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| Subject: Former HD216 Owner|
By: Anonymous (xxx.xxx.127.53)
Date: 11/23/2002 07:14:56 pm PDT
The Ceravolo HD216 (8.5") f/6 is not a Mak-Cassegrain as stated at the top of the page (specs). It is a Mak-Newtonian, as is all the other shorter f.l. Maks Peter Ceravolo made during the 90's. It is not very useable for astrophotography or imaging because of its short back focus (uses a low profile helical focuser) and a small secondary (with 20% obstruction). It is very superior as an observational telescope and easily outperformed A-P 155 EDTs and Takahashi 225 SCTs. It held its own against a Tak FS150 and A-P 180 EDT. These tow scopes did provide better planetary contrast, though. I owned a HD216 for a few years and can also attest to this scope's observational superiority over other similar aperture standard Newtonians with regards to offering an excellent flat field with pin-point stars without noticeable coma right to the edge of 2" eyepieces (the fov of 31mm Naglers and 35mm Panoptic eyepieces). Mak's offer 1/5 the coma as compared to standard Newtonians. Planetary performance can be awesum provided you barlow up the f.l. and allow the tube to cool down properly and use superior eyepieces (short f.l. Zeiss Abbe's, Takkahashi LE's, etc.). Running the mirror fan helps quicken the cool-down time. The tube lacks any interior baffeling which lowered the stray light blocking ability a bit, esp. while observing faint objects near brighter objects like the moon or Jupiter. The level of contrast was a bit lower than what I was seeing with neighboring 6" and 7" APO's, though. This might of been due to the lack of baffeling and/or mirror light scatter. It is most difficult for any reflective/catadioptic system to offer as much contrast on a planetary view as a top notch APO can. The laws of photon physics prevail here.
The Sital primary mirror substrate keeps the inside tube temperature up for too long, in my opinion. This was my chief complaint with an otherwise excellent telescope. The HD216 took at least twice as long to cool down as a neighboring A-P 180 EDT one warm summer evening. On some summer evenings, it never cooled down enoughh to achieve the diffraction views I was waiting for. The low-slung primary mirror at the bottom of the tube being only a foot off of the ground would always pick up the warm ground heat for much of the night. Sital holds its heat too long, but telescope makers use it because its a soft material and easier to work when figuring. Quartz is far superior but its a very hard material and takes a lot more time to figure a mirror made of this.
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