In the three years that I’ve been playing around with this hobby I’ve acquired five telescopes. Three are Dobsonian mounted Newtonian reflectors and one is a catadioptric, a Maksutov-Cassagrain (the ETX). The last is a refractor, a 80mm Orion Short Tube that I got as a finder scope for the obsession. It also doubles as a "quick look" scope. Now I own one of each type of telescope. I started with the ETX succumbing to pretty ads and a dream of a cheap computer operated telescope …
There they are. The tiny Meade 90mm ETX on the floor. The Discovery telescopes f5.6 10" ‘sky cannon’ on the left, and the giant Obsession f5.0 20" on the right.
The Mak/Cas or ETX is a small portable telescope. Yet, to my mind it suffers from a long focal ratio (about 12.6 I think). What this means is that it is biased towards more magnification and can’t deliver wide field views. You might think that magnification is good, yet there is a limit to how much magnification you can use on a given telescope. You see, magnification dims the image by a power of two, and if the telescope can’t gather enough light to overcome the image dimming that comes with increasing magnification, well the image gets to dim to see and you lose all detail. Yuck. And magnification limits your field of view. So you are limited to looking at bright,smaller objects. Large open star clusters and star fields will not fit in the field of view, and you can’t push the magnification far enough to look at really small objects. I’ve spent a lot of time looking at open star clusters so I have begun to favor lower power and wider fields of view – which the ETX cannot deliver. Yet, for a scope in the 80-90 mm range it yields nice ‘contrasty’ views. And, unlike the Orion Short Tube, it does not produce lots of false color around bright objects at high power. And, it’s cheaper then a "nice" refractor. But in my opinion less versatile then a high quality f5-7 refractor (ex: TV Pronto or TV 85mm). A good refractor in this size range will have more flexibility in terms of field of view vs. magnification. With a 32mm 1.25” EP you can get about 1.3 degrees of field of view with this scope. Not bad but vastly different from most other scopes in this size range which have shorter focal lengths and faster focal ratios. This is related to the Mak-Cass design.
The ETX looks cute. It is a nice telescope for medium to high power views of bright objects: the moon, the planets, double stars, and the brighter star clusters. And if you can get to dark sky’s (Hawaii was nice) it can see the brighter nebulous (diffuse) deep sky objects as well (from our hotel balcony in Maui I could see the ring nebulae – I could never do this from my driveway or porch). The ETX is the small thing on the floor in the photo. These days I use it as a solar observing telescope. It tracks pretty well when set up as an equatorially mounted scope: I have a "Thousand Oaks" solar filter for it. It is very portable and can be set up on a table. Our wine appreciation club was meeting and I was asked to bring a telescope. The event organizer forgot that in the summer it stays light late. I set the ETX up on the bar and defying convention we observed the sun (appropriately filtered of course) through the window.
The ETX is "under mounted". Mine used to shake when the drive was tracking the earth’s rotation. The "jitter" that the drive produces has really diminished since I got a set of those Celestron anti vibration pads. I’d recommend them to any one with vibration problems in their tripod (can’t hurt). In addition, I bought the ScopeTronix Aluminum Tripod Leg Clamp Set. The clamps make the legs stiffer (as there are now upper and lower clamps on each leg) while the pads damp vibration. These "fixes" make it a usable scope. However there is "slop" in the altitude drive of my unit. When you put a heavier eyepiece in it the telescope moves. At high power the desired object may move out of the field of view. This is frustration at higher powers. The ETX doesn’t get out much anymore. But, this is what I began to explore the sky with.
If it wasn’t so portable and If I didn’t use it when teaching to be and example of a catadioptric telescope I think I’d sell it. Perhaps if I could put it on a good Alt/Az mount …
The 10" f5.6 Discovery telescopes Dob is called the "sky cannon" by my daughter. I believe that the following photo explains why.
