For those who are not already aware, the DGM 'scopes are Newtonian 'scopes that use off-axis parabolic mirrors whose focal axis is not coincident with center axis of the primary mirror, permitting the secondary to be placed out of the incoming light path. This yields the same unobstructed image as a refractor, is inherently apochromatic (as all Newts are), and is much cheaper to produce than apochromatic lens systems. In short, you get all the performance of an apochromat at a fraction of the cost. (Nearly all because good refractor lenses tend to transmit more light, while mirrors scatter a bit…)

Firstly, let it be said that Dan McShane offers first-rate customer service. He is very helpful, and very quick to respond to questions both before and after purchase, both by e-mail and by phone.

I purchased my OA-4 in late October 2001, and received the 'scope December 16 2001, a little quicker than normal, I hear.

The OTA and mount are first rate in craftsmanship. The split-yoke mount is very simple and extremely effective in allowing balance and rotation of the OTA. It's shocking that no other tube-dob makers use the same idea.

The primary mirror mount is very simple, easily installed/removed. and collimates with a wonderful buttery-smooth feel, due to its nylon construction. The secondary is collimateable, but is more or less permanently placed, negating the need for collimation, and making it difficult if it does end up needing tweeking (you have to bend the support bracket with pliers… mine didn't require collimation of the secondary). The focuser on my OTA is a Lumicon 1.25" helical, functions wonderfully, and has very tight tolerances on the threading.. I wasn't sure I'd like a helical focuser, but found the accuracy of the focus adjustment to be superior to rack and pinion and most Crayford focusers I've tried. (Hint: slide eyepiece barrel in and out of the focuser for coarse focus, tighten the eyepiece set screw, then use the helical focuser threads for fine focus…) The PVC OTA is very black inside, and painted very nicely (red, in my case) on the outside.

The 'scope and mount are easily transported, owing to the 4' tube length. The mount is a little taller than many other dob mounts, as the center of gravity of the tube is very near the middle, requiring extra rocker height, while negating the need for springs or counter weights. While this does make the mount a little heavier and bulkier than other dob mounts, it is superior to conterweighting, in my estimation. Altitude and
Azimuth motions are the smoothest of any dob I've tried (I've tried a lot!). Tracking is a pleasure. Anti-vibration pads make an already steady mount rock-steady.

Daylight collimation is very easy. The final star-collimation is a touch tedious, but absolutely necessary for the 'scope to deliver worthwhile images. It takes a bit of practice, but is very easy once the process is understood and repeated a few times. (Hint-- Use Polaris!)

I find the 'scope takes 20-30 minutes to equalize before delivering sharp images (in freezing weather, going from my warm apartment), and performs relatively poorly until the tube currents disappear (the converging off axis light cone hugs the side of the tube, where the warm air flows the most). The final star collimation can not be done until the currents are gone, as the diffraction rings are too distorted until such time. Touching the side of the tube radiates heat through the PVC and into the light cone. This is evident when focusing in cold weather, but clears immediately when the hand is removed. Most of the thermal issues should nearly disappear in warmer weather.

Once cooled and well collimated, the 'scope delivers absolutely crisp images, very easily to 200x, more on nights of better seeing. Contrast is outstanding. Star testing shows the best intra and extra focal patterns I've ever seen. I think the test patterns indicate a very, very, very slight amount of under-correction (just a very, very subtle dimming of the extra-focal rings), and Dan told me the mirror had a slight zone, though I was not able to detect this. Still, it's nearly the best diffraction pattern I've seen, and I test every 'scope I look through (only the 5" Starfire had slightly better patterns…). Jovian moons consistently resolve into little disks of varying size (though I must admit they are probably not resolving into the actual disks as much as Airy disks of varying size), Jovain banding nearly always shows festoons, and lots of layering. Saturn always shows the Casini division, the C ring (faintly), banding, and several moons cast about on a deep black sky. Zeta Orionis splits quite easily and cleanly, each star getting it's own disk and diffraction rings, while showing the blue-white and yellow of the respective components very well. The trapezium shows as 4 distinct stars even at low powers, and another OA-4 owner mentioned resolving all six of the brightest components, though I have not done so myself. Open clusters are a joy to behold; saphires on black velvet, with the occasional red or yellow gem thrown in for good measure! M81 and M82 showed surprising detail for the aperture.

At first glance, $750 (mounted) seems like too much for a 4" Newt. Look through one, and you won't feel that way, especially when you pit the price/performance against the cost of an apochromat. I can offer no negatives on the 'scope, and can recommend it without hesitation, though I'd have to say I think the OA-5.5 or 7 would be a superlative purchase if finances permit!

Overall Rating: 10
Optics:10 Mount:10 Ease of Use:10 Value:10
Weight: 10
Date: 03/17/2002 06:24:35 pm PST

Replies: 0

I was skeptical of a comparitivle cheap ep of this focal length when I bought the Siebert 3.9mm. Any skepticism was removed uppon my first use of the ep.

This ep provides my highest power, without a barlow, on my DGM OA-4 and my Pronto. The image is very sharp and focuses with relative ease. The 13mm eyelense is very nice; it feels like looking through normal a 13mm ep. Images are sharp to the edge. The image is superior in sharpness and contrast to a 7mm Nagler/Celstron Ultima 2X Barlow combo, or RKE 8mm with the same barlow. Also far superior to a 12mm Nagler and 2.5x Powermate combo. Very light ep, which is nice for balancing the 'scope.

Minor ghosting, hence the 9 for the optics. Otherwise, a very, very great ep. Other than a Radian of similar focal length, this ep would be hard to match, let alone beat. A pleasure to use. Gotta give it a 10.

Overall Rating: 10
Optics:9 Value:10
Weight: 10
Date: 01/16/2002 06:56:04 pm PST

Replies: 0