The DAS had
their our monthly Open House at the stunning and historic Chamberlin Observatory. This is a beautiful 1894 building with a 20-inch Alvin Clark refractor inside:
The Open House is the society’s regularly scheduled outreach session, which takes place in Observatory Park, just in front of the observatory itself. (They also have other outreach sessions that they do all over the metropolitan area for groups that request them.) And what an outreach session it is! In New York, the AAA has regular outreach sessions at about half a dozen locations all around the City, one in each borough, and two in Manhattan, with between about half a dozen to a dozen scopes at each location.
In Denver, it’s done quite a bit differently. Everyone brings their scopes to the one location, about 5 miles southeast of downtown as the crow flies. While the largest gathering of scopes I’ve ever seen before is about 30, at the once per year Urban Starfest in Central Park, there were about 25 scopes on the lawn at the monthly Open House. Pretty impressive! And the public also gets to go inside the observatory and look through the 20-inch refractor, which this night was pointed at Saturn. I took advantage of the opportunity to look through the largest scope I’ve ever seen.
I have to say, for all the vaunted quality that the late 19th century Clark refractors are supposed to have, I was not impressed. A couple of years ago, I had previously looked through a 6-inch Clark at the Custer Institute in Southold, NY (way out east on Long Island) – that time it was Jupiter. And then as now, the view, while okay, was certainly not stunning, or amazing, or chock full of details that I otherwise couldn’t see. Yes, I saw four of Saturn’s moons through the 20-inch, while I could only see three out on the lawn nearby through a member’s Sky-Watcher 180 Mak. (More on the Mak in a second.) But Saturn itself wasn’t glorious or stunning. The view through the 180 Mak on the lawn – or even through my 5-inch Mak on my apartment building’s roof back in Manhattan – was much sharper than what I saw through the 20-inch scope.
My explanation would be that current 21st century computerized optical grinding techniques, along with modern anti-reflection coatings, and the use of better optical glass that has been developed in the past century or so, combine to result in a vast improvement in current optics over those from the 19th century. The improvement is so vast that even
cheap inexpensive amateur instruments, like my $400 Mak, can easily best the finest professional instruments, like those from Clark, who was building the ne plus ultra in refractors 120-odd years ago. I’m sure the 20-inch is a beast in terms of light collection and its ability to split double stars, but it is lacking in sharpness to modern eyes.
I was picked up and driven down to the Observatory by the DAS new member coordinator, Digby, and helped him a little with his setup and teardown. Because he had the car, I had the opportunity to easily bring my scope down with me to the observatory for the outreach in his van. I decided against it, because for a first time visit, I would rather meet the members and talk with them than spend the night with the scope. I got to meet and talk to a few of them.
Interestingly, I met one member in particular with whom I had been corresponding for over a year: Sorin. Who’s he? He’s the author of a blog with a great article, containing terrific advice for newbies trying to figure out what scope to buy. Sorin breaks down the various types of scopes, giving the pros and cons of each, so that someone just first looking into getting a scope can have at least a general, basic understanding of what’s going on. We’re pretty simpatico in terms of the first scope advice we give, so I recommend his article to newbs looking for a first scope at the Facebook clubs I belong to all the time. I’m also subscribed to his blog so that I can help guide any new people posting to the article asking for advice. Funny how I forgot that he was in Denver.
I also met another new member of the Society, Sam, who had just bought that Sky-Watcher 180 Mak I mentioned, the one I’ve been drooling over for a coupla years now. He explained his eyepiece philosophy to me – buying all 2-inch EPs, from 41mm down to 17mm, so that he doesn’t have to switch out to 1.25-inch EPs. Interesting way to do it; I wouldna thoughta that. With his 2-inchers, he’s able to go from a 42mm at 64x all the way up to a 17mm at 159x. Not a bad spread, but the 180 is easily capable of 300x or more – if the seeing conditions support it. Perhaps a good 2-inch Barlow is in Sam’s future? Checking Agena’s site, there are also plenty of dual 1.25/2″ EPs out there as well.
