LX80 vs LX90
Posted 20 December 2012 - 10:46 AM
About a year ago I dusted off my, ashamedly unused, ETX-90 and within minutes became hooked on backyard astronomy. Within weeks however, I equally became frustrated with the mechanical limitations of the 'scope (I have made posts to this effect elsewhere in 4M).
I expect to purchase my second - and last - 'scope over the coming months. Having grown used to the ETX-90, I am mainly considering the LX80 or LX90, and would appreciate your advice. In the UK I can purchase the LX90 (8"ACF) for approx £1900, whereas the LX80 Mount and 8" ACF OTA (when brought separately) would set me back around £250 more.
In short, what would you recommend? I live in a light polluted area, but my ETX provides beautiful planetary views. Regretfully, deep sky images, such as M31, are a faint blur even when using a sodium filter - something I would like to improve upon. In due course I would also like to explore astrophotography, and as such I am drawn to the flexibility of the LX80 mount over the fork mounted LX90. However, the reviews of the LX80 appear less favourable than the LX90! Assuming I stick to an identical OTA, which mount is the best, i.e. doesn't possess the same mechanical limitations of the ETX (plastic gears, lots of play in the gear train, rubber banding, excruciating noise etc)?
This will be my last 'scope so getting this decision right is absolutely critical as I've yet to find a tree growing £100 notes...
Best regards and clear skies, Matt Steele
Posted 20 December 2012 - 03:22 PM
From what I have heard, the LX-90 is a proven scope that is known to be good with imaging. Not as good as an LX-200...just not as heavy and maybe some differences in gearing, etc. I
doubt if the LX-80 has been out long enough for any really
firm opinions to be formed yet. In terms of what it can theoretically do, it is a valid option......Just no confirmation yet from a large number of people. If it proves out, it will be more adaptable for less over-all money than the LX-90. The LX-90 needs a wedge for the most serious types of imaging that the scope can do.
If you would only be concerned with casual imaging or none at all, you might want to consider either the LS or LT models. The LS has many bells and whistles and has worked well for me so far. However, I have not tried all the "on the edge of the envelope" things that a person might try with it. It is a very convenient scope. The other option is an LT. It would be like the LS except with bells and whistles more like the LX-90 except that it cannot be put into an equatorial mode. (Same for the LS) Both the LS and the LT are going to have more wiggle than an LX-90 due to the lighter design. The LS and LT 8 inch ACF weigh in at about 39 lb., while the LX-90 is about 53 lb, and the LX-80 about 80 lb or so.
The LT-8 ACF sells for about 80% of the cost of the LX-90 ACF or the LS-8 ACF.
With the price of a wedge for the LX-90, assuming you might want to do some serious imaging sometime, the LX-90 and the LX-80 would be very nearly a wash in terms of total expense. If you are only interested in visual astronomy, the difference in price between the LT and the LX-90 would buy you a pretty good set of eyepieces.
Just thoughts.....hope it helps.
Posted 22 December 2012 - 12:50 PM
Thanks for your reply.
I have looked at the LS as a possible replacement for my ETX - it does appear to be Meade's natural successor. My concern is the quality of the mount versus other options - hence my interest elsewhere.
Here's another question for you, if you're up for it?! In your opinion, and with all other considerations being equal, would a 10" SCT OTA perform as well (or out perform) an 8" ACF OTA? Is there really that much of a noticeable difference between an 8" SCT and 8" ACF?
Thanks in advance, Matt
Posted 23 December 2012 - 09:04 AM
Here are my opinions and what facts I can give you. A lot will depend on you, your eyes, and what interests you. I am mentioning a lot of technical stuff, but don't get all confused or bothered by it. For most of your viewing and astronomy activities, it is not going to make a whole lot of difference. It will make a difference mostly when you push your telescope to the outer edge of its capabilities. I normally end up doing that, one way or another.
There will be a slight improvement in resolving power, but I do not think you will really be able to tell much difference, unless your start trying to split double stars with separations below 1 arc second.
Light gathering power will go up in a 10-inch, with the square of the diameter. The difference will be about a quarter of a magnitude. There may be a few objects that you can see better visually, but with imaging, I understand that you take longer exposures to make up that difference.
The potential magnification will go up from maybe 400x to 500x for visual observation, but for most things, other than maybe the Moon, people that I know rarely go over 300x anyway. Atmospheric instabilities, transparency, etc. become larger factors as one goes up in magnification. I normally use only my 26, 14, and 9 mm series 5000 five-element plossls and a 2-inch 30mm fl, 80 degree, AFOV eyepiece. I do use an f/6.3 focal reducer for wide views with the 30 mm eyepiece some. (I have a 2-inch diagonal on my LS-8 now. I very rarely feel the need to go above 200x, but know that I can if I want to under the right circumstances.
