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Old 10-06-2019, 05:03 PM
Wavytone (Nick)

Wavytone is offline
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 3,884

For the 5"-7" aperture range mak-newtonians are underrated, IMHO. The basic geometry is the same as any newtonian, ie can have a smallish secondary, however the use of spherical surfaces means no coma, and a flat focal plane is possible. Nirvana for visual observers and not bad for imaging, either. Secondly the big corrector effectively closes the OTA so the mirror coatings will last a very long time if it is well cared-for.

In particular Intes garanteed their complete scopes to 1/6 or 1/8 wave, something very few manufacturers ever did. When properly collimated, these scopes do perform extremely well.

Another aspect to consider is that Intes built theirs like a T34 tank optimised for maximum contrast - a full steel tube OTA in battleship grey with internal baffles, and a screw-on black dewcap with more baffles. The original Intes Crayford focusers were very agricultural - worked but very crudely made. Heavy, but they last. A long time.

And in case you don't know, Intes ceased production a few years ago so these scopes are now only available secondhand. The big downside was their cost, and that they were (are) only viable in apertures around 5" - 7" - the Intes 130mm MN56 being the baby, vs the Skywatcher 190 being the largest in this country. Intes made a few 8" and 10" versions but none exist in this country AFAIK.

The alternative is a newtonian with coma corrector of one kind or another near the focal plane. This solution is cheaper but inferior, and there's no practical limit on the aperture (big cheap mirrors are available).
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