone swayed by this, or does everyone still line up on the same side of the fence?

Thanks for your thoughts,
-- Jeff.

I used to have an ETX Mak-Cass. My primary memory is that the contrast was far better than my SCTs.

Is this because of the smaller CO? Seems unlikely, as the SCTs I've viewed through were 33% - 38%, and I think the Mak-Cass is 32%.

Is it because of the long focal length? In other words, I'd usually be using more magnification per inch of aperture and so the background looked darker?

Or is the Mak-Cass easier to baffle because of its design?

Or is the design somehow easier to get a higher polish on the glass, and therefore less scatter?

Or something else?

Thanks,
-- Jeff.

I'm likely upgrading my SCT to a 16" f/10 Mak-Cass. I've got some questions about cooling such a beast.

It will be observatory mounted. The temp inside my observatory in the evening is typically 2 - 4°C warmer than the outside temp. Not a lot, but enough to see tube currents in my current SCT.

While the Mak has a considerably thicker corrector, my SCT will dew up in an hour or so (even with dew heaters) if I leave the dew shield off. So I think the corrector will probably lose heat to the sky plenty fast enough. I imagine the primary is the main concert. (But if you disagree, please speak up.)

For the primary, do I want to promote a laminar airflow all the way up the tube from the back (the tube is 20-1/2" in dia, and has pierced baffles, so I've plenty of room outside the light path), or would it be better to scrub the front of the mirror (fans from the side), and let the warmer air rise out of the tube on its own?

Thanks for any comments,
-- Jeff.

Specialization tends to bloat collections considerably.

I view mostly DSOs. I'd say 75% of my viewing is done through 4 ultra-wides. I've got a 5th for when the seeing supports higher mag, but that's rare.

But when Mars or one of the gas giants is well-placed, I like to use a binoviewer. That's a problem to start with (double the eyepieces). But the rings of Saturn and the moons of both Saturn and Jupiter are better in "cool" eyepieces, while I prefer the planets' disks in "warm" ones. So I've got 4 cool focal lengths (8 eyepieces) and 3 warm ones (6 eyepieces), and suddenly my 5-eyepiece collection has ballooned to 19. And this is all for one scope!

-- Jeff.

Mewlon 250 (non-S), AP600EGTO, AP pier.

I recently replaced my portable rig's OTA (previously a C9.25) with a Mewlon 250. I use it visually only, so I got the non-ring-mounted (ie: non-S) version, but wanted a neater (and lighter) mounting than the wedding of the Tak hockey puck and the Losmandy dovetail.

Several folks have deleted the hockey puck and mounted a dovetail directly to the OTA, but I kind of like the hockey puck mouting, and so went the other direction.

I couldn't find anything comercially available, so it was time to make some chips.

First pic: a 3/4" aluminum plate mounted up in the 4-jaw, being radiused for the hockey puck clamp.

I don't know if there are any other amateur machinists out there, but I just posted a long thread in the Takahashi section detailing (with lots of pictures) my efforts to fabricate a custom adapter for mounting a Mewlon 250 on an AP600E.

http://www.astromart.com/forums/viewpost.asp?forum_post_id=536496

Here's a teaser picture to whet your appetite:

I haven't posted here in ages, but I thought I'd share a couple of recent observing reports.

Over the summer I acquired both Alvin Huey's and Kanipe & Webb's Arp Peculiar Galaxies books, but I'm a bit embarrassed to say that I hadn't used either in anger yet. I finally got a chance to correct that on Thurs night.

This was a short session due to having spent all day cooking. I was also using a plan that had been prepared for October (which lay unused due to cloudcover) -- so I just picked a couple of the later (RA-wise) ones which were still fairly well-placed.

Conditions were mixed: better than average seeing for my site, but worse than average transparency.

Arp331 11/27/2008 23:30UT; Pickering 8, NELM 5, SQM 20.4

5 direct-vision plus 1 averted-vision galaxies. NGC383 the brightest, but 380/379 readily visible to the NNE and 384/385 somewhat less so to the S. NGC386 could only be teased out with averted vision; holdable perhaps 35% of the time but easily re-acquired.

NGC380 has stellar core; NGC379 slightly larger but with a less condensed core. NGC383 has largest core, elongated somewhat NNE-SSW. NGC384 and 385 show very weak semi-stellar cores under averted vision.

Note: Arp331 is probably an example of where I should have gone out with a chart -- I expect with a bit of prompting and knowing where to look I could have found NGC387 and possibly even 388. But I still have a preference for taking my first observation of a target "blind" (so that I'm unbiased in recording what I see).

Arp158 11/27/2008 23:55UT; Pickering 8, NELM 5, SQM 20.4

Small elongated streak, but without the swelling so typical of edge-on spirals or lenticular galaxies. Two brighter knots, both semi-stellar in appearance; one at E end and second about 2/3 of the way to the W end, connected by a brighter bar. Dimmer knot between the other two not seen, nor was the exceedingly faint extension to the ESE.

-- Jeff.