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Review of Portable Telescope Mounts
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In late 2004 my old portable astrophotography mount died. It was an equatorial mount from my old 4.5" Newtonian. That mount was one of the better mounts in it's range but not ideal for astrophotography (no DEC motor, bad backlash, bad periodic error, etc). So given the chance to upgrade I jumped at it, and in doing so spent several months investigating the available options in some detail.
My intended use of the mount would be 70% astrophotography and 30% casual viewing/other.
Over all I have been very happy with the mount to date. It definitely is built strong, easily handling any payload I have put on it. The Gemini system is quite easy to use once you get to know it's menu system. The whole package is all easy to use, always ready to go, and quick to set up. |
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It is also worth mentioning that while I have an SBIG CCD camera and do use autoguiding in my observatory, I don't use it out in the field and only want the capability for future expansion. Typically my portable astrophotography is all long exposure film with me manually guiding on a guide star. I prefer this as a relaxing activity when out and about in the country as compared to messing with cables, cameras, alignments, calibrations, etc.
This page outlines what I found in my search for a good portable astrophotography mount for a light to medium weight setup. I have put it here for the info of others going through the same search. Written in February 2005 the information here was correct at that time and may have of course changed since.
The Requirements
First, the requirements. What was I looking for in a telescope mount? Well it could be summarised as the list below:
- Capacity to handle two small refractors and a camera or two. Total weight around 5kg not including counterweights.
- Easy and quick setup/dismantle time.
- Relatively compact and lightweight for easy transport when the car is already packed with lots of camping gear.
- Accuracy. The most accurate (periodic error, backlash) I could afford.
- Computer interface (for those times I want to casually view with GoTo/computer linkup) and for autoguiding.
The Options
Given the above set of criteria I narrowed my selection down to the following telescope mounts:
- Vixen GPDX with Skysensor 2000
- Vixen Sphinx
- Takahashi EM-11 Temma
- Losmandy GM-8 with Gemini
There was a large number of mounts I choose to exclude, particularly the lower cost ones such as Sky-Watcher, Celestron and Meade. I knew I could have GoTo, PEC (periodic error correction) and backlash compensation with any of those but I didn't believe they would be up to the accuracy and reliability. I could be wrong, I'll leave that up to you if you wish to investigate them.
It is worth mentioning that my experience of the above mounts is somewhat limited and goes like this. I know (in person) 2 people who own a Vixen GPDX with Skysensor 2000 but I have never used it myself. I know no one with the Vixen Sphinx and can only go by reviews, internet forums and the like. I know one person with the Losmandy GM-8 and have had a good introduction from them on it's operation and features. I know no one with the Takahashi EM-10 and so can only go by information provided to me by suppliers, reviews and internet forums.
Prices are rough figures (I expect them to change a fair amount over time so best you check for yourself). The prices are mentioned largely to give you an idea of what I was comparing at the time of my decisions.
So now let's look at each of the options in more detail.
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Vixen GPDX with Skysensor 2000
This was the telescope mount I always intended on purchasing before my old one died and I actually looked in to the available options in more detail.
The GPDX has a large amount of respect amongst the amateur astronomy community. Many people use it for high quality CCD and film imaging with small to medium sized refractors and SCT OTA's.
The critical statistics:
- Price: AU$4200 (mount, counterweights, HAL tripod, Skysensor: AU$1999 for Skysensor alone)
- Periodic Error: Stated at 20 arc seconds or less however there are measurements out there of up to 26 arc seconds of PE.
- PEC: Skysensor has built in PEC which has been known to take PE down to as little as 5 arc seconds.
- Weight Capacity: 10kg
The Skysensor appears to be a robust computer interface allowing great PEC, backlash compensation and autoguiding capabilities (in addition to the usual GoTo capabilities). One aspect I felt was a slight negative of the Skysensor was it's computer interface (not serial like most mounts are) and the fact that it's not as widely supported by computer software as other mount types. The price of the Skysensor also seems a little extreme for what it is, compared to what you get in other systems. The hand controller is a little bulky and has 3 cables coming out of it.
The autoguiding and PEC available in the Skysensor allow very accurate astrophotography there is no doubt of that. Long unguided exposures are apparently possible and with autoguiding there is no problem.
The capacity of 10kg seems to be well respected with people happy to put a little more weight on it without noticeable performance degradation. Certainly the ones I have seen in person have been fairly solid units with vibration dampening times of around 1 second. The Vixen dovetail plate design is well respected also and well supported. This makes mounting various telescopes on the mount quite easy and relatively inexpensive especially if you are prepared to get something made by a local engineer.
The Vixen GPDX has 3 cables coming out of the hand controller. A separate cable goes to each of the motors which are external to the mount (attached on the outside). The main thing I couldn't understand with the GPDX is that the Skysensor costs AU$2000. This takes the mount over the AU$4000 price range and above that of the Losmandy GM-8 which to me appears to have better specifications and support than the GPDX/Skysensor combination.