The down side, other than weight and cost of the 10-inch will be a loss in field of view. Both scopes are f/10, so the focal length goes up with the diameter. For any given eyepiece, the magnification will go up proportional to the increased focal length and the true field of view decreases proportionately. For objects you want to look at that are large, you will be fighting more to get the wider field of view with the ten inch scope than with the eight inch.
If I have out a really short f-ratio telescope, such as an f/3.8 144 mm reflector I have, I find myself fighting for more magnification to get up to the potential of the scope. With a high ratio scope, such as my f/10 LS-8, I find myself fighting to get a wider field of view to see things that I am used to viewing. It is just part of the trade-offs with different scopes.
As far as light getting into your eye, which has a lot to do with the limits on magnification and field of view, about the smallest image that is useful going through the lens of your eye is somewhere in the 0.4 to 0.5 mm range. To figure out that top magnification, divide the diameter of the scope, in mm by, let us say, 0.5. That gives you the 400x for the eight inch and 500x for the ten inch that I mentioned earlier. On the low magnification end of things, the largest diameter of the image will be somewhere between about 5 mm to 7mm, depending on how far open your iris will go. For me, at age 61, I use 5.5. For a young person with good expansion of the iris, that figure could be 7 mm. For me, the lowest magnification with my 8 inch would be about 36X. With the 10 inch and its 254 mm diameter, that magnification would be a about 46X. With a given eyepiece you divide the apparent field of view by the magnification to get the true field of view. Again, A lot depends on what kinds of things you want to look at or image as to which scope would be best.
John Graham, who has done a lot of imaging as well as a lot of visual astronomy for many years, posted about this in another string in this beginners section. In it, he recommends an 8-inch scope in the LX-90 series for imaging. For anything larger, he thought someone would need to go to an LX-200 mount to get away from wiggle. There are, however, other people who do use a 10-inch LX-90 for imaging. It may depend a lot on what kind of imaging standards you set. John also stated that with visual astronomy, "apperature rules." He recently bought a Lightbridge 16 for visual work.
As far as the comparison between a traditional SCT and an ACF, I think either one is good. The selection of which one, right now, depends again on what you want to look at. The ACF is really sharp all the way to the edge for all but the widest configurations. The SC will have some coma around the outside, but it is not objectionable in my somewhat under experienced opinion. For work at a lower focal length, there is not a reasonably priced focal reducer for an ACF right now. One company has one that is in the $400 range, while the f/6.3 reducers for an SC cost around $100 and there are many brands out there. With a standard SC, there is additional coma correction in the focal reducer that takes out the coma around the edges. This is very important since coma gets worse as you go down in magnification and increased field of view. With the ACF, using a standard focal reducer with coma correction injects some coma at lower magnifications by going too far in the "opposite direction." The ACF does need some correction "way out there," but not nearly as much as a normal SC. In the selection of which one to get, right now, you have to chose when you want the coma to show up. The coma is not really objectionable, in my opinion, but it is definitely there. If some company comes out with a focal reducer specifically for the ACF at a reasonable price, then I will buy one and have the best in both worlds. Compared to what you probably have with your ETX 90 and its limits on magnification, field of view, etc, I doubt that you will really have a problem either way.
If you are the type of person that would have regrets down the road, simply because the ACF is a little bit better on the edges in its natural mode and has the potential of being just as good in a powered down mode, you probably should spend the extra $200 or however many pounds that is, and get the ACF. If you do not think that will bother you, then the extra money could buy you several good eyepieces.
I noticed your comments on trees with 100 pound notes on them. Those are very rare and extremely hard to grow. Next spring, try planting shillings in a row at six inch intervals, three inches deep in a flower bed. Water well for six weeks and see what comes up. (smile)
Hope this helps,
Posted 02 January 2013 - 05:33 AM
Many thanks for this post, and Happy New Year to you.
You have given me plenty to think about, which is going to require further research. I think joining an Astro Club would be of much use; to practically encounter the kind of issues you have raised.
In terms of what I hope to reasonably achieve (that old friend 'work' does get in the way on clear nights!), I want to maximise my visual experience, and treat AP casually, at first. If I get seriously hooked on AP, then I will pursue idc, assuming whatever scope I eventually purchase offers this degree of flexibility.
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