A Eulogy for Russian Mak-Newt Telescopes Part Two – optical analysis
Optical Analysis
The optical recipe for the MN76 was examined using our lens design software.
In this F6 system, coma and astigmatism are small. While distortion and field curvature dominate, they are also quite small. By using an APS-C sized sensor, and ignoring the field beyond 34mm diameter, the aberrations are reduced significantly through refocusing. We also see that there is a little scope for LSA reduction by refocusing between filter changes.
Refer to Fig 11 and 12. The MTF worsens with diagonal size increase, as expected, due to diffraction effects. This instrument, when fitted with a small diagonal, is a high-resolution ‘planet killer’. But irradiance is too low for good imaging, at the third field point (0.7 degrees). Also, aberration starts to be significant at the extreme edge of the field, but this will not be apparent visually, due to low irradiance and/or low magnification respectively.
With the larger 66mm diagonal, flat-fielding will be less critical in APS-C sensors, while contrast at the full-field for high spatial resolution targets will be poor. For deepsky imaging, this is not very important, but it implies the need for an interchangeable diagonal if planetary imaging performance is to be retained.
Setting up the instrument for a nominal 2 degree field, corresponding to a 45mm diameter image circle, and deprecating the image quality outside of the 34mm diameter central region relevant to APS-C sensors, allows deepsky visual performance to be assessed. Figure 13 and 14 document spot size across an optimally curved image surface (R = -2700mm) computed for the central 1.4 degrees of the 2 degree field.
The Airy disk diameter of this scope is 0.0098mm at the C-line (656.3nm). Note the implied excellent image quality.
Now, with deepsky imaging in mind, let us consider a flat focal plane, not one optimally-curved.
Looking at the spot sizes over the extended (photographic) wavelength range below, and enforcing a flat focal surface, indicates the imaging potential as a 1100mm EFL, 2 degree astrograph.
The key takeaway is that, despite received wisdom about the small Petzval radii of MN designs, it looks perfectly possible to produce diffraction-limited imagery across at least an APS-C sensor, without additional corrective optics.
All optics in a MN telescope are spherical. Consequently, no optical axis exists on the primary that needs alignment with the optical axis and only translations rather than tilts need to be addressed for good collimation. In effect, there are only 2 degrees of freedom to worry about.
The corrector lens is a different matter. Both decentration and tilt need to be controlled. However, as is seen in the spot plots below, fairly significant deviations from nominal positioning are possible with relatively small effect on image quality.
The optical arrangement of Intes-Micro Mak scopes is admirable for portable equipment. The closed tube, strong corrector, generous tube diameters, management of tube currents and insect-excluding cooling duct filters enable rough handling, while the push-pull primary cell holds tip/tilt collimation well. The diagonal mirror mount is not so well engineered, and can rotate under vibratory loads e.g. transport. The corrector lens is stable, provided it has been radially shimmed properly.
Verdict on suitability for deepsky astroimaging
The strengths of the Intes-Micro 7” Mak-Newt astrograph will be for APS-C and small chip imaging, at medium apertures, where refractors are becoming expensive and heavy. The 1100mm EFL fills the image scale between Celestron’s RASA line (610mm EFL) and various RC and Dall-Kirkham reflectors.
The MN is easily portable, tends to have stable collimation, and some tolerance of rough handling. There are no chromatic issues, even well into the IR range where many APO lenses and refractors would struggle to deliver low aberration images.
The significant image extraction distance of 123mm enables various types of low profile focuser to be used, for example, large aperture helicoids, the Clement flexural focuser, or possibly a low profile large-bore rack & pinion model. It should be possible to allocate 65mm of backfocus to the imaging train, which is sufficient.
Exchanging diagonals is not simple, as it involves re-collimation, which for a Mak-Newt takes more time than for a simple Newtonian, and is absent in most APO’s. It may be worthwhile engineering a better secondary mount that is a modular install into a hub permanently fitted to the corrector lens.
Much stiffer rotating rings will also be needed for supporting the OTA than provided by Intes-Micro.
The benefit of this project will be the absence of diagonal spikes on images, i.e., ‘refractor-like’ images, combined with relatively short FL compared with SCT’s and small RC telescopes. The relatively small central obscuration and absence of ghosting and veiling glare may or may not be apparent in imaging results.
