A 6 inch F2.8 telescope

by Mel Bartels, November 2013

I built this scope for three reasons:

Observe with a 4+ degree field of view with 6 inches of aperture
Gain more experience grinding sub-F3 mirrors
Test how a Newtonian telescope behaves at F2.8

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 with 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 and diagonal obstruction.

The results speak for themselves. My first object, the Pleiades, I discovered very faint dust clouds surrounding the Pleiades. Stars are pinpoint from center to extreme edge.

Later I (re)discovered Integrated Flux Nebulae, sketching a great number of them. And I've begun stitching together multi-field drawings into grand mosaics. For me it's always the view through the eyepiece. More of my sketches are here.

The telescope's combination of field and aperture shows objects that can be hard to see in larger scopes.

NGC and IC emission nebulae stand out framed by large swaths of black background. Thanks to the pinpoint star images, I can resolve clusters at low magnifications into sprinkles of glitter.
I see large scale objects in a way not possible before. The Pleiades and the nebulosity that waterfalls off Merope look to be embedded in the dark center of a larger ring of faint nebulosity. The entire Andromeda Galaxy including the extreme side extensions is beautifully framed in the field of view. The twist in the Galaxy's outer reaches is obvious.
Dark nebulae standout and strands of nebulosity of varying brightness fill the eyepiece as the scope is panned across the Milky Way. I pick up the Horsehead Nebula without a filter and can place it in the same field of view as the belt of Orion or M42. Barnard's Loop is a nice fit; I can trace it across many fields of view.
I enjoy 100 degree eyepieces. Using a 80 degree or worse, 60 degree, eyepiece feels constricting.
Most of all I forget about the fast F2.8 focal ratio and lose myself in the views. Every night I've use the scope I've seen something new, something unexpected.

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.

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	Visual sketches of IFN; 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.
Light throughput compared to refractor	Equivalent unobstructed aperture is 5.5 inches (6 inches aperture / refractor throuhput of 95% * enhanced coating 96% primary * enhanced coating 96% secondary * 0.94 (aperture reduction due to 1/3 diagonal obstruction))
Field curvature caused by short focal length	Not an issue at 17 inches focal length.
Contrast	The contrast appears exception under dark skies.
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 wavefront.
Coma corrector intrustion into the light path.	The P2 corrector in my 13 inch f/3.0 intrudes 1/4 that of the diagonal's obstruction (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). This does not affect observations. For more on my thoughts, read this.
Vignetting caused by the coma corrector	Another concern is vignetting caused by the P2's barrel. Slightly defocusing the star's image into a disc and moving it around the field, I tested for and found no vignetting caused by the bottom of the coma corrector.
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.
Focusing	Requires an precision focuser able to position the eyepiece to better than 0.001 inches.
Exit pupil thoughts	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. I must place my eye exactly at the correct location centered and above the eyepiece using the rubber eyeguard. 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 or about 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. Aperture is an important consideration seting the overall size and weight of the telescope. 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 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.

More information:
Richest Field Telescopes
My 10.5 inch F2.7 RFT
My 13 inch that started it my renewed interest in RFT sketching
How I make mirrors
My Telescope Making

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