henry link
Well-known member
Last week I had an opportunity to briefly examine a just purchased 95mm Zeiss Harpia. The last time I reviewed a scope (the Zeiss Gavia) I found two days was insufficient for a complete review. This time I had about one hour. The design of the Harpia is so unusual that testing it really demands even more time and energy than a conventional scope. For instance, usually a single star test and a single resolution measurement at high magnification are all that’s needed. That’s not true for the Harpia because the objective’s configuration, focal ratio and aperture change with magnification, so it can be reasonably assumed that aberrations and resolution will not remain constant over its entire magnification range. Unfortunately, I didn’t have enough time or enough control over the test conditions to make the multiple tests at different magnifications needed for this scope, so the results here are barely more than suggestive of areas that need closer examination.
I used the flashlight method to measure clear aperture. That confirmed that the aperture remains constant from 70x down to about 40x (my measurement for the largest clear aperture was about 93mm rather than 95mm). At magnifications below 40x the aperture appears to shrink in a linear fashion until it reaches something between 55 and 60mm at 23x. I can’t be more specific because my test method resulted in the internal obstruction that causes the loss of aperture below 40x to cast a fuzzy shadow rather than a sharp one on my ruler.
Very bad (Hurricane Florence) weather made outdoor testing impossible, so I was only able to star test the scope indoors at about 10m and rather than measuring its resolving power I could only roughly compare its resolution and sharpness at 70x and 40x to a 92mm Astro-physics Stowaway APO (with Baader Hyperion Zoom) using a USAF 1951 glass slide at about 7m. At those distances I expect any scope properly corrected for infinity focus to be somewhat under-corrected, so my experience can only really be applied with much confidence to the Harpia’s close focus performance.
In a 70x star test at 10m I found the Harpia’s spherical and chromatic aberrations to be clearly worse than the reference scope. The Stowaway shows an essentially textbook perfect star test at infinity and is slightly under-corrected at the 10m distance of this test. The Harpia did much better at 10m than the Gavia I tried last year, but it still showed considerable asymmetry between the intra-focal and extra-focal diffraction patterns, with hard rings and an overly bright outer ring outside focus and soft rings with an overly bright central spot inside focus. There was a narrow red fringe of longitudinal chromatic aberration around the focused test star that was pretty minor by spotting scope standards, but still indicated less than true APO performance. There was also some mysterious spiking around the entire periphery of the intra-focal diffraction disk, which resembled the appearance of surface roughness in a star test of a mirror. Other potential defects like coma, pinching, astigmatism and roof edge anomalies were very low or absent. However, unlike “normal” scopes the Harpia’s star test appears to deteriorate as magnification is reduced. Evaluating star tests accurately becomes more difficult at lower magnifications, so I didn’t even attempt it at magnifications below about 40x. The one change I was sure I could see was the gradual development of some astigmatism in this particular Harpia unit as magnification was reduced from 70x to 40x. I’m sure this wasn’t related to my eyesight because astigmatism was completely absent at all magnifications in the Stowaway set up next to the Harpia. This suggests to me that the aberrations and optical defects of the Harpia are indeed not constant with magnification and could grow worse as the optical configuration changes from about f/5.6 at 70x to f/3.2 at 40x.
Using a USAF 1951 glass slide I found the image quality of the Harpia to be a bit soft and gauzy at 70x compared to the Stowaway, with a loss of at least one Element of resolution on the chart. Unfortunately I couldn’t quantify the resolution because I didn’t have a way to measure the exact distance to the chart and besides 70x is a little low for my eyesight acuity to reliably detect the Stowaway’s true resolution. But, since I know the Stowaway’s resolution as about 1.25” using the USAF 1951 I can say that this particular Harpia was no better than 1.40”, probably not quite that good. That was no big surprise given the higher star test aberrations, but unlike every other telescope I’ve seen the Harpia didn’t appear to sharpen up as the magnification was reduced. The image appeared to be no sharper, perhaps even slightly less sharp at 40x, than at 70x. Since the Stowaway follows the usual pattern of appearing sharper as magnification is decreased that made the difference in image quality between the two scopes starker at 40x than at 70x.
Visual comparison to a reference scope of known quality is better than no reference, at all but it still mostly falls into the realm of subjective impressions. I couldn’t do any better than that at 40x because in the time available I couldn’t find a way to firmly attach my 3x Zeiss Tripler to the Harpia eyepiece. The booster would have allowed testing the Harpia’s 40x magnification at 120x, plenty high enough to measure the true resolution and properly evaluate the star test. In the end I suspected from the subjective appearance of the 40x image that higher aberrations at 40x probably degrade the actual instrument resolution at 40x compared to 70x even though the aperture is unchanged. That wouldn’t be a surprise given the f/3.2 effective focal ratio at 40x, but without real measurements I couldn’t confirm it.
One last thing I noticed was that the Harpia’s image was dimmer than the Stowaway’s at all magnifications but particularly, as you might expect, at the lowest power. The Harpia’s actual light transmission was almost certainly lower than the Stowaway’s since, as configured for this test, the Stowaway had only 10 glass to air surfaces and one dielectric mirror reflection vs. the Harpia’s 32 glass to air surfaces and 4 internal prism reflections. But, the increasing difference in image brightness between the two as magnification fell below 40x clearly resulted from the Harpia’s shrinking aperture. The window light I was using was approximately equal to what would frequently be experienced outside on a dark overcast day. Under those lighting conditions the reduced exit pupil of the Harpia at lower magnifications is a definite disadvantage compared to conventional scopes.
