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Switching to iOptron: What I learned from my new GEM45
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Switching to iOptron: What I learned from my new GEM45
Michael Covington is the author of Digital SLR Astrophotography and other books. By day, he develops artificial intelligence software in Athens, Georgia.
These are short notes on things I learned about using my new iOptron GEM45 equatorial mount, moving from Celestron mounts (AVX and CGEM). The CEM40 is very similar to the GEM45, and other iOptron mounts are also rather similar. I assume you have the instruction manual for your (actual or planned) iOptron mount; manuals are available on line.
The main differences between iOptron mounts and other brands are the following:
- The instructions are less complete and less suitable for beginners. iOptron seems to be going for people who have already used another mount and are upgrading.
- The gears are spring-loaded and must be protected from accidental damage.
- Like an observatory mount, the iOptron mount thinks it is always star-aligned; that is, to use Celestron terminology, it always hibernates when turned off. It does not to prompt you to align it when you turn it back on because it does not know it has been moved.
- "Set zero position" confused me; it means "tell the mount it is in zero position now." More about this below.
My first impression of the GEM45 was that its fit and finish are more precise than my AVX or CGEM, but also that it is a bit more delicate. The tripod arrived with rubber pads on the tips of the feet; I removed these because they would quickly be chewed up by my concrete driveway. I also added some hand-made labels to make it more obvious what to plug in where.
Am I going to break it?
The first thing you see in the instructions and the quick-start guide is a set of stern warnings about the risk of damaging the gears. iOptron's gears are spring-loaded. If you handle the mount awkwardly or bump the counterweight shaft while the gears are locked for tracking, the gears will "cam," meaning teeth will slide over each other very roughly, making a "brrrrrrrrAP!" sound and doing damage. The mount will still work, but tracking will be less accurate.
How much of a risk is this? Well, it hasn't happened to me, and I don't hear about it happening to other iOptron owners. Once you're aware of the risk, it's easy to avoid. The basic principle is not to force anything (just as with any other delicate machine) and not to do stupid things such as putting the telescope on without the counterweights.
Note by the way that if you run the mount into an obstacle under motor power (such as if your telescope hits the pier while slewing), the mount will make an awful noise but not be damaged. What you hear then is not gears camming or stripping; it is a stepper motor trying to step and not being able to. The result is awful vibration but not mechanical harm. In fact, this is one of the advantages of stepper motors: when they can't move, they just shake in place and do not burn out.
The gear locks
Instead of clutches that uncouple a shaft from the mechanism, iOptron mounts have gear locks for declination and right ascension. Each of them engages or disengages gears. Unlocked, the mount moves freely but does not track the stars; locked, it moves only under the control of its motors, and if forced, will cam the gears (as just described).
The most important thing about gear locks is: never halfway. Each lock must be locked or unlocked. If it will only lock halfway, the tips of the gear teeth are touching, and you should jiggle the mount a tiny bit to get the teeth to go in between each other in the proper position.
It's somewhat confusing to know whether the locks should be on or off at each step of setup and takedown. The RA lock is more important because it affects movement of a much larger part of the mount, and because the performance of its gears is more critical (so you want to avoid damage).
Locks should be off when the mount is transported in its case, on while you are setting it up (and taking care not to overload it), and so on. Summing up several pages of the instruction manual, here is my lock protocol:
- Locks OFF when transporting the mount head in the case.
- ON when lifting the mount head out of the case and putting it on the tripod. (If you try to do this with it unlocked, it's like handling a squirming octopus.)
- ON when leaving the mount head on the tripod, with or without the counterweight bar, without counterweights or telescope. (This is to avoid the upside-down problem, below.)
- ON when attaching the counterweight bar.
- OFF when attaching the counterweights. (A heavy counterweight might apply torque and cam the gears.)
- ON when attaching the telescope. (You can't have it twisting around freely.) As with other mounts, start with too much counterweight — that is, with the counterweights at the end of the shaft — and then move them toward the telescope as you balance.
- OFF when adjusting balance (of course).
- ON in operation.
- ON when removing the telescope. (Counterweights must be straight down.)
- OFF when removing the counterweights.
- ON when removing counterweight bar.
- And reverse the earlier steps as appropriate.
Zero position is a physical position of the mount — counterweight shaft straight down, declination axis pointing toward the pole — and cannot be redefined as anything else. The position you can redefine is called park position.
The mount does not assume it is in zero position when powered on. It assumes it is in the same position as when it was powered off (which can be zero position if you make a practice of going to zero position before powering down). Actually, of course, you unlocked the locks when installing the counterweight and also when balancing it, so the mount isn't exactly where it was.
That is why you are advised to put the mount into zero position carefully and then choose "Set zero position" before proceeding with alignment. "Set zero position" means "tell the mount it is in zero position right now, because it has been moved and doesn't know."
Curiously, unlike Celestron mounts, the GEM45 has no index marks for you to look at when placing it physically into zero position. You can add index marks (I did). But there is a better way.