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Vixen Sphinx SXW
This mount really caught my attention. It is the ultimate in portable simplicity with one cable to the telescope, one cable to power, built in star chart software, internal motors (no external boxes or cables for each motor), built in counterweight and a nice clean look.
The critical statistics:
- Price: AU$3000 (mount, counterweights, HAL tripod, GoTo controller)
- Periodic Error: Stated at 20 arc seconds or less with most measurements coming out to about 16 arc seconds of PE without correction.
- PEC: Proposed as a future capability (software upgrade) but not available at the time I investigated this.
- Weight Capacity: 11kg
There are several things I liked about this mount but it's the general simplicity and "all in one" aspect - Everything you need is there with only 2 cables that I really liked about it. I would still rate this mount as my choice for an ultra portable visual observing mount. I looked forward to the ability to use it on a tabletop with my 80mm William Optics Megrez refractor. Put it on the table, align quickly using onscreen prompts and you're observing with a package consisting entirely of OTA, Mount, built in counterweight, 2 cables, hand controller and batter - how simple is that!
There are two reasons I chose the Losmandy GM-8 over this mount. One is the instability of it's software (and hardware?). At the time of choosing the software was at build 24, going on build 26. Some people were having success and enjoying astro photography while many others were having problems with jittery viewing, overshooting objects when guiding using small corrections, large backlash in DEC, etc. The second reason was periodic error. While better than the Vixen GPDX (roughly equal, arguably better) it was still "better than 20 arc seconds" and the PEC software was still in it's early days with people having problems using it.
I have no doubt this mount will continue to improve with successive versions of software and hardware, and give it another year it would be a much more stable and reliable solution capable of astro photography better than the Vixen GPDX. But right now it's not that.
A couple of other small issues I had with the mount (which wouldn't have stopped me buying it) were the lack of a numeric keypad on the controller and the battery consumption. Without a numeric keypad and the ability to use cursors to select digits there was (at build 26 of the software) no way to enter a specific RA and DEC to slew to. Battery consumption would have been a problem for me, I don't mind having a 17amp/hr battery for observing but it could have been a problem if I chose to camp remotely with the setup for several nights.
It's a shame the Sphinx doesn't have engraved markings on the RA and DEC axis. While in theory they aren't needed and would be of little use with the whole mount being computerised, I just prefer having them.
The Sphinx uses a twisted pair network port for it's connection to computers for link-up and software upgrades. This is good and bad. It's bad because it's not standard in the amateur telescope world where most telescopes use RS232 serial ports. This and it's young age means there are very few programs that support an interface to the Sphinx. This may not be such a problem as it has a very comprehensive planetarium program built in to the hand controller. I remain undecided on this.
To summarise for this mount: If you're keen on visual observing it would be great even now. If you're keen on more astro photography then check out the Yahoo! Vixen-Sphinx group and see what the software issues are like, if they have improved at the time you read this. I look forward to the day I can highly recommend this mount as it seems to have so much potential. If you're considering the Vixen GPDX I'd wait a while for the Sphinx to develop a little because I bet the Sphinx will soon take over.
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Takahashi EM-11 Temma II
This is without a doubt the most accurate and capable mount in this category. It is strong and solid, has built in motors with no external cables except power and data to external devices), 5 arc seconds of PE or less, etc.
The critical statistics:
- Price: AU$6500 (mount, counterweights, tripod, 4 direction hand controller)
- Periodic Error: Stated at 10 arc seconds or less with most measurements coming out to about 16 arc seconds of PE without correction.
- Weight Capacity: 10kg (apparently this is conservative and I believe it would be)
To tell you the truth there are very few reasons why I didn't purchase this mount. They are cost (it was simply more money than I was willing to spend at the time), the compatibility of the computer interface and the lack of a GoTo hand controller (one was due to be released soon but I expected even when it arrived it would cost a significant amount of money - being Takahashi).
I at first didn't investigate this mount expecting "Takahashi? there's no way that will be in my budget that's just not worth investigating.". I then found the Takahashi EM-10. At the time I didn't understand the difference between that and the Temma (Temma being the version with computer interface). Without the Temma this mount is reasonably priced and a very good option (About AU$4500 I believe). I simply wanted the computer interface and so was up for the full AU$6500.
There are many good features of this mount. Without a doubt the best has to be the periodic error and general strength. With this mount I was looking forward to never worrying about using PEC. Rather than having to train my portable mount to get PE down to about 5 arc seconds I was going to be able to just set it up and straight away have that level of accuracy. Sure PEC works but why bother with it when you don't have to? After all, I was after a simple, portable, fun and easy mount. Another nice feature of the mount is guide speeds between 0.1 and 1.9x in 1.5 arc sec/sec increments. This would provide an excellent level of control (but perhaps more than you would really use?).
