Posts Made By: joseph cannavo

Much to my surprise, a bit of information that is hard to come by is the point of focus, for a given EP, relative to the focal plane of the primary. One way of quantifying this would be to specify the location of the seat of the eyepiece relative to the focal plane of the primary when the eyepiece is in focus. "Seat" here refers to the point along the EP that sits on the plane defined by the end of the drawtube. (Obviously there are other ways to quantify this)
When setting up a low profile/short travel focuser, this information would be quite useful to guide placement of the shoulder of the focuser relative to focal plane of the primary. It would also bear on future eyepiece purchases (That 31mm nagler is often out of reach of both the pocket and the drawtube!)
I have found bits and pieces of this info scattered around the web. Anyone know of a site that deals with this in some detail?
Joe
PS, Of course with EPs where the field stop is ahead of the optical elements this can be determined by inspection, but you still need the EP in hand.

Does anyone know how much in-focus the meade series 5000 30mmUWA requires, as say compared to a 31mm Nagler (which will not reach focus with my low profile-1.25"travel setup
Joe
(kineoptics helical on w/16"f5 Royce conical)

Thought I could start a thread on most recent weather updates for Cherry springs Pa. (Black Forest this weekend) So far I have seen extended forecasts for partly cloudy skies...
Joe
Royce 16"

After much more time spent than I anticipated my front collimating 16" Royce conical dob structure is finished. I am awaiting a clear night - tonight - to do some final adusting of the focal plane location and then await first light. The following link provides some pics...
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=atm&Number=1125282&Forum=All_Forums&Words=royce&Searchpage=0&Limit=25&Main=1119526&Search=true&where=bodysub&Name=&daterange=1&newerval=1&newertype=w&olderval=&oldertype=&bodyprev=#Post1125282
Joe

I have a question I have been wondering about. Generally - and especially with metal tubes - too small a clearence between mirror and tube walls is a set up for thermal effects to cause problems. With dobs with wooden mirror boxes, is the same issue as significant. As an example, obsessions use significantly smaller boxes for a given size mirror than starmasters. Is this ever an issue?
Joe

Please excuse the somewhat off topic post, but I think you all would be most likely to have an answere for me. Last night I was observing the south east lunar limb (16"F5 Royce , 667X) and was observing a beutifil sunset over a large carter - Bailly, Lamark???- just on the SE limb with a central peak. Between 10am est and 12pm est the crater slowly filled with black and the central peak gradually dissapeared. It was spectacular, as if black water was rising and the peak submerged just before the level rose to the crater walls.
I am trying to identify which crater this was. It was roughly 250-300km across, slowly swallowed up as the terminator crept from east to west. Anyone know??
Joe

Question for the group.
Last night I was observing the south east lunar limb (16"F5 Royce, 3mm Radian , 667X) and was observing a beutifil sunset over a large carter. Stupidly, I didnt consult an atlas and I am unfamiliar with many of the craters near the limb. This was just on the SE limb, very largte, viewed nearly edge on and with a central peak. Between 10am est and 12pm est the crater slowly filled with black and the central peak gradually dissapeared. It was absolutly spectacular, as if black water was rising and the peak submerging just before the level rose to the crater walls.
I am trying to identify which crater this was. It was roughly 250-300km across. Watching it slowly get swallowed up as the terminator crept from east to west was spectacular. Anyone know??
Joe

I have two questions on star testing for the group:
My own experience with star testing is that I can recognize a near perfect test, as well as a the common defects that manifiest in a less than excellent mirror. However, I do not have a "feel" for how such defects translate into wavefront error or Strehl.
I recently tested a mirror (12.5" F6) at ~40X/inch. Seeing was 7/10. The mirror was touted as having a 0.94 Strehl ratio. By my test, however,the mirror yielded sharp,even, outside of focus concentric rings with washed out nearly undescernible inside focus rings - possibly suggesting some combination of spherical aberation and a turned edge. Some tube currents were visible as fell, but these in no way obscured the outside focus pattern. Nor did the effects of seeing. My first question then is this:
Is the star so very sensitive that a >0.94 Strehl mirror could yield such a test.
Second, due tube currents effect the inside focus image more than the outside focus image?
Joe

Does anyone know of a luar atlas (on line?)that will give the precise appearence of the moon from a given location, date and time?
Joe

Mike
Right on! The photo you took very much dupliates what I saw visually through my 16" Royce at 667X, during moments of stillness. I had given up on IDing this crater. Thanks! It would be something to see some sort of time-lapse on what I saw visually. Thanks again mike. PS wpould love to see more images of this if anyone has any.
Joe