CN FORUMS ARCHIVE Telescope Reviews: Orion 150mm Mak arrived today!

Daryl,

Don't think that's what Mike's talking about. The primary mirror in a Maksutov (and even an SCT) needs to be a bit larger than the meniscus. The meniscus is an aspherical corrector plate (to generate an ashperical aberration that is then 'corrected' by the spherical mirror -- quite ingenious that Dmitri, wasn't he?) But think about it, the meniscus is concave, and not convex like in a refractor. So the light is actually being spread out and 'unfocused', if you will. Select Diagram here for a picture worth all my words and then some ...

http://www.dp.net/~jplatania/telescopes.htm#m-cassegrain

Notice how the little red rays are curving OUT, and not in, as they move from the corrector meniscus to the primary mirror. Obviously, if they're the same size (the corrector and mirror), then some of the outer portion of the light will be lost into the sides of the tube. This is exactly what happens with the 127mm model, so its effective aperture is closer to 121mm (the rest is lost). The only way a Maksutov can function at 'full' aperture is to have an oversized primary. Meade charges a premium for this, but if you notice they advertise that their Maks have an oversized primary, so the stated size is correct on the Meades by utilizing this oversized mirror specification.

Personally, I don't think this matters as much as Meade charges for it. I just ordered a 5" Orion Mak, and though I know it will only produce a 121mm obstructed view, it produces one nice view (this is my second one to own)

The only issues with larger Maks are cooldown time (they need a fair amount) and weight (quite heavy for their size). The 5" is plenty heavy enough for me, but no doubt Mike's six inch will outperform mine on Mars, etc. But if I can recommend to Mike and all other Mak owners, take the time to let it cool down a while, and if you can't wait, start your observation session with DSOs at lower powers. The longer focal length eyepiece views are more forgiving of the heat dissipation going on inside the tube. Turn to Mars or the moon after hitting all the DSOs, and you'll be glad you waited.

In the case of my 127mm Mak, the amount of light it gathers is the same as an F/12.7 114.5mm APO (121/2*Pi squared - 39mm/2*pi squared). Of course, the large obstruction means it wouldn't have the contrast of a 114.5mm APO, and it doesn't. Most folks with the 5" Mak say they do somewhere between a 90mm APO and a 102mm APO on planets, and that sounds about right. Ironically, for brighter DSOs (like the Dumbell & Andromeda, e.g.) they perform closer to the 114.5mm APO. Their 121mmm resolving power brings them very close to the full aperture 114.5mm APO in this regard. This is a curious aspect of the Mak design. All in all, I like to think of it as a 5" powerhouse -- portable and packing a lot of punch. The lack of a large TFOV is the main drawback, due to the F/12.7 focal ratio.

I've owned all three of the smaller Maks before, and they were all excellent performers. Although I treated all mine gingerly and never dropped one, the collimation was dead on for each of them and was the same when I sold them. The sweet spot diameter for these Maks is over 45mm for all of them! That's an F/5 collimator's dream come true -- 2.75mm for him, by comparison.

Looking forward to hearing about the 150, though, Mike.

Good luck,

CDS

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