What kind of new small scope
Posted 03 July 2003 - 10:19 PM
To give a bit of background: I have an etx90-ec, that I have gotten a lot of use out of. Trying to raise a son and trying be a husband limits the amount of observing time I have. I have not been to a dark site since September 2002 and only then because I was a guest speaker. I do most of my observing between 11:30 and 12:30 from a light polluted backyard. The etx90ec has been a good scope because I can download a lot of observing lists to the hand controller and then when the opportunity strikes I can go out the door with the scope in hand and be setup in under 3 minutes (the deck has markers where the tripod legs go). The only problem is the etx90ec's limited size. I have a buyer for the etx90ec, but what scope to replace it with? Meade 6" lxd-55 reflector, Meade etx125 or a nexstar 5? I've even considered a JMI ngt-6 as it only weight 35 pounds and has digital setting circles. Ron says I'm setting sights too small. Does anybody have a suggestion for another scope.? Money is a critical factor so don't suggest a 5" short focal length APO as I don't have the money for that, Besides I have to support my own and my son's habit of buying high-performance computers for home
Posted 03 July 2003 - 10:39 PM
Just from what I've read, a C5 should do the job quite nicely. However; may I suggest a binocular? I was out with mine again, many times I prefer it to the Pronto, and I was free handing it then I mounted it on the mirror (one of those ex-tank periscope mirrors oberwerk sells so folks with stiff necks like myself can view in relative ease).
To keep the costs down you are probably looking at 70 to 100mm (www.bigbinoculars.com) with the 22X100 running in at $395.00. Should be able to afford it if your kid gets a paper route.
Should be interesting to see what others come up with for suggestions. You'll find we love spending others money as much as we like seeing the dollar sign, I mean stars.
Posted 04 July 2003 - 02:34 AM
All the scopes you mention seem fine. I think there are only slight differences between the ETX125 and the nextstar5 so you might go for the meade since it resembles the ETX90 most. It's the same scope but with more aparture.
A 6" scope will show you even more but will be heavier and larger.
If your budget and size tolerances allow you could look after a second hand nexstar8 or a 8" LX10, LX90 or LX200
But things are getting a little big now.
I hope I've been of some help.
Posted 04 July 2003 - 05:43 AM
My suggestion would be exactly what you suggested yourself; get the larger etx model; you seem to be happy it, using it's functionality (goto, observing lists), the only problem is aperture.
If you want alternatives (and still want GoTO; take a look at the new Celestron series; C5-SGT for instance.
If GoTo is not a pre; take a look at the 127mm MAK from Orion, i'm very pleased with it!
Ron; every time I see those 22x100 it's harder to resist them.. now i have to go to bigbinoculars.com again and dream away....
Posted 04 July 2003 - 06:25 AM
Posted 04 July 2003 - 08:37 AM
Posted 04 July 2003 - 11:36 AM
Just had a brain storm (had a different word in mind but I barely know you) and the Questar came to mind. Money issue doesn't meet your criteria but that seems to be everybody's problem!
My 15X70 barely show the rings of Saturn and that was after I recollimated them (they appear to have been collimated upon shipping so the only thing I can think of is that I'm gaining experience with them). Of course any clear optics can show the moons of Jupiter but I can make out the larger bands of Jupiter. Everything else, to my eye, are shiny dots or faint fuzzies.
M31, Andromeda Galaxy shows up best in my binoculars, I think it's the two eye thing working again.
Like many I drool over the larger bino's; e.g. 22x100's but there was some discusion earlier about field of view and something on the order of 11X42 had the widest. Having a nice tool chest full of a range of bino's is not far from the truth. Since I have the 20mm (don't ask), 35mm & 70mm, it would not take much to add the 42 and 100. I'd spend more money getting eye pieces for a binoviewer - one pair!
Have you visited a telescope store to handle the units you are considering? Star Party? An old Atmospheric professor of mine was big on navy surplus for toys. Or perhaps you could write one into a grant.... Since you're always upgrading, like renting a car, you get to try out a few before you commit person funds. :0
Posted 04 July 2003 - 02:04 PM
Of course, different people live in different conditions and look for different objects, so advice may not always be suitable for your conditions and preferences. You mention that you live in light polluted skies, and want to be able to see planets and double stars. In my skies, 50x is about the lowest I can go with my 5" Mak before skyglow reduces the contrast. Sounds like Binocs are completely out here.
I have seen thru a C5 a number of times back in the 80's, it was a nice scope, decent resolution, and so-so contrast. When I got the 5" Mak, I expected pretty good resolution and so-so contrast. In fact, the contrast turned out quite good, and I'm very pleased with it. I think it is better than a Schmidt at lunar/planetary, but the SCT (with shorter fL) can go lower, with more field, and may be a little better at deep sky.
