Urban Observer’s Survival Guide, Part II
Posted by Brian Ventrudo
Today, the second of three installments of my interview with Rod Mollise, author of a just-published book Choosing and Using a New CAT: Getting the Most from Your Schmidt Cassegrain or Any Catadioptric Telescope. In this installment, Rod guides you through the best mounts, eyepieces, and accessories for urban astronomy, and gives an unedited view of why go-to mounts are indispensable for city-based observers. The first installment of the interview is here.
(NB: Yes I know this is a much longer article than usual. But it contains valuable advice for urban amateur astronomers… and all of you will learn how big a telescope it takes to see the elusive Horsehead Nebula in Orion!)
One-Minute Astronomer (OMA): Now how about the mount of the telescope. Is an equatorial mount essential for this city? Or does it matter?
Rod Mollise (RM): If you have an open backyard and you tend to just remain in one spot and look in one direction, an equatorial mount is fine.
For many of us though, and equatorial mount isn’t so usable. Many of us have to use three or four different spots in our backyard over the course of the evening. You might be able to go off in one spot and see off to the east, and to see in the south you have to go a little bit farther to the back of your yard so that a shed is blocking Aunt Matilda’s security light, and to see to the north you have to move to another place still.
If that describes your situation, I would not recommend either a German equatorial mount that you want to polar-align and use for go-to computerized operation, or even an altazimuth go-to mount like the moderns SCT’s. Both of those are very nice things, but unfortunately if you have to pick up the scope and move it around during the course of the evening, you’ll have to stop and realign your telescope every time.
OMA: And you’re looking at another 5 or 10 minutes.
RM: Right. And with a manual German mount that’s not so bad… you just vaguely point it north. But if you want to use go-to then you have to run through, like you say, five or ten minutes of aligning. Maybe for the average person who has to observe like that, a Dobsonian is best. And maybe not just any Dobsonian, maybe a solid-tube type Dobsonian. One of those can basically be picked up at smaller apertures and waltzed around the backyard. And in larger apertures, maybe in two trips and it doesn’t require a lot of alignment or fussing around.
OMA: This is a solid-tube as opposed to a truss-tube type telescope.
RM: Exactly. By a solid-tube I would mean something like the Orion XT telescope from telescope and binocular center here in the U.S. By truss tube we mean the classic obsession telescopes done by Mr. Kriege.
OMA: Or the new Meade Lightbridges, for example.
RM: Right. Or a semi-truss like the Meade Lightbridge. Those are a little bit more difficult to move around without taking apart in the larger sizes.
OMA: OK. Now you mentioned go-to mounts and go-to telescopes before. They’re fairly easy to find these days at a pretty reasonable price. Some purists say that go-to mounts will degrade your observing skills and take away the pleasure of learning your way around the sky. What do you think?
RM: Well they’re full of crap with a capital “K”. Nothing helps the urban astronomer more than go-to. As I said a moment ago it maybe that if you’re continually moving around your property to observe, go-to isn’t very practical. But if at all possible, if you can use go-to, do so.
There’s a very important reason for that. There are basically two ways to find things, by using go-to or by star hopping. There are other variations too like setting circles and digital setting circles, but today basically it’s go-to, digital setting circles (kind of the same thing), and star hopping.
Star-hopping is when you use a finder scope or zero-power finder like the Telrad to put objects in your telescope. You use patterns of stars and constellations and asterisms and locate objects that way, and it’s fine and it can be a lot of fun if you’re out where you can actually see stars. If you’re in the city and you’re trying to find the Virgo galaxies and you look inside the arms of the maiden, where the realm of the galaxies lies, you won’t see any stars hardly at all even with a finder… an optical finder… much less a Telrad.
And how are you going to find galaxies if you can’t find guide stars and signposts along the way? You can’t. Far from making you a worse observer, if you can’t ever find anything you will probably be made into no kind of observer at all. Your telescope will go in the closet.
OMA: You just give up.
RM: Go-to makes it possible for the urban observer to actually see stuff. No ifs, ands, or buts about that.
OMA: Alright. Let’s talk about binoculars for a second. Now just about every book says how useful binoculars are for astronomy. I’ve find that binoculars in the city don’t show me much except for grey sky. What do you think about using binoculars in the city?
RM: Like a finder, they can be… there’s no doubt that they’re limited by skyglow. That said, I’ve used binoculars in the city quite a bit, maybe because I’m kind of lazy. And it has to do with living down here on the Gulf Coast too. You wouldn’t believe what summer’s like here. It’s hot, it’s humid, it’s hazy, and there’s nothing but mosquitoes. If all I could do was take out an 11 or 14-inch SCT to observe, I wouldn’t observe all summer long. But right over by the back door there’s a table, and on the table is a pair of inexpensive 15×70 binoculars and an Orion Star-Blast 4-inch f/4 Newtonian. All summer long, I use those two and I’ve seen a lot. I’ve seen bunches of little comets in both the binoculars and the short-focal-length telescope. Neither one is really optimum for city use… lower powers are not what you really want in the city.
And yet… and yet… if you work at it, even with a pair of binoculars, you will be amazed at what you see in the city. Bottom line… you will see a lot more in the city skies with a pair of binoculars than you will sitting in your chair watching American Idol. End of story.
OMA: (Laughs). Fair enough, fair enough. We’ve talked about telescopes and binoculars. Now when you’ve got your telescope set up, do you use it exactly the same way as you would under a dark sky…?