This mount appeared to have the simplicity and clean look of the Sphinx but the accuracy of something light-years ahead of the Sphinx. It's just a shame it's cost put it out of my budget. Also you have to consider the options. A Losmandy GM-8 will cost AU$4000 complete and give you under 5 arc seconds PE once corrected. The Takahashi is AU$2500 more. Is that AU$2500 worth it or would that money be better spent on something else like a new OTA or laptop to match. I believe that given the AU$6500 I would have chosen the Takahashi. Why? because it would be the best base from which to start building a portable astronomy system. But it would be a tough choice if given that AU$6500.
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Losmandy GM-8 with Gemini
Well, we finally come to the mount that I ended up choosing. This mount appears to me as the best all-round option from the above selection, for the money. It has a 15kg weight limit, GoTo, excellently rigid and solid build, well supported and reasonably accurate.
The critical statistics:
- Price: AU$4000 (mount, counterweights, tripod, Gemini GoTo system)
- Periodic Error: Stated at 20 arc seconds or less with most measurements coming down to +/- 3 arc sec with PEC enabled.
- Weight Capacity: 15kg (although recommended 10kg)
At first I was dead against the Losmandy. I just didn't like it. For some reason I always preferred the Vixen GPDX and Sphinx. After a couple of months investigation I ended up swaying completely the opposite direction, towards the Losmandy.
First to the negatives. The mount uses external motors and cables, with 2 control boxes minimum plus battery. This means you have lots of cables, motors visible and boxes. The look of this is very nice and technical but not clean, simple and indestructible looking like the Sphinx and Takahashi mounts.
To make the mount easier to dismantle and setup it's recommended you buy the Tripod Knob Set. It would be nice if this set and some other usability features were included in the base unit.
The Gemini computer system isn't the easiest to use - with the small screen and relatively in-depth menu system it is obviously going to be much harder to use than something like the Sphinx. But it is accurate and packed with features, some features that the Sphinx and other mounts miss.
People say that if you are going to reach near the 15kg limit you should get a stronger mount, possibly the Losmandy G-11 and they may be right. It seems from my reading that Takahashi under-estimate their mount capacity leaving room to move while Losmandy slightly over-estimate their mount capacities. Having said that, I think the GM-8 would handle the 15kg without any problem, you just won't be able to use it on those breezy viewing nights so easily and may risk more strain on the mount (although I'm sure they have designed it to handle the 15kg without damage). Interestingly the same is said for the Sphinx and Vixen GPDX that you should not use them with a load near their capacity. The only exception to this rule is the Takahashi which apparently handles it's 10kg with ease and even up to 15kg with ease (reportedly).
There are of course many good points of this mount. As I mentioned earlier it is rock solid and strong. If you ever come across one of these ask the owner if you can try rocking it or tapping the mount. It just won't move. I tried this with one before I purchased my mount and I was very surprised at how it simply didn't move. The Gemini system is very compatible with many features. It uses largely the same protocol as the LX200 which instantly makes it very connectable to many devices and computer programs. Being a serial port based system you can plug a Bluetooth transmitter on the port and instantly have wireless connection to your PDA or laptop. Built in autoguiding and PEC allow for a very accurate solution, PEC easily getting the error below +/- 6 arc seconds and capable of +/-2 (visual limit). The Gemini system also allows you to use modeling to improve pointing accuracy when your equatorial alignment is not perfect (which it never is). I have not tried this myself but it is reputed to be extremely useful, accurate and easy to use with only a few quick iterations required before pointing accuracy is improved out of sight. I don't believe any other mounts in this range have that modeling although I'm not sure if the Sphinx does (the full details of what the Sphinx software seem a bit illusive).
I mentioned earlier that the GM-8 has many cables. While this is true, it does only have one cable coming from the hand controller. This and the nice buttons and size of the hand controll make it quite nice to hold, grab from the tripod and handle.
The Losmandy mounts have a strong following and for this reason support, spare parts and options seem relatively easy to come by (everything astronomy is hard to come by in Perth (Western Australia) hence I say "relatively"). The dovetail system is also very well supported although I chose to have my won adapter plate made up locally as it would be significantly lighter, cheaper and more what I wanted than the Losmandy solution to mount two telescopes on the GM-8 with movement in one (guide scope).
The Losmandy has accurately engraved markings on the RA and DEC axis. Not something I would often use but something I like to have when all else fails.
To Summarise
Of course the GM-8 mount is not as good a mount as the Takahashi, but at AU$2500 cheaper and with good PEC, pointing modeling like that of the well known T-Point software, lots of other handy software features plus a GoTo hand controller, I don't think you can beat the Losmandy GM-8 for value for money. The Sphinx would be so much fun visually as a portable mount. The Vixen GPDX seems to be falling behind compared to the others. Having said that, as you can see above every mount has it's good and bad points. Take your pick! I would certainly be very happy with any one of them and would choose to make the most out of what I had. Even with the Sphinx I am sure I would achieve good long exposure astrophotography, just look at what I have been doing hour long exposures with up until now: an EQ2 equivalent mount with only an RA motor that had immeasurable periodic error. |
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