I have the Orion 127 Mak, which does not have GoTo feature. I don't know how the Meade compares, but I personally don't care to buy Meade products. Being that I fix electronics for a living, I am paranoid of the stuff breaking, so I avoid computerized scopes. Besides, I got used to finding things for my own back in the 70's - 80's, and I don't want to lose that skill.
Posted 04 July 2003 - 05:17 PM
Posted 04 July 2003 - 09:55 PM
Mr. Pasken, I'm sorry for not reading your post carefully. I also use Vixen 20X80 binoculars, but for a quick instrument I perfer my 80mm ST refractor. It is much more comfortable observing near the zenith with a diagonal and the magnification can be changed. For this particular scope the useful range would be 12.5X to 44X. Over that the images are soft, but this is a subjective limit.
I also use a 5" Russian Mak. I think it is a good compremise between portability and aperture. My Mak is an f/10 and so similar to the C5. It can use 2" eyepieces and with my current collection I have a magnification range of 25X to 336X. 336X is only practical on the moon. If higher powers are not a problem for you, I would probably go with the ETX Mak over the C5 Schmitt. When I was doing research to purchase my scope, a few reviews found the Schmitt images a little soft, this was never mensioned with Mak reviews. However, if you like working at lower powers, the C5 will be more versatile. As for cool down, leave the scope outside during dinner and it will be ready when you go out, but I'm sure you know this by your experience with the ETX 90.
Posted 05 July 2003 - 07:22 AM
Posted 05 July 2003 - 10:51 AM
Blair the APM site refers to the ability to see low contrast detail on planets. In this case indeed the 5" with a 39% obstruction resolves low-contrast detail like a 3.5 scope. In terms of light gathering power the central obstruction reduces the light gathering power to a 4.75" scope. There have been several articles in S&T about this over the last few years. This issue has been the debate between refractor and reflector owners for as long as I have been in astronomy (37 years)
Kent I lean towards the GOTO simply because it allows me to see more in the limited time available. I can star-hop but have always preferred to use analog/digital setting circles. I don't want to star a flame war over star-hopping versus setting circles/GOTO, but for me an equatorial mount and setting circles is how life should be. Having inside the etx90ec I am also concerned with the design and build quality of the etx series, which is why I was looking at the Celestron.
Still looking for the $600 40" equatorially mount reflector with 1" accuracy GOTO in trailer, oh and throw in 40 acres in the adirondak mountains
Posted 05 July 2003 - 12:34 PM
If you have at least a 23mm hole in the mirror to allow the light to get through to the diagonal how does this equate to 4.7". Physically, it appears it is really 104mm (127-23) on Orion's model anyway. I've always been a refractor user and just recently bought a Mak (Orion's 127mm) which I returned because it did not perform that much better than my Vixen made 90mm refractor on anything I looked at including DSOs. I guess portability is the key advantage of the Mak design but I consider my refractor portable enough for home use. I do not use GOTO. But then because of my seeing and light pollution I have a very short list of favorite objects (the brighter Messier, Caldwell, and lower power double stars) that I can zip through much quicker than GOTO can slew but if I was at a much darker sight then I might need it. I've been primarily a binocular user (15X80) the past years and just about a year ago decided to own a telescope again. So, scope theory and real use experience I'm definitely in the learning curve.
Posted 06 July 2003 - 12:39 AM
I don't know about the 39% obstruction, my Mak (which is over f/12) does not seem to have such a large obstruction. But the diffraction rings are still evident, and can be annoying on tough doubles. I have noticed that light throughput is low, and this seems common. Brightness seems more comparable to 4" Newtonian (to me)! Nevertheless, I have seen M1 in town, but star clusters (including Glob's) seem to do better than nebula. I think the Mak's resolution helps there.
The Mak has given me better planetary images than any other small reflector that I have seen. I have not seen through too many good refractors lately, but in an email to Orion, they told me that the only refractor they have that can beat their Mak is the long 5" (and they called it "slightly better"). That is big, and way too big for the mount, IMHO! So for me, I don't see any good alternative to what I have now. And I'm not gonna buy an 80mm no matter how good it is, I'd rather keep the Mak.
Okay, back to GoTo's.... (I can believe a PhD would need a computer)... JUST KIDDING BOB! Please don't get mad at me. I can't say anything about the Meade, because I don't know. But I did get to use a C5 spotter on a nice tripod for awhile back about 10 years ago. On deep sky, it was better than a 4", so it had good throughput, a good scope for DSO's! It had decent resolution too. I could see the faint outline of the Red Spot, and a little variation (detail) in equatorial belts on a good night. But contrast was low, my Mak makes those subtle things pop out better. I think color filters could help, just as it does on any scope. All in all, I think a C5 is a good general purpose scope. Only if you want max performance for planets or tough doubles (at the expense of deep sky) is it a less than ideal choice.