That’s pretty much all I could gather from my hour with the Harpia. If I ever get another crack at one on my own turf I would certainly want to star test and measure the clear aperture and resolution over the entire magnification range by placing my 3x booster behind the eyepiece. That’s unlikely to happen with this unit since the owner is not local, so for now I’ll be happy to turn things over to my few colleagues here who are inclined to do the same kind of testing.
Henry Link
I used the flashlight method to measure clear aperture. That confirmed that the aperture remains constant from 70x down to about 40x (my measurement for the largest clear aperture was about 93mm rather than 95mm). At magnifications below 40x the aperture appears to shrink in a linear fashion until it reaches something between 55 and 60mm at 23x. I can’t be more specific because my test method resulted in the internal obstruction that causes the loss of aperture below 40x to cast a fuzzy shadow rather than a sharp one on my ruler.
Very bad (Hurricane Florence) weather made outdoor testing impossible, so I was only able to star test the scope indoors at about 10m and rather than measuring its resolving power I could only roughly compare its resolution and sharpness at 70x and 40x to a 92mm Astro-physics Stowaway APO (with Baader Hyperion Zoom) using a USAF 1951 glass slide at about 7m. At those distances I expect any scope properly corrected for infinity focus to be somewhat under-corrected, so my experience can only really be applied with much confidence to the Harpia’s close focus performance.
In a 70x star test at 10m I found the Harpia’s spherical and chromatic aberrations to be clearly worse than the reference scope. The Stowaway shows an essentially textbook perfect star test at infinity and is slightly under-corrected at the 10m distance of this test. The Harpia did much better at 10m than the Gavia I tried last year, but it still showed considerable asymmetry between the intra-focal and extra-focal diffraction patterns, with hard rings and an overly bright outer ring outside focus and soft rings with an overly bright central spot inside focus. There was a narrow red fringe of longitudinal chromatic aberration around the focused test star that was pretty minor by spotting scope standards, but still indicated less than true APO performance. There was also some mysterious spiking around the entire periphery of the intra-focal diffraction disk, which resembled the appearance of surface roughness in a star test of a mirror. Other potential defects like coma, pinching, astigmatism and roof edge anomalies were very low or absent. However, unlike “normal” scopes the Harpia’s star test appears to deteriorate as magnification is reduced. Evaluating star tests accurately becomes more difficult at lower magnifications, so I didn’t even attempt it at magnifications below about 40x. The one change I was sure I could see was the gradual development of some astigmatism in this particular Harpia unit as magnification was reduced from 70x to 40x. I’m sure this wasn’t related to my eyesight because astigmatism was completely absent at all magnifications in the Stowaway set up next to the Harpia. This suggests to me that the aberrations and optical defects of the Harpia are indeed not constant with magnification and could grow worse as the optical configuration changes from about f/5.6 at 70x to f/3.2 at 40x.
Using a USAF 1951 glass slide I found the image quality of the Harpia to be a bit soft and gauzy at 70x compared to the Stowaway, with a loss of at least one Element of resolution on the chart. Unfortunately I couldn’t quantify the resolution because I didn’t have a way to measure the exact distance to the chart and besides 70x is a little low for my eyesight acuity to reliably detect the Stowaway’s true resolution. But, since I know the Stowaway’s resolution as about 1.25” using the USAF 1951 I can say that this particular Harpia was no better than 1.40”, probably not quite that good. That was no big surprise given the higher star test aberrations, but unlike every other telescope I’ve seen the Harpia didn’t appear to sharpen up as the magnification was reduced. The image appeared to be no sharper, perhaps even slightly less sharp at 40x, than at 70x. Since the Stowaway follows the usual pattern of appearing sharper as magnification is decreased that made the difference in image quality between the two scopes starker at 40x than at 70x.
Visual comparison to a reference scope of known quality is better than no reference, at all but it still mostly falls into the realm of subjective impressions. I couldn’t do any better than that at 40x because in the time available I couldn’t find a way to firmly attach my 3x Zeiss Tripler to the Harpia eyepiece. The booster would have allowed testing the Harpia’s 40x magnification at 120x, plenty high enough to measure the true resolution and properly evaluate the star test. In the end I suspected from the subjective appearance of the 40x image that higher aberrations at 40x probably degrade the actual instrument resolution at 40x compared to 70x even though the aperture is unchanged. That wouldn’t be a surprise given the f/3.2 effective focal ratio at 40x, but without real measurements I couldn’t confirm it.
One last thing I noticed was that the Harpia’s image was dimmer than the Stowaway’s at all magnifications but particularly, as you might expect, at the lowest power. The Harpia’s actual light transmission was almost certainly lower than the Stowaway’s since, as configured for this test, the Stowaway had only 10 glass to air surfaces and one dielectric mirror reflection vs. the Harpia’s 32 glass to air surfaces and 4 internal prism reflections. But, the increasing difference in image brightness between the two as magnification fell below 40x clearly resulted from the Harpia’s shrinking aperture. The window light I was using was approximately equal to what would frequently be experienced outside on a dark overcast day. Under those lighting conditions the reduced exit pupil of the Harpia at lower magnifications is a definite disadvantage compared to conventional scopes.
That’s pretty much all I could gather from my hour with the Harpia. If I ever get another crack at one on my own turf I would certainly want to star test and measure the clear aperture and resolution over the entire magnification range by placing my 3x booster behind the eyepiece. That’s unlikely to happen with this unit since the owner is not local, so for now I’ll be happy to turn things over to my few colleagues here who are inclined to do the same kind of testing.
Henry Link
