Sassenberg 80mm

OTA disassembled and the paint stripped from the tube. Rather than polish it up, I think I'll leave it with a Satin finish. It should age nicely and in another 60 years, the patina should match that of the tailpiece and objective assembly. 

First light (for me at least) 2 January 2011

The OTA was mounted onto a modern day GEM (Atlas EQ-6). Seeing was pretty bad (very unstable due to cold air and city environment). Temps were in the mid 20s and transparency not much better than mag two. There was a considerable amount of moisture in the air.

Airy disk:

·        Inside focus:

Rings were uniform with a hint of slight out of roundness. The outermost ring is two to three times as wide and as bright as inner rings. The center spot appeared very violet.

·        Outside focus:

No rings at all. I think this means the objective is undercorrected.

Useable magnification was < 200 due to such small exit pupil above 200x (<.41mm). When using the University Optics 7mm eyepiece (199x), detail was lost due to dust motes and floaters in my eye interfering with the view.

The scope performed very well with the 8mm Astro Techs so the maximum mag so far would appear to be 174x with top end at 199x.

No "color" typical of low end or fast achromats was noted - A good sign.

The finder has a very wide 7 ½ degree field of view but low light gathering power at 42.5 mm. Few stars were seen. Lower limit appears to have been about mag four to five this night. Stars do not resolve to pinpoints at center of field. Distortion sets in about 2/3rds of the way to the edge of field. Cross hairs are very large.

Viewed several objects:

• M42 – in 25mm paradigm. Fairly good detail. Not as bright as larger scopes but great contrast.

• Trapezium – Three of the four trapezium stars were easily visible with the fourth needing a little effort. Soft at 174x but all four stars were visible but large.

• Jupiter - Not much detail due to seeing. Brief periods allowed several cloud bands but no red spot although it was supposed to be visible.

• R Leporis - Showed good red color.

• M41 – Showed a few stars. Not all that impressive in Light polluted skies - Need dark skies

• M35 – Takes up half the FOV of the 25mm eyepiece.

Yep, it’s an 80mm telescope alright!

I did discover a problem with this size scope: It won't take high magnification due to the very small exit pupil. And, at the opposite end (low magnification), the view is sensitive to eye position relative to the eyepiece. All due to the size of its exit pupil and my entrance pupil.

Basically here is what happens:

Using my 25mm eyepiece yeilds a magnification of 56. Exit pupil is aperture / magnification (82/56). This yields an exit pupil of 1.47 mm. My pupil size is 4.5mm dark adapted. This means the beam coming from the eyepiece is about 1/4 the diameter of my eye pupil size. This means I can not look to far off the center line of the eyepiece before the field goes black (The view is more sensitive to eye position relative to the eyepiece than in larger scopes). This is not too bad for older folk like me where our maximum pupil size is small to begin with but it is worse for younger people whose dark adapted pupil size is on the order of 7mm! 

At moderate to high magnifications (say, using my  8mm Astro Tech at 174x), the exit pupil is .45mm (about 1/10 my pupil size). This is so small that the beam of light from the eyepiece is only falling on a very small part of my retina. Any dust on my lens or "floaters" in my eye blocks most of the image so I can't see much detail. Now this does not denigrate the optical quality rather it is an example of what you can expect from any optic with this high of a focal ratio. This won't bother younger people since they don't have the buildup of floaters that us old farts have accumulated.

I wonder what he'd think