Some others may disagree, but these are my obsevations,
I hope this helps.
Posted 06 July 2003 - 12:50 AM
Posted 06 July 2003 - 09:03 AM
the resolution of any telescope is based on the effective DIAMETER of the objective. Hence for a telescope with a central obstruction (Deffective = Dprimary - Dsecondary) on the other hand the light gathering power is based on the effective AREA of the objective (Aeffective = Aprimary - Asecondary) Using a 5" SCT and a 4" refractor as examples:
Primary: 4" Secondary: 0" Effective Area: 12.57 in^2 (4") Effective Diameter: 4"
Primary: 5: Seconary: 1.9" Effective Area: 16.80 in^2 (4.62") Effective Diameter: 3.1"
To quote William Zemak (Sky and Telescope; 1993 pg 92) "In fact a 6-inch Newtonian with a 1" diagonal is really a 5-inch telescope, as far as its potential performance on palnets is concerned. The diagonal is less harmful to light grasp, where the unobstructed area is what counts. Hence the effective aperture is about 5.9" Sky and Telescope ran a two part article on resolution and light grasp in 1993. The focus of the article was the role the secondary plays in resolution and light grasp. Just as a side note the performance of an obstructed telescope is slight better than a unobstrucuted telescope FOR FINE DETAIL CLOSE TO THE RESOLUTION LIMIT.
OK now back to work on that proposal [list]
Posted 06 July 2003 - 11:08 AM
Posted 06 July 2003 - 11:51 AM
If you took a drafting instrument, say a protractor and drew two circles, one inside the other - to represent the maximum diameter of the objective; the smaller one to represent the hole, so you have a donut and I could come along and say "Are you designing a newtonian?".
From the front, you really can't tell if that obstruction you drew is a hole or the back of the diagonal or the back of a secondary mirror. The light can't tell either. For all practical purposes they are the one and the same.
Behind that obstruction you can place any kind of mirror. Either a focussing mirror like in the SCT, flat angled one like on a newtonian or one that flips so the scope doesn't know what it is like some cool ones Takahshi make.
In the case of your question and that hole in the mirror (like a SCT I assume) something has to get the light from the mirror thru that hole. That something is the focusing mirror placed right behind that obstruction you drew moments ago. The light passes by the obstruction, off the mirror up to the secondary which bounces back thru the hole in the mirror to off who knows where. That obstruction will be roughly the same size as the hole. OK?
So what Bob explained still applies. Of course light has the dual nature of being a particle and a wave but this is the final day of a three day weekend so I propose we leave that part till later!
Is this all clear as mud, beating a dead horse, clear things up or do you feel like taking up bowling for dollars?
Posted 06 July 2003 - 11:58 AM
Anyway to answer your question, consider that light rays are entering the telescope all in a straight line parallel to the tube. The Secondary creates a shadow on the the center of the primary mirror, so whether there is a hole there or not is irrelevant. In a conventional Newtonian design, the center of the mirror is just wasted space as the mirror reflects the image back up the tube to the secondary mirror and out the side. In the cassegrain, whether a Mak, Schmidt or others, the light reflects off the primary mirror back up the tube to a secondary mirror facing back down the tube instead of out to the side. This mirror then reflects the light back down through the whole in the center of primary mirror. The design could be varied through the use of additional optical elements but this should get you started. I have purposely avoided adding the complications of the corrector plate etc into this, as it is not pertinent to what your asking.
Posted 06 July 2003 - 12:11 PM
Posted 06 July 2003 - 01:32 PM
Posted 06 July 2003 - 10:06 PM
The central obstruction has two effects - first, a slight reduction in light gathering of the scope and second, to increase the effect of diffraction at the aperture by shifting *some* light from the center into the outer rings of the diffraction disk, which reduces contrast a little, but not resolution. Thus, a 6" Newt resolves like a 6" scope, but with a little less contrast than a 6" refractor, and gathers light like a 5.9" one.
My 11" SCT with a 3.75" central obstruction resolves like 11", not 7.25", gathers light like a 10.1" unobstructed scope, and despite the reputed loss in contrast performs quite well on planets though not the same as if it were a 11" APO (or Newt) I suppose
Posted 07 July 2003 - 03:51 PM
Bob, have you decided yet on the scope or are you still not sure? Can we actually help you with anything at this point if so let us know!
Posted 07 July 2003 - 08:53 PM
He has a lxd-55 6" refractor. THe refractor is too large bu the 6" SN might prove to be a useful alternative.
Posted 08 July 2003 - 01:16 AM