Astronomy and astrophotography are demanding fields with a lot of variables waiting in line to ruin your results. While we can’t do much about some of them, we can strive to achieve perfect focus and maintain it.
Focus plays a large role in image quality, so the focusing mechanism of an imaging telescope is one of its key components. I had a long list of requirements that the Optec Gemini had to meet.
The Gemini is a precision focuser with an integrated instrument rotator. It has a combination of features that is almost unrivaled, and I’ll be highlighting them here as well its drawbacks.
Profile and Clear Aperture
Some optical setups require a short or “low-profile” focuser. Furthermore, as imaging sensors are getting larger in size, a larger clear aperture is preferred.
The Gemini is only 55.9mm (2.2 inches) thick fully retracted, with a generous clear aperture of 95mm (3.75 inches).
Weight and Payload
Given that it’s both a focuser and a rotator, the Gemini is lighter than I expected the combo to be, weighing just 2.9 kg (6.4 lbs).
While it’s good to be light, you definitely want the focuser to be extremely rigid; there must be no flex or tilt under the load of the imaging train. Tilt will lead to distorted stars in the corners of the image, and that distortion will change location as the telescope tracks the sky and the angles change.
The Gemini has a flex-free payload capacity of 10kg (22lbs). I was unable to detect any flex with my 36cm (14-inch) long imaging train with several components.
For the system to obtain the sharpest images possible, the focuser must be able to make extremely small movements. Faster optical systems (low f/ratios) require higher focusing accuracy; this is defined as the critical focus zone (CFZ). Being slightly outside this zone will quickly bloat the stars and blur the image.
For example, a typical Schmidt-Cassegrain at f/10 would have a CFZ of about 250 microns, while the CFZ of an f/4 system would be only 40 microns. That’s plus-or-minus 20 microns, or 0.02mm, on either side of perfect focus.
The Gemini is able to move the camera in 0.11-micron steps. For reference, a single wave of light is about 0.5 microns and human hair is about 75 microns thick, meaning the Gemini can move a hair’s width using 700 steps!
Filter Offsets – Even with parfocal filters, most telescopes will exhibit a small difference in the focal point from one filter to another, known as the offset. With a very precise and repeatable focuser you can confidently measure those offsets once and apply them to every filter change. This is what I’ve done, and it saves a lot of time by not having to refocus with each filter change. This also renders sharper stars when combining color channels.
Focuser travel is the distance the focuser moves from fully retracted to fully extended. The travel range of the Gemini is relatively short, only 12.7mm (0.5 inches), which is one of its main shortcomings (literally).
This means you have to know, or calculate, exactly where the focal plane will be, and place the Gemini there with adapters/extensions. No guess work here!
Fortunately, there were a surprising number of adapters and extensions offered by Optec (139 in total, at reasonable prices) and I didn’t have to resort to custom adaptors ($$$). All the adapters fit using dovetails and set screws, making the connections very rigid and straightforward.
A rotator is not essential, and you can get by with a manual rotator if it’s not a remote setup. However, there are several advantages of using one like the Gemini that has a very fine resolution of 600 steps per degree (that’s over 200,000 steps for a full circle).
Framing – The most obvious use of the rotator is to get the target framed the way you want. With longer focal lengths this becomes even more of an issue. Also, if you have more than one target per night, the imaging software can auto rotate from one target to another in seconds. This not only saves a lot of time but, because it’s repeatable, you can go back and forth between targets from previous sessions with perfect alignment.
Off Axis Guiding – This is a big plus here for guiding. There are times when you may be centered on your target but there are no suitable guide stars. The rotator can turn slightly to allow the guide camera to find a star. An added bonus is that guiding software like PHD2 doesn’t need to be recalibrated when the rotator moves; it can work out the orientation and guide accordingly (I have tested this and found no loss in tracking performance). Optec includes standalone software, and it is ASCOM compliant. There is a physical hand controller included, too.
Field De-rotator – The Gemini can also act as a field de-rotator. Alt-Az setups need this feature to counteract the rotation of the earth. The Gemini connects to the mount and creates a “server” that calculates the correct rate. (I use a German equatorial mount so I disabled this feature).
The optimal focal point almost never remains at a fixed position. Ambient temperature dropping throughout the night causes minute changes in telescope tubes and truss materials as they contract with lower temperatures. The degree of contraction depends on the material’s coefficient of thermal expansion. The Gemini’s temperature sensor can be trained to attempt to counteract or minimize this drift of the focal point.
Besides the somewhat short focus travel, the main drawback is its relatively high cost; it’s probably one of the more expensive focusers available. Optec could cut the cost by not including an expensive Pelican case ($150) that’s not needed for those of us who don’t need it for later transport. The hand controller ($169) might also be made optional since many users control these with a computer. I wasn’t aware the external controller (which is small and light) doesn’t include a mounting bracket ($35) that I would have liked.
Overall, the Optec Gemini has been a very pleasant upgrade. The performance is excellent with accurate, relativity quick, silent, and virtually backlash-free operation. Its list of features vastly increases the telescopes capabilities.
Once I got past its price tag, I really felt it was a well-engineered piece of art that opened many doors for me!
Kit MSRP: $3,499