If you choose "Search zero position," the mount will use internal electronic markers to find zero position (or what ought to be zero position if it's level). I do this at the beginning of every session, and then I press "Back" to cancel calibration. By pressing "Enter" I could actually calibrate the internal index marks — tell the mount to store in memory a position other than the one currently stored. If "Search zero position" seems inaccurate, calibration is what you should do.
By far the best way to polar-align an iOptron mount is with the iPolar camera and a Windows PC. This is normally done with the telescope in place and balanced, but before powering on the mount and doing any initialization. The iPolar camera is powered by the PC.
The field of the iPolar camera is about 8 by 11 degrees, and only part of it needs to be open sky. A tree that blocks the view of Polaris from my driveway is no obstacle to it. You can manually exclude parts of the field if you want to, but I have not needed to do that; it might be necessary if there were lights in the area excluded.
When aligning, you center a cross on a circle. The circle is 8 arc-minutes wide, so even with poor centering, you can easily polar-align to 2 arc-minutes or better, at which point polar alignment will not be the limiting factor in your guiding accuracy (flexure will take over). Contrast that with Celestron's All-Star Polar Alignment, which is reproducible to about 4 or 5 arc-minutes.
Three iPolar hints: (1) Besides iOptron's own iPolar software, the iPolar camera is also supported by Sharpcap. (2) Along with version 2.5 of the iPolar software, iOptron also continues to offer version 2.2, which some people find more reliable. (3) If the iPolar absolutely can't find the pole, or seems to find it only fleetingly and then gives up, the problem may be exposure time or gain, but another possibility is that you've put in your latitude incorrectly so that it's looking for the wrong celestial pole!
iOptron's polar scope, if that's what you opted for, is one of the best in the business; expect an accuracy of 5' to 10' with it.
If you can't see Polaris, you can use a procedure that iOptron calls Polar Iterate Align, but it's rough, apparently sensitive to the starting position. I prefer to use three-star alignment, read off the reported polar alignment error, and correct it. Drift alignment, aided by PHD2 or other suitable software, is better yet.
I find that after precise polar alignment with the iPolar camera, one-star alignment is almost always good enough. If I find that the go-to is poor in some area of the sky, I sync on a star in that vicinity, and that's good enough.
One-star alignment assumes the mount is perfectly polar aligned and has no internal errors. Two-star alignment enables the mount to measure its polar alignment error and correct for it when going to celestial objects. Three-star alignment does that and also measures axis perpendicularity. Two- and three-star alignment fail if the polar alignment error is too large or you mis-identify a star.
My experience is that two- and three-star alignment on iOptron mounts are very exacting; be sure to use an eyepiece with crosshairs. I actually get better go-to performance with one-star alignment. Also, don't be scared by the polar alignment error that is reported after a two- or three-star alignment; it is likely to be imprecise. When iPolar and alignment disagree, in my experience, iPolar is right.
If you use Celestron mounts and do 2-star alignment, you're accustomed to the second star being found much more accurately than the first one. In effect, a Celestron mount does a one-star alignment on the first star, then uses the second star to make further corrections. iOptron mounts don't work that way. All three stars in a three-star alignment are equally far off when the telescope goes to them. The alignment does not have any effect until you've aligned on all three.
The upside-down hazard
If you put the GEM45 mount head on the tripod and don't add the counterweight bar, you'll find that it is top-heavy and will turn upside down when the RA gear lock is released. This is not a risk with the CEM40.
Letting the head turn upside down is not good because it can unplug internal cables in the GEM45. Contrast this with the Celestron CGEM, which can turn around and around because all its motors are in one unit, and the Celestron AVX, which would only tangle its external declination cable. Contrast also the iOptron CEM mounts, which cannot turn upside down because the structure of the housing prevents it.
The GEM45 has stops that should stop it from turning upside down, about 120 degrees from straight up on either side, but it can go past them. If it does, be sure to reverse the motion it went through, rather than trying to complete the circle, so that you don't pull the internal cables even further. And if it's stuck on the wrong side of the stop, there's a trick to release it. Set your mount to the highest latitude (polar axis altitude) that it can reach, and then remove the two big altitude lock screws and lift it even higher. This will reveal two small screws in the housing, one on each side. These are the rotation stops, and you can loosen one of them and get the mount back to right side up.
If a GEM45 has been upside down, you will probably need to remove covers and re-seat internal electrical connectors. The symptom is a "Mainboard connection error" message on the hand box. Detailed instructions are available from iOptron about all the connections to check.
The DC power connector
The DC power connector of the GEM45 is apparently a standard size, 5.5 mm o.d., 2.1 mm i.d., positive center. But in fact, some plugs that are supposed to be this size won't fit!
