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Orion 190 mm Mak-Newt

A few months ago, Orion Telescope and Binoculars announced a new 190 mm (7.5”) Maksutov-Newtonian telescope. Russian Mak-Newts have long been held up as excellent instruments with APO-like performance by many observers. The Orion MN, unlike most of the Russian (but not all) is designed with a relatively large central obstruction. While this can rob the image of some contrast in the low frequency portions of the MTF curve, it is needed to illuminate larger imaging chips or wider FOV eyepieces. Thus, it is billed as a $1300 astrograph.

Round 1
Having just unpacked the scope yesterday, all I have time for is some first impressions and a few quick tests. More will have to follow.

The Package
First, the scope is bigger and heavier than I expected. Yes, the specs are on the website, but going from a 12 lb Vixen R200SS to this is a big jump. With rings and a dovetail bar, the rig comes in at 26.5 lbs (Orion specs it bare at 22 lbs). It’s also 37.5” long, a good 10” longer than my old R200SS. Having it physically here, it seems bigger than that by comparison. I think with the Vixen, I was able to think of it as just a touch bigger than a C8 and mentally that’s how I thought of it. But really, this is about the size of a number of 8” Dobs, so keep that in mind when you’re thinking about the scope.

The scope comes as a bare OTA (no finder, eyepieces, rings, etc) but does include tools (screw driver, allen wrench, collimation plug) and a copy of a “lite” version of Starry Night. I’ve not installed that yet and probably won’t. They know they’re not selling this to total novices, so keeping these out and their cost out is fine by my book.

Anyone doing photography will want to know about the focuser. This is a single speed Crayford with somewhat oversize knobs on it. While I’d have preferred a dual-speed setup, I was able to focus with the existing one just fine. I used a Canon XSi’s LiveView mode and I used the Fine Focus tool in Nebulosity both without trouble. The focuser was smooth and had no problem with the DSLR. For visual use, a 2” extension tube is provided to let your eyepieces reach focus.

Collimation is done with hard push-pull screw pairs (screwdriver for “pull”, allen wrench for “push”) on the primary. On the secondary, there is a metal cap that covers the three collimation screws and there is a retaining ring that lets you twist the secondary to line it up. The primary has a nice center spot (ring) and the secondary has a very faint one (also a ring).

Under the Stars
I had a chance to get it out for some quick tests last night. This was in a lit parking lot, so don’t expect accounts of faint fuzzies. My goal was to see how well it will perform as an astrograph, so I wanted to see what the stars really looked like across the field and what kind of vignetting to expect.

Visually and in LiveView, it was clear the scope had a very nice “snap” into focus feel. The intra- and extra- focus images looked nice but I didn’t have a chance for any formal evaluation. Things looked pretty “textbook” to me. Panning Vega around in an eyepiece showed nothing odd, just a nice crisp star that took magnification well. All I had was a Baader Hyperion 8-24 mm zoom with me, but a quick visual look there showed nice images so far.

Sticking the camera on, things look good as well. Here is a full-frame (stack of four 30s shots) image around Vega:

Clicking on the image here should get you the full-sized version so you can scan around for yourself. In the lower-left, stars are a bit off but elsewhere they look quite good. We’re not 100% perfect at the corner
s but really I don’t think we can complain about this. The lower-left is probably the result of my collimation not being spot on. We’re not badly off at all but it could be tightened up a bit I’m sure and there’s little else that would cause one corner to be worse than the others. Something isn’t perfectly lined up / squared.

So far, so good. I took a series of flats using my “Elf” flat-fielder. This gives a known-flat image (it’s an EL-sheet) and works very well to give me flat frames that correct the vignetting in my scopes. I binned the image 8x8 and have plotted the intensity relative to the maximum here:

Keep in mind the x and y-axis labels are in 8-pixel steps. It’s probably best to just realize that this is an APS-sized chip (22 mm x 15 mm). What we can see here are two things. First, the secondary isn’t perfectly centered on the focuser. This is what is causing the shift to the right of the hot spot. Second, even once centered, the best we’ll be getting is probably just around 80% (plus or minus a few percent) illumination in the corners of the frame. If you’ve got a smaller chip, you should be able to use this to estimate what kind of vignetting to expect. Note, this was with the supplied 2” to T-thread adapter and a standard T-ring. I will have to try this with my CCD Labs Q8-HR that places the same-sized APS chip a lot closer to the T-ring to see if that is part of the vignette I’m seeing.

There is one other thing worth noting. I was fighting balance and flex last night. My mount seemed to be tracking just fine despite the imbalance. PHD was keeping the star centered and the errors were never very large. Yet, the frames were showing drift from frame to frame. Ah, the dreaded flex. While I can’t be 100% sure I found it as I’ve not tested it yet, I did find one really bad spot where flex was creeping in.

Orion supplies a 2” to T-thread adapter. This is a very nice idea given the target audience for the scope. They also supply a nice compression ring setup on the focuser. Individually, these are nice things but together they are ... insert some not so nice word here. The problem is the angled cut on the 2” to T-thread adapter (so it won’t fall out if you’re using set-screws, but we’re not using set-screws...) and the compression ring don’t play well together. The net result is that you can wiggle your DSLR around easily when attached this way. It’s in no danger of falling off but it’s not exactly solid. After taking things down for the night, I tried a different 2” to T-thread adapter (an Antares unit with a normal, flat undercut barrel) and the compression ring fit right into the undercut. The camera was held very solidly with this and there was no flex in this joint.

That’s all for now...