Visual impressions through Astro-Physics Refractors - 180EDT StarFire and 7" Starfire
I recently had the unique opportunity of comparing side-by-side my 7"f/9 Astro-Physics Starfire (1989 vintage) vs. my friend Mike Dunphyís 180EDT Starfire (vintage 1995). Due to Mikeís moving, I "took care" of his scope for over two weeks. Unfortunately during this period two hurricanes decided to visit the area and I was only able to observe for three nights. But the views on those occasions were memorable.
Sunday August 29,1999
Under clear skies in Philadelphia with temperatures in the low 70's and excellent seeing (8-9/10) conditions I had the pleasure to observe through two truly fine telescopes. Another friend, Alan Daroff (a.k.a. Mr. Sol Man) joined me for the Ďstar testingí portion which was performed using a 4mm Zeiss Abbe Orthoscopic eyepiece yielding magnifications of 405x for the 180EDT (1620mm f.l.) and 400x the 7"f/9 Starfire (1600mm f.l.) respectively. Alan has been an avid observer for over thirty years so I value his input. We briefly used a pair of 4.8mm Naglers to do quick back and forth comparisons but didnít feel these eyepieces were good choices Ė it was just the fact that we had two of them. We chose Altair as the subject for its brightness and position and viewed straight through for optimum performance. My 7" Starfire lens has recently been tested on Roland Christenís interferometer and he refigured, repolished and replaced the original cement spacing with the latest oil spacing and multi coatings on the surfaces. The scope is working better than ever.
* Just a note to say that Jim's 178 had gone through a flood and the 178 had been submerged under water requiring a complete rebuild. Scott
At focus, both instruments showed a hard white airy disk with a faint 1st diffraction ring. The 7" Starfire exhibited a faint violet background while the 180EDT showed no color. Raking the focus inside and outside of focus revealed round concentric fresnel rings with sharp edges in both scopes. The outer ring in each was significantly brighter than the inner rings as well. The noticeable difference between these scopes was the obvious but subtle color in the rings of the 7" Starfire. The colors varied from a yellowish green to magenta. The colors first appeared when just slightly tweaking the scope out of focus. Blue was the first color observed followed by green and yellow.
We spent about two hours comparing the instruments and both came to the same conclusion that both scopes gave very sharp images with perhaps a very slight edge to the 7" Starfire. Color error correction obviously went to the 180EDT. The faint violet background in the 7" Starfire leads me to believe that bright stars may tend to bloat compared to the 180EDTís in long exposure photographs. We also weighed the two telescopes with the 180EDT weighing in at 39# vs. the 7" Starfireís 33#.
About 3:20am I began the second phase, that of performing high power critical planetary observations. Since 90% of my observing is planetary, this if where my curiosity is piqued. As Jupiter and Saturn were high in the sky, I used 2" diagonal mirrors in both, the 180EDT with a Tele Vue unit and the 7" Starfire with an Astro-Physics model. Identical A-P Convertible Barlows were used in both as were 11mm Tele Vue Plossl eyepieces yielding magnifications of approximately 250x. I also used shorter focal length eyepieces in addition to a Zeiss binocular viewer with magnifications ranging from 240x to 350x. Iím most comfortable and experienced using two eyes so this is where my most critical observing was done. For 2-1/2 hours I was treated to some awesome views!!!
My first objects were the faint moons of Saturn and faint background stars. I couldnít see any difference in either scope. What was threshold with averted vision in one was the same in the other and vice versa. Call it a draw. The next area was sharpness using Jupiterís moons and Saturnís rings. Both were extremely sharp as I could detect the Cassini division all the way into the planetís shadow and see Jupiterís moons as clear tiny disks. Low contrast was the last area which I examined. Again both telescopes appeared equal. As I observed pale blue festoons and soft white ovals with subtle brightening/darkening in belts, it got to the point where I couldnít tell which scope I was observing through. What I saw in one scope, I saw in the other. At one point I thought the EDT showed a slightly darker blue in a festoon and the 7" looked a smidgen sharper on Saturnís rings. That was the only impression I had of any difference and another night may yield different results.
During the next two weeks, I was able to perform this comparison two more times. With a combined total of nearly nine hours at the eyepieces, I was still unable to detect any significant difference visually through these excellent instruments.
Conclusion: A great time!!! The 7" Starfire was easier to handle and setup being 1" narrower and 6# lighter. The 180EDT Starfire would probably have an advantage with long exposure photographs while visually the performance of both appeared for my purposes, identical.
Update: March 28, 2003
My good friend Mike passed away suddenly on Jan. 5, 2003 from a massive heart attack. As Mike and I agreed several years ago, the survivor would take care of liquidating the deceasedís astronomy equipment inventory.
Since then, Iíve had Mikeís telescope and mount set up on my deck next to my 7" Starfire on an identical 900GTO mount. Weíve had a rough winter and it wasnít until the past two weeks that Iíve been able to do any decent observing. In that time, Iíve been able to compare the two scopes on five occasions.
As previously mentioned, the first thing I notice is the difference in handling the two scopes when setting up or taking down. The extra 1" diameter and additional 6# of the 180EDT makes it significantly more cumbersome to handle. My age/weight ratio certainly enters the equation too.
Iíve performed star tests twice recently, both times using Maxbright diagonals in both scopes with a 4mm Zeiss Abbe Ortho eyepiece and a green filter in front of my eye. I learned that little trick in the writings of Roland Christen and Thomas Back. The first thing I notice when raking the focuser inside and outside focus is that the 180EDT seems to snap into focus more abruptly whereas the 7" requires more iterations to zero in on a star. Itís not significantly different but there is a difference. It was only after going back and forth so many times between scopes that I noticed this difference. It would usually go something like "tweak in, tweak out, tweak in with the 180EDT. The 7" normally would take an additional tweak or two. Sounds like John Madden-ese. I used Capella early on, followed by Arcturus later in the evening.
With the green filter the star images in both scopes appeared nearly identical. Tube currents were visible a couple of times in the 7", probably due to the smaller diameter tube. The outer circumference of the first diffraction ring in the 7" appeared at times to be a smidgen smoother than the 180EDT while the same ring may have been a smidgen brighter in the 180EDT.
Views of Jupiter and Saturn revealed the same observations as I noted previously. The minimally slight edge in brightness seemed to go to the 180EDT while the 7" seemed slightly sharper, most notably on Saturnís rings. Low contrast features appeared to be a draw. What I saw in one scope was duplicated in the other. This went back and forth and I couldnít give an advantage to either scope. Again, after awhile I couldnít tell which scope I was viewing through. Perhaps If I were in the Florida Keys with the great Ďseeingí conditions, I might be able to draw more definitive conclusions but this is the best I can do here.
In a conversation with Roland Christen, he told me that I probably wouldnít see any improvement visually in the 180EDT and that appears to be the case. Of course for deep sky photographs, the EDT has the edge but I don't use the scope for that purpose.
Itís been a real treat having these two great scopes side by side to compare. If I thought I would eventually get into deep sky photography in a permanent observatory, Iíd probably opt for the 180EDT. But after 11 years with my 7" f/9, I know that Iíll most likely continue as a visual, primarily planetary observer. With that in mind, Iíll keep my 7.