A 6 inch F2.8 telescope

by Mel Bartels, November 2013

 

I built this scope for three reasons:
Topsy turvy; that's my super-fast telescopes. I began amateur astronomy in an era where the major expense and effort was split between the primary mirror (and why I've ground my own optics all these years) and the mounting. Eyepieces were small and not that expensive with a field of 45 to 60 degrees. Look at my telescopes now; dominated by huge, expensive 100 degree eyepieces and coma correctors.

Having built a 13.2 inch F3.0 two degree field of view telescope, I determined to build a scope with more field. I settled on a 6 inch because of concerns over field curvature with such a short focal length and diagonal obstruction (the TeleVue P2 coma corrector also cuts into the light path slightly).

The results speak for themselves. My first night out, my first object, the Pleiades, I discovered very faint dust clouds surrounding the Pleiades. Surprised, I found deep CCD images of the Pleiades that showed dust clouds in the locations that I had observed. Star images are pinpoint from center to edge as they roll through the 4+ degree field of view (close to binocular fields of view).

The telescope's combination of field and aperture shows objects that can be hard to see in larger scopes.
I love this little telescope - it gives me views otherwise unobtainable. Here are some sketches made with the 6 inch f2.8.

 

I discovered that grinding and figuring a F2.8 mirror is no different than a F3.0 mirror. The figure is certainly touchy - a few seconds of polishing will subtly alter the star test image at high powers. For more on parabolizing the mirror, see http://www.bbastrodesigns.com/JoyOfMirrorMaking/Parabolizing.html.

To see the scope's design, please go to the Google Sketchup here

Here's a summary of design considerations and my conclusions.

Combination of aperture and field of view
Can see objects otherwise invisible; see common objects in a larger composition.
F2.8 primary
Make it yourself. If you have a couple of mirrors under your belt, then this should not be out of reach. Barring that, purchase from a mirror maker who also observes with short focal ratio telescopes.
Field curvature caused by short focal length
Not an issue at 17 inches focal length. Stars are pinpoint at the center and edge at one focus position.
Contrast
The contrast is exception under dark skies. My observations of large scale very faint dust clouds appear unique. Based on many years of building scopes and observing with them, I attribute the inky black backgrounds in the eyepiece to good baffling. 
Central obstruction due to relatively large secondary mirror
The central obstruction ratio between the diagonal and primary mirrors is about one-third (2.1 inch diagonal), which causes optical quality to drop by about 1/6 wave (not a problem because the optical quality of the 6 inch primary is top shelf). The resolution of stars in globular clusters and open clusters is quite good with this scope, made compelling because of the dynamic 100 degree apparent field with the Ethos eyepieces.
Coma corrector intrustion into the light path.
The smaller the scope the greater the percent of light loss and diffraction caused by the necessarily minimal distance from focuser to diagonal resulting in the coma corrector's barrel intruding into the light path. For my 13 inch f/3.0, the intrusion is 1/4 that of the diagonal's obstruction (or 1.5% of the overall light available). The 6 inch's intrusion is 1/2 that of the diagonal obstruction (or 7% of the overall light). Observationally this does not materially affect observations, though it is an increasing concern for wider angle smaller aperture Richest Field Telescopes for example a 4 inch.
Vignetting at F2.8 in the coma corrector
Another concern is vignetting. The diagonal that I use with the diagonal distance to the focal plane means that I have a fully illuminated field across much of the field. By slightly defocusing the star's image into a disc and moving it around the field, I can see where the primary mirror's edge begins to be clipped. This matches the calculated edge of the fully illuminated field, meaning that there is no vignetting in the F2.8 primary to P2 coma corrector to the 21mm Ethos eyepiece stack. Remember that the field to care about is the 1.4 inches of the 21mm Ethos and the 1.2 inches of the 17mm Ethos; most definitely not the full 2 inches possible given the 2 inch focuser. Also don't forget that the diagonal vignettes some of the light before reaching the coma corrector. There's no such thing as double-vignetting!
Optical alignment
Optical alignment proved no more challenging than my 13.2 inch f3.0 scope. A good laser collimator and very accurately centered notebook ring on the primary proved sufficient. The alignment cannot sag or change at all (maximum deviation is a fraction of a mm) as the scope is moved from a horizontal to a vertical direction. That's a matter of design and construction.
Focusing
Requires an expensive focuser able to position the eyepiece to better than 0.001 inches.
Exit pupil thoughts: how to test and positioning issues.
How to test for exit pupil without ruining dark adaption and causing the pupil to shrink? I test by slightly defocusing the star image so that it forms a disc. Since I can see the mirror's edge, I know that my eye's pupil is not truncating the mirror's aperture. However, I must place my eye exactly at the correct location centered and above the eyepiece (the rubber eyeguard helps). Consequently I also use the 17mm Ethos with its 3+ degree field of view at slightly higher power where my eye doesn't need to stay exactly centered.
The 21mm Ethos eyepiece with this scope gives an exit pupil of 6.4mm. My eye opens to this size. But what would be the consequences if my exit pupil were smaller?
If my eye opened only to 6mm then that's equivalent to stopping down the aperture to 5.6 inches, or a drop of about 10% illumination. This sounds significant. But the eye works logarithmically which means that the loss expressed in the eye's unit of measurement is 0.1 magnitude. And keep in mind that both the object and the sky background are equally affected, leaving the ratio between the two or the contrast the same. I've not been able to observe this difference or find anyone else who can in a blind experiment.
Aperture is an important consideration since it is the primary expense (unless you make the mirror yourself) and sets the overall size and weight of the telescope. But field of view is equally important. Try considering field of view first. With this approach you determine the aperture based on the lowest power eyepiece in combination with your eye's exit pupil, giving the widest possible field of view. This makes the eye the limitation, not aperture, not field of view. That's as good as it gets.

For more on Richest Field Telescopes, see http://bbastrodesigns.com/rft.html
For more on the 10.5 inch f2.7, the larger brother of the 6 inch, see http://bbastrodesigns.com/10.5inchF2.7/10.5InchF2.7Telescope.html
The 13 inch that started it all http://bbastrodesigns.com/ZipDob/ZipDob.html
How I made the mirror is at http://bbastrodesigns.com/JoyOfMirrorMaking/JoyOfMirrorMaking.html
For more of my telescope making see http://bbastrodesigns.com/tm.html

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