The problem is that Asian manufacturers interpret 5.5 mm as the size of the hole the plug fits into, and make the plug itself slightly smaller. The hole in the case of the GEM45 is indeed 5.50 mm in diameter, and plugs that fit it are about 5.45 mm, common in Asia. American-made plugs that are 5.50 mm in diameter (plus or minus a tolerance) often won't fit. That's why the plug that fits Celestron mounts generally does not fit iOptrons.
When building additional power cables, you want a plug as much like the one on iOptron's power supply as possible: true outside diameter about 5.45 mm, and in the center, not just a hole but rather a two-sided spring to grip the pin. This cable fills the bill, and so does this plug, although each of them is still a tight fit on my GEM45. Caution: Other iOptron mounts may be different, even very similar models.
According to iOptron, the operating voltage range is 11 to 14 volts.
"Mainboard connection error"
In my experience, this message on the hand box means any of three things:
- A loose connection between the hand box and the mount. (You can substitute the cable from the GPS unit; it is the same cable.)
- Loose connections internally, such as from letting the mount head turn upside down. In that case you need to remove covers and re-seat connectors.
- Very brief interruptions in electric power, such as from a loose power connector.
- Possibly a firmware problem, correctable by updating the firmware.
Saddle and saddle knobs
The GEM45 saddle can be set up for V-size or D-size dovetails, but not both at once; to change, you have to back screws out and reconfigure it.
The GEM45 saddle actually tightens onto the dovetail with two separate clamps. That is a good thing, because if there were just one long clamp, it might not be aligned with the dovetail, and you could then have looseness without knowing it.
The saddle knobs are rather small, but they accept hex wrenches to help you tighten or loosen them (especially the latter). If you'd rather have bigger knobs, I recommend the GN 6336.5-TE-32-M6-25 from this company. The important thing is that the treads are M6 [corrected].
Things to wish for
Sideways is not an option
With a Celestron mount, you can turn the saddle 90 degrees from its usual position and simply tell the mount that you've done so. This enables you to use a single dovetail bar carrying two cameras side by side.
iOptron does not offer that option (except by removing 4 screws and physically turning the saddle sideways). This rather expensive ADM adapter can solve the problem if you're desperate, or you can build something similar out of separate parts.
"Precise GoTo" is not available
On Celestron mounts I rely on the "Precise GoTo" option, which slews to a bright star for you to sync on before going to the selected object. I do my focusing on the star. iOptron does not offer that feature, so when I prepare for each deep-sky session, I write down not only the objects I intend to photograph, but also the HIP numbers of moderately bright stars near them, and go to those stars to focus and sync.
Having also installed an iPolar camera on my Celestron AVX mount, I wish iOptron's software provided a convenient way to store settings and dark frames separately for two iPolar cameras (and, preferably, recognize the cameras automatically). As it is, you need to take a new dark frame and do a new position calibration (which takes half a minute or less) whenever you switch from one camera to another. Or you can create multiple Windows accounts on your PC and use a different one with each iPolar.
Cables and connectors
Just like Celestron and many other brands, iOptron does two things that I consider dubious electrical design: The mount can be damaged if you plug things into the wrong sockets, and also if you apply power with the wrong polarity. Protection against both of these would be simple; for the first one, just use different connectors for things that must not be mixed up! But nobody does it. Beware that some SBIG products have power connectors that will plug into iOptron and Celestron mounts but have the opposite polarity.
Two things I particularly like
The menu structure on the iOptron control box is straightforward and well-organized. Contrast that with the traditional Celestron menus, which obviously have been added to over the years, with a variety of new functions placed under "Utility." On an iOptron, it is easier to remember what functions are under each menu choice.
Also, the iOptron mount has the entire Hipparcos (HIP) star catalog, rather than a portion of SAO or some other catalog. You don't have to wonder whether a star is included; if it has a HIP number, it's in.
My GEM45 has no trouble carrying a 22-pound telescope, but then it shouldn't; it's rated for twice that. Surprisingly, it seems to be smoother with 22 pounds than with 10 pounds, indicating it could carry appreciably more. Nonetheless, it is a very small, light mount, and at home I can easily carry the mount and tripod around as one unit.
Tracking is appreciably smoother than with CGEM and AVX mounts; according to the autoguider, there is much less roughness on a scale of 0.5 to 1 arc-second. This is not just because of better worms and worm wheels. It's also because the GEM45 uses stepper motors, which are microstepped so that they run smoothly at a slow speed. The AVX and CGEM use servo motors, which inherently run faster and require a more elaborate gearbox, introducing more roughness.
The periodic error graph that iOptron supplies with the mount is measured at the factory and may be too optimistic as an estimate of actual performance under load. But that's not what matters. The important thing is that with autoguiding and/or PEC, you can correct the periodic error, which usually takes the form of a smooth sine curve rather than irregular hash.
In short: My GEM45 does the job. It's a bit less beginner-friendly than my AVX, but it has everything it needs, and it performs well.
- Joe Bergeron, dswtan, eros312 and 9 others like this