This is the telescope that I’ve used to learn my way around the sky. It has a larger aperture (its wider and gather’s more light) and that allows you to see fainter objects and to push the magnification. It has a theoretical limit of 500x, I have the eyepieces to push it to 406x, although usually the sky conditions here limit me to 200-250x. When the sky is good and it is mounted on its equatorial platform (allows the telescope to track the rotation of the earth) it delivers good high power performance on the planets. It has enough light grasp to observe the deep sky. The views of the Veil nebula through the OIII filter at the Blackwater Falls Star party were amazing.
I had thought about the Obsession for a long time. I bought the 10" as a learning scope to see if I could live with a dob. I bought it as a 10" f5.6 (many 10’s are f4.5 so the tube is about 48" long rather then about 60" long for the f5.6) as that is a bulky size (that black OTA weighs about 70-80 pounds) and is a little difficult to move around. I wanted to see if I’d tolerate some inconvenience (i.e. actually use it) before getting the bigger (and more expensive) 20".
I learned some things about dobs with that 10". First, you need to check and confirm that the telescope will fit into your car. The 10 f5.6 is too long to lie across the back seat of the Camry. Finally, I discovered that if I laid the passenger seat down flat I could fit the OTA (the black tube) in the car facing front to back. Until I figured this out, I couldn’t take the telescope in my car. Bummer.
Second, a Dob is easier to assemble than an equatorially mounted telescope. Yet, it cannot track the earth’s rotation, so if you don’t push it along what ever you are looking at eventually wanders out of the eyepiece’s field of view. This is not a problem at low power. But, as you increase the magnification, the eyepiece’s field of view decreases and objects begin to move through the eyepiece’s field of view faster and faster. One partial solution is to buy expensive eyepieces with larger fields of view, but this only gets you so far. The other solutions are to motorize your telescope (This technology is still in its infancy; the motor controller has to drive the two axes at different speeds which vary depending on where the telescope is pointing and thus needs a computer to control the drives.) or to get (or make) an equatorial platform. The latter is simple, elegant, and lets you track the earth’s rotation for up to an hour (with a simple motor). You can see the equatorial platform under the 10" (I like simple).
Still at least half the time that I use the scope I don’t use the tracking. If I roll it out of the garage for quick solar observing (I made a 10” full aperture white light solar filter for it) I just pop in the EP, focus and observe. I’m usually at 64-115x so tracking is not a big deal. If I use this for quick observing secessions at night, and if I’m not going after a challenging object of planning a long term planetary observation I’ll just use it with out the tracking.
The motor and control box are on the right. There are two wheel bearings in the front. One is motorized; the other is not.
Notice that the 10" dob sits on a wheeled platform. I made this by screwing together 2 pieces of 1" plywood and attaching wheels (?castors) to the bottom. This makes scope set up a breeze. I leave it set up in the garage, and when I want to observe I just wheel it out onto the driveway. I can be ready to go in about a minute. Usually, I look at the sky’s on the way home. If I’m not in the mood to drive to a dark sky site, I wheel it outside and set up a 20" house fan to blow air at the back of the mirror so that later, when its dark, I can just walk outside and observe. No fuss. No muss. If you think about making a rolling platform for your dob (I stole this idea from Ed Ting by the way), don’t cheap out on the wheels. Go for big with sturdy mounting hardware. Make sure the wheels can handle the weight load. Bigger wheels go over bigger cracks and bumps. If you don’t believe me, check out Ed Ting’s site, its down the page under avoiding aperture fever. (Why would you want to do that?)
Any way it is great to have a scope of this size that can be easily moved out onto the driveway for a quick look observing secession.
The Obsession has wheel barrow handles. But to move it assembled (only recommended on flat pavement) is a two person job. I have yet to make it a platform. I don’t think that I ever will. There is too much local light at my house to make that worth while.
The Fan that came preinstalled on the Obsession is smaller then the 4.5" fan I installed on the 10". Mirror cooling is now thought to be important to allow your reflecting telescope to create the best images possible. The idea is to cool the main mirror so that its temperature is within 1-2 degrees of the air that surrounds it – hence the fan.