Doing this at the lower end of the magnification scale is painless and very inexpensive – he has a 42mm GSO Superview that’s only $70, which is very cheap for a 2-inch EP; and the scope comes with a 28mm 2-inch widefield EP at 96x, so he’s got the “lower” magnifications locked down. The 42mm Superview is “just” an Erfle, an older wide-field design which is known to be not as well-corrected as the more modern (TeleVue, Explore Scientific, etc.) widefield designs. Erfles were the original 68-degree EPs developed 100-odd years ago, but with the dob revolution, their shortcomings in short focal ratio (f/5 and less) scopes becomes apparent. As you move towards the outside edge of the field of view, the sharpness deteriorates, and the view towards the edge gets progessively more and more out-of-focus. This was not the case in the 180 Mak, because, as I’ve often said, that looong f/15 focal ratio makes any EP look very good.
I was also concerned that at that “low” magnification of the 42mm (even though it’s still relatively high at 64x), we would start to see the shadow of the secondary. Any scope with a secondary mirror has a minimum magnification that you should not go beyond, because the lower you the magnification, the greater the chance that the secondary can be seen blocking the center of the view, or at the very least, the shadow of the secondary would start dimming the center of the view. But we used the 42mm to look at both Saturn and the moon, and all was well – no dimming of the image at all across the entire field of view. Plus, at 68 degrees AFOV, the 42mm delivers a 1.06 degree field of view – a smidge more than the about 1.04 degrees I get in my 5-inch Mak. Why, that’s practically widefield! Not bad! Not bad at all!
But as EP focal lengths get shorter, the number of pure 2-inch EPs out there get fewer and fewer, leaving Sam to buy two very expensive TeleVue Naglers at 22mm and 17mm. Unfortunately, that’s really overkill and a waste on this scope. As I mentioned, you just don’t need to spend that kind of money on those premium EPs with an f/15 scope, because cheaper EPs will look damned good. But it’s Sam’s EP philosophy , and it works for him.
Right next door to Sam was Greg, who had the club’s 7-inch TEC Mak on loan. Yup, the DAS owns a $4000 scope and just loans it out to members. Man, is it suh-weet! It’s a gorgeous white thing, fully baffled down the tube. The TEC has a Rumak design, where the secondary is separate from the meniscus. In a regular Gregory Mak, like mine and the 180, the secondary is an aluminized spot on the back of the spherical meniscus. Therefore, the secondary’s curvature is fixed by the curvature of the meniscus. But in a Rumak design, the secondary is on a disc that is attached onto the back of the meniscus – like an SCT secondary.
This means that the Rumak secondary can be shaped differently – resulting in a different focal length – from the meniscus to which it is attached. So, while the 180 has a focal length of 2700mm at f/15, the TEC has a focal length of 2350 at f/13. This results in a slightly wider field of view with the same eyepiece. Where the 180 could get a 1.06 degree FOV with his 42mm in the 180, that same EP would yield 1.22 degrees in the TEC.
We compared and contrasted Saturn in both Maks and I couldn’t see any differences. I’ve often held that most of the differences between how good things look through a scope mostly come right down to seeing. And since the seeing was only just average out there on the lawn, the TEC definitely couldn’t show off its four-thousand-dollar optics over the basically equivalent SW 180 Mak right next to it. However, this won’t prevent me from borrowing the TEC from the club as soon as I have a mount that can handle the weight. Maybe I’ll be buying that iOptron AZ Mount Pro sooner than I thought.
In fact, this is one of the – relative – drawbacks to Denver astronomy – that the seeing is mostly just average most of the time. Which of course, is a tautology, contrary to how things are at Lake Wobegone. My understanding is that the wind coming over, and more to the point, through the peaks of the Rockies makes the atmosphere relatively turbulent, with the result that things are a little murky seeing-wise most of the time, so that you’re basically limited to 200x. But then again, it’s those handful of nights when the seeing is better-than-average that a scope like the TEC would really shine. But I would bet that the 180 Mak, at less than one-third the price, would still look really good then as well.
But I still go back to the Vixen VMC200L. I’d like to have that extra aperture, plus it’s lighter, cheaper, and it has an open tube for faster cooldown. Should I go with the devil I know or the one I don’t? Hmmm. Well, it’s not as if I’ve got the dough right now for either, so this isn’t a decision I have to make any time soon.