Quote:
Originally Posted by astro744
ServoCAT is no doubt a better option especially for the large Dobs but the OP mentioned a 12 Dob. Get a quality f4 to f4.5 or perhaps shorter mirror on one of these and the 6 extra height of the platform is welcome especially if you prefer the eyepiece at eye level at zenith. The platform of the EQ mount can become the baseboard of the Dob as a permanent fixture. For a 12 or 14 its not a bad option but anything larger then you would want to consider all options carefully.
I consider perfect Dob to be 12.5 designed with focal ratio and EQ platform in mind such that eyepiece is at eye level when the telescope is pointing to zenith when standing up. I prefer to stand and move freely around the telescope from target to target. I personally dont like GOTO but tracking is nice but I understand some people cannot be without GOTO for whatever reason.
A seat is OK at some angles but definitely not for all and that is the nature of the Dob. Anything low can be awkward standing but anything high just out of reach can be achieved with a $5 folding 8.5 step from Kmart.
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I always try to set my observing program for a night so that I am observing targets with reasonable elevation (above 45 deg). Below that, the image quality and light throughput is significantly reduced. Obviously there are a lot of targets that don't get above 45 degrees and you have to observe them down low, but those that I can observe at good altitude I do.
A $250 Observing chair has enough height adjustment to allow for 90% targets to be observed seated with a compact dob. I use my 10"/F5.3 mostly seated. A taller person could use a slightly larger dob seated, like a 12"/F5. Observing seated effectively adds 10% to 15% to the aperture of the scope, at a minimum. I don't worry about sitting down if I am out doing a tour of bright globulars or the like, but if I am observing moon, planets, double stars, threshhold galaxies with the 10" scope I always observe seated. Without question, you extract more detail than you can standing.
Another thing to consider is that as the scope gets faster, the depth of focus reduces. This is the range that the image is in focus. With sub F4.5 dobs this is very shallow and in marginal seeing higher power images tend to continually phase in and out of focus. With slower scopes this is something that is a lot less noticeable due to the greater depth of focus. Notwithstanding that the Paracorr II has fixed the coma issue with sub F4 scopes, it hasn't and can't fix the depth of focus issue. This is the specific reason none of my scopes are faster than F4.5. Having observed with just about everything over the past 50 years including 36"/F5, 30"/F4.6, 25"/F5 and a 28"/F3.3; I will climb the ladder and take the 25"/F5 over the 28"/F3.3 every day, for high power work. With the scopes Greg has owned over a long period and the scopes he currently owns, he would probably not not have experienced a scope with very shallow depth of focus. On that basis I think he is better off looking at a 12"/F4.5 to 12"/F5 scope, which without an EQ platform, would be the perfect height to use seated, or standing and slightly bent over.
FWIW all my scopes have Servocat on them,
including my 10"/F5.3, and I wouldn't have it any other way.
Cheers
John B