Notice that when you have a Dob you can feel free to use as much duct tape and bailing wire as possible when making after market upgrades. Yes, that fan is attached with real wire and little brass colored cup hooks. I use only the finest materials. Imagine the pride that I felt when the TV85 (TeleVue 85mm refractor, about $2k) owner came up and told me that the best views of Jupiter that he’d ever had were through my scope (not that I wouldn’t mind having a TV85). The humble cardboard "tube dob" coated with duct tape. Aperture wins, at least when the sky is steady … and your mirror is cooled … and the collimation is good.
I added a dew heater to the secondary on the Obsession. Rather than using the heat rope supplied by Kriege, I glued one of Jim Kendrick’s secondary heaters to the back of the secondary. I used the thin copper tape (metal foil on one side, sticky stuff on the other – what would you call it?) that he supplied to run electricity out the spider arms. There is a strip on each side of the spider vane (one + one -). My son did the soldering. (They try to keep me from handling dangerous tools.) A detail photo follows, notice the tool less collimation of the secondary (thumbscrews) which is a new feature on the Obsession:
The obsession moves like a dream. It looks nice. The components are well made. The few DSO’s I’ve seen are bright and show more detail than in my 10". A telescope of this aperture requires dark skys. At home there is too much local and reflected light to look at anything other than stars or star clusters. Globular clusters are beautiful in a scope like this. From a dark sky site, Planetary nebulae that were hard to find in the 10" look like small headlights. The ones that are about magnitude 9-10 that is. I haven’t pushed the power on these yet. Galaxies also look great. I view that I had of the Orion nebula was amazing. I’ve never seen such detail or extension in the nebula. Wow. The one night that I had it out we saw over 30 objects, not bad for a hand driven scope. My observing notes from that night are found here.
One thing that is nice with a scope this size is to pick a constellation and say “I think I’ll look at all the NGC open clusters in this constellation”. As long as the clusters are smaller then a degree most are easily found. Can do. This happens to me a lot. I’ll go out with an observing plan and end up noticing that one constellation seems to be well positioned in the sky and I’ll just try to see how many NCG objects I can find there.
But what about aperture fever , ok I had it, but now I have a 20" so its under control. There is a concept, and I may have gotten this from Ed Ting too (I have few original thoughts.), that there is a aperture limit for any given location. At my house, I’d say that it’s around 10". Or more accurately it’s greater than 10" but less than 20". If you live in the city it may be less, 6", 90 mm, 80 mm – I don’t know. What I do know is that if you have access to "dark skys" where you can see stars that are dimmer than magnitude 5 (ideally dimmer then magnitude 6, no 6.5, no 7, … ok you get the idea), then a large aperture telescope begins to sing. I’ve seen the Orion nebula from home and a dark sky site in the 10". There is no question that these dim, extended objects look better where the sky is dark. When you take your 20" to a dark sky site and look at the Orion nebula, well it’s enough to make your jaw drop. It’ll give you religion. So, in my mind, if you can get your big scope out beyond the aperture limit imposed by a city’s man made light, you will be rewarded. It makes you start to think, if only I’d gotten the 25".
(Want to know more about light pollution? Visit my web page on it. It’s your gateway to the IDA web site (i.e. there’s a link to the International Dark Sky Association web site there.))
But a man’s got to know his limitations (I’d love to speak only using lines from various movies, wouldn’t you.), I felt that a 20" was the biggest scope that I could move and use by myself. I was afraid that a 25-30" would be a 2 man job or require a dedicated trailer with electric winch. As these things get bigger they get heavier.
Alas, now I have a fourth scope. I bought an Orion "Short Tube"
80mm to use as a "quick look scope" and as a finder scope for the Obsession. I
got it for low power views. With a 32mm Plossyl for the eyepiece, I hope to have
a field of view of about 4 degrees at 12.4 power. It weighs 3.1 lbs,; it’s going
on the UTA. After Christmas, on a cold night, I peaked at the Pleiades, the
Messier clusters in Auriga (smudges), the double cluster (adjacent smudges), as
well as Jupiter and Saturn, all at 16x.
I think that all of these f5 80mm short tubes come from the same factory in China. Orion sells them. Apogee and Discovery sell them. I got the equatorial mount from Stellarvue. It is sturdy. No need for any fixes. I have the "back packer" model. This guy is moving away from the cheap 80mm refractor and carries a whole line of 80mm refractors in various focal lengths. These have more exotic lenses and should have less problems with false color than the cheap f5.0 refractors like the "short tube". I’d like to see how his new f6.4 refractor does.