RM: Let me add one thing to the binocular story. If you have the choice, in the city, instead of 7×50′s get 10×50′s or even better 15×70′s (and we’ll get to that in a minute) but a little bit of magnification really helps in the city… and that kind of falls in line with your next question too: How do you use a telescope in the city?
Well the first thing to notice in the city is that low-power eyepieces look horrible. That is because of the skyglow. And what do you do about that? How do you fight that? Easy. Increase magnification. In the city, we tend, as I said earlier, to use longer focal length telescopes. the reason we do that is because a longer focal-length telescope will give you higher power with a more comfortable eyepiece. An f/10 8-inch telescope will give you a decent magnification of 80x with a 25 mm eyepiece.
Why do you use higher magnification in the city? Contrast with a capital “C”. What a higher eyepiece does in the city is spread out the skyglow background…
OMA: So the sky looks darker at higher power.
RM: It darkens the sky a little bit and helps your deep sky objects pop out. For example, you might use a 35 mm eyepiece for hunting objects, for finding objects. In the city you might want to use a 25 mm or a 20 mm eyepiece instead.
And going to higher magnification doesn’t necessarily mean you’re looking through a peephole anymore. There are 70-degree eyepieces (apparent field of view), 82-degree apparent-field-of view eyepieces, and now 100-degree apparent-field-of-view eyepieces.
OMA: Yes, from Televue.
RM: And they give a bigger true field as well and make it easier to find things in the city while keeping your magnification up. I have both Ethos’s and I swear by them, although they’re not for everybody obviously, they’re costly and they’re something that longtime and most advance amateurs will be interested in. But you can buy an 82-degree apparent-field-of view eyepiece for less than $100. They may not be as good as Naglers, but they can be a big help for city observers. Again, you get fairly high power, but you get fairly high true field of view with one of these eyepieces and it helps objects pop out while giving you a decent swath of sky to help you find it.
OMA: Another obvious accessory of course is as light-pollution filter. As I understand it, there are three kinds of these filters, broadband, medium-band (which filters out more light), and narrowband line filters that look at a particular spectral line of the object. Do these filters help for city observing?
RM: I guess the first thing we ought to talk about is what they won’t do. What light-pollution filters (LPR… light-pollution reduction filters) will not do is help with anything but nebulae. They will not help with anything made of stars. That includes stars. That includes open star clusters. That includes globular star clusters. And that includes galaxies. Unfortunately, the light of the stars falls into the same wavelength as earthly light pollution. So if you block out earthly light pollution, as a consequence, you are going to block out the light of stars. That being the case, the only thing a light pollution filter will help on is a nebula. But that’s a good thing. Because nebulae are harmed by light pollution more than any other object.
Now there are three kinds of filter you can buy. A broadband filter like the Lumicon Deep-Sky. A medium-band like the Orion Ultra-Block, or a line filter like the Thousand Oaks or Lumicon OIII. You can make is easy on yourself by x-ing out the broadband, the Deep Sky.
OMA: They don’t help?
RM: They don’t help enough to do pea-turkey for you. Some photographers find that the wide filters help trying to do imaging from the city, some don’t. But they don’t do enough to improve the contrast to make them worth bothering with for a visual observer. For most visual observers, those who want to start out with one filter, a medium-band filter like the Ultra-Block or the UHC from Lumicon is what you want. Not only do they provide an incredible gain on nebulae, both in the city and out in the country… but gain I mean an increase in contrast; they don’t make anything brighter, that can’t… they work on many, many objects. An Ultra-Block or UHC filter will work on most nebulae.
Now the next step up is the line filter, the OIII. Those are incredible filters; they have a couple of drawbacks. Some people will say they’re very dense filters, they don’t work well with small-aperture telescopes. I’ve actually had fairly good luck with them on small-aperture telescopes. Their main drawback is they don’t work with everything. For an OIII filter to work on a nebula, that nebula must be radiating a certain range of frequencies. From doubly-ionized oxygen, the “forbidden lines” in other words. A lot of nebulae radiate those wavelengths and a lot don’t. For example, I’ve never found M42 to be overly improved by the OIII. Many planetary nebulae are helped a whole lot by them. OIII can be an incredible filter on the Veil Nebula. Probably an OIII is best as your second filter.
Then of course you can go to the narrowest line filters like the H-Beta. Mostly those are useable on a few objects like the Horsehead Nebula and the California Nebula, objects that you won’t see from the city anyway, filter or no filter, big telescope or no big telescope. So consider an H-Beta only if you want to chase dim ones from dark country skies.
OMA: Have you had any luck with hydrogen-beta filters? With the Horsehead?
RM: Oh yeah. I just got back from a stargaze where it was quite easy to see the background of the Horsehead, (the emission nebula) IC434 with an H-Beta. And with just a little looking “Mr. Horse” popped right out. He didn’t look like a horse, but I did see him the weekend before last with direct vision with an H-Beta filter in Chiefland, Florida. But the catch is I was using a 42-inch Dobsonian.
RM: But that said, he did look like his pictures with direct vision. No averted vision or imagination was required.
OMA: A 42-inch.
RM: A 42-inch known as “The Beast”.
OMA: Yes, yes, I’ve heard of that telescope.
RM: It rocks!
OMA: I wouldn’t mind having one of those myself, or at least taking a peak through it.
RM: I hope you get a chance one day. It’s worth it.
(In the last part of this interview… the best celestial objects to see from the city; why skyglow isn’t the worst enemy of the urban astronomer, and Rod’s favorite things to see in the night sky …)