What is false color? Well, with standard glass lenses, its hard to get all of the various wavelengths of visible light to focus at the same point. (Remember how white light is made up of colored light and that each color has a different wavelength? It’s ok, I was absent that day too.) The telescopes with standard glass can’t do this so that when you look at a bright object, say Venus, at high power it gets surrounded by the colors of light that aren’t in focus yielding a purple (or to me other colored ring) about the planet. (You try rubbing and scrubbing but you still come out with ring around the planet.) These types of telescopes are called achromatic (for without color which is not true – salesmen). In the olden days, when all they had was cheap glass, they tried to get around this problem by making telescopes with long focal lengths (f10-13) as false color is less of a problem at longer focal lengths (at the cost of higher power views and smaller fields of view). With a f5.0 Short tube false color is a real issue if you try to push the high power. But it’s designed for low to medium power views. More modern refractor designs, using exotic (aka expensive) glass or exotic fluorine (or who knows what) lenses can focus the light, have a short focal ratio, and produce images with less to no false color at high or low power. Yet they are more expensive.
Recently I got a Televue Telepod head and a nice photo tripod. I like this for use with the ST at low powers. Viewing is a snap. I need to get a new Rigel Quick finder as the tiny finder scope is a pain to use. The equatorial mount is nicer if you plan on looking at one thing for a long time or plan to increase the magnification. I acquired a “Tak” Sky Patrol II (Tegul II) mount. This is so much smoother and easier to use then the old “back packer” there is almost no comparison. The clutches are easy to move. The knobs for guiding are like butter.(cold butter). Recently, I’ve been keeping the Short Tube on the Skyn Patrol or the Telepod and grabbing it for quick observing secessions of double stars and brighter DSO’s.
So what’s a guy with 4 telescopes doing buying a fifth? Well it was on sale. $299 including 2 eyepieces! Well, I’ve been pushing the 6" as a good beginners scope. So I got one to use when I teach kids. It is a nice size for kids, not as tall as the 10". And the tube is light. It makes a good quick look scope. The 80mm is just a finder or the obsession right? The views are nice and planets look good. But I got it primarily to teach with. If the kids bang it around I don’t cringe. I had 5 8th graders over to the house the other night and I set up the 6", the 10" , the 80mm and the ETX. I let the kids play arround moving and seting up the 6". When the EP fell out and hit the concrete I did not cringe. That’s what it’s for and it gave me a chance to drive home the point about removing the EP before moving the OTA. I’ve added a Rigel Quick finder (from the ETX) as the supplied finder is less then ideal.
I put a Quick finder mount on this and it is now easier to use. It has too much motion in the altitude dimension. When I change EP the scope tends to swing up. I need to make one of those spring things like on the Orion Scopes. I got to use an Orion 8” dob recently and I like its focuser and the spring tension in the altitude dimension.
The
6” remains a teaching scope. I had it out the other night with the Short Tube
and the binoculars with Larry Oyster when we went star gazing with his class.
Yet, once your used to a f5.6 10” with a FOV of up to 2 degrees the 6” seems
like the ETX, it has too small an FOV and compared to the 10” it gathers too
little light … but it is lighter to carry and easier to set up.
The EP page has tables giving the magnification, field of view, and exit pupil for each of my scopes with my currently available EP. There are also links to several EP calculator sites if you’d like to compute similar info for your scope and eye piece collection! This gives you an idea of the differences in field of view and magnification of scopes of different aperture and focal ratio.
In general as aperture increases the field of view decreases and the magnification increases. You can off set this some what by going to a low focal ratio scope but you can only buy back so much FOV by doing this.
Compare the f5 20” Obsession to the 80mm f5 Short Tube to see the effect of aperture (which increases the focal length). Compare the 80mm (fast) f5 Short tube to the (slow) f14 Meade ETX to see the difference in FOV between a wide field scope and a large focal ratio scope that are of similar apertures.