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Suburban observing report, ES 127 Mak w/2" diagonal

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#1 Redbetter



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Posted 23 September 2020 - 04:29 AM

A frequent question is whether or not a 2" diagonal will work with a mid-size Mak or SCT.  That has been covered elsewhere (yes, it works at least from about 5" to 8"--although specific mechanical interferences/issues can arise, so research first to understand what you are getting into with your model.) There are pro's and cons, and in many cases a reducer/corrector can be employed to avoid 2".  So the next question often asked is "why try to get a larger true field of view with a Mak or SCT anyway?"  Folks will give various reasons why it isn't needed, or some often mistaken comments about the level of vignetting to be expected, etc.


To refresh my memory and to provide some real world evaluation, I thought it might prove useful to do some suburban backyard observing to give folks an idea of what a 2" set up can do for visual under less than optimal conditions.  (I have already done a number of these before with this set up in dark sky and mentioned that in another thread.)  Stuck in the Valley again tonight due to smoke/fire/Covid closures, but with some relatively clear skies in the backyard, I decided I would do a tour of a few targets.  I watched the Io transit and its shadow before switching to bigger targets with the early 1st quarter Moon still in the sky.  All early nebula targets were overhead in 18.6 MPSAS condtions (measured via SQM.)  Seeing was poor.  I was only going to about 4.5 NELM but was using stars at this level for pointing with the RDF, so I wasn't trying hard to go further.  There was still some mild smoke scent in the air.


System:  ES 127 Mak, f/15 (1900mm nominal focal length, ~27mm rear baffle diameter)  2" Astro-Tech SCT threaded 2" mirror diagonal (99% dielectric.)   Skywatcher AZ5 mount.

  • TV 55mm Plossl that combined with this diagonal results in ~2080 mm focal length for f/16.4, 3.4mm exit pupil, 1.27 deg true field at ~38x.   
  • 2" TV Bandmate II OIII filter employed for emission nebulae w/55.
  • 13mm T6 Nagler that combined with this diagonal resulted in ~2105 mm focal length for about 162x  (remember, seeing was poor.)



First up was the Veil.  This is a tough one for small aperture in town, particularly for long focal length Maks and SCT's because of its large overall size, the two primary arcs are over 1 degree in their brighter portions. The 55 Plossl provided enough exit pupil for the OIII to do its work even in red zone skies.  With a narrow filter, it was necessary to use an observing hood/cover to eliminate some mild sources of glare, plus the suburban sky.  The Western Veil around 52 Cygni was seen rather dimly as my eye was still adjusting, but I could trace the major contours and they were contained within the 55's FOV.  The Eastern Veil is always easier to see, but requires a bit of a hop without a magnifying finder.  This broader arc was reasonably well seen considering conditions, spanning nearly the entire field.


The North America and Pelican Nebula were less impressive.  Both could be identified, but were dull, almost amorphous versions of their usual selves.  A larger part of the problem is that the NAN is still so large that panning is required.  In dark sky this is less of an issue for actually defining it.


I did a quick hop/jump over to the Ring and to then to the Dumbbell Nebula with the the 55 to get an appreciation of the scale.  Both are bright with the filter, but the Ring is quite small at 38x.  Then I turned to NGC 281 (Pac Man Nebula.)  This one was reasonably well seen in its central regions, although it trails off at the edges in bright sky.


Then I removed the filter and took a look at the Double Cluster (well-framed in the 55 Plossl), the Andromeda Galaxy, and the Pleiades.  It is tough to see much of the extended disk of M31, but at 38x M32 stands out noticeably non-stellar and a brief search across the disk revealed the subtle glow of M110 framed by field stars--all in the same field of view.   Likewise the primary members of the Pleiades were all visible together in the 55's FOV.  I had some slight indications of the broad fan of the Merope Nebula...but marginal at best, not like in dark sky.


To test suburban star hopping using only the RDF mounted on the OTA, and the 55 Plossl, I tracked down Neptune.  This did not prove difficult as it was near the meridian.  It is a lot tougher with just a 1.25" max field eyepiece.  Neptune wasn't steady, but it was clearly a gray blue at 162x and appeared as tiny shifting disk, lacking good definition in the seeing, but definitely non-stellar.  (I confirmed it by doing the hop in the 20" afterward, but I knew from the pattern panning the 55 that I was on it.  Triton was visible in the Dob.)


Then I turned to Uranus.  This again required a hop, about 5+ degrees from my 4+ mag RDF pointer star in these conditions.  But the 55 Plossl made the hop manageable.  Switching to 162x revealed a larger, better defined disk in a pale turquoise hue.  It wasn't steady though, moving about a bit.  Turning the 20" on Uranus revealed what appeared to be two moons at 227x (confirmed positions as Titania and Oberon.) 


I finished with a brief look at Mars.  The seeing was not really sufficient, so only a few details were seen: SPC, southern Mares, vague indications of the Solis Lacus structure.  I've seen better with this scope in recent nights.  The 20" of course showed more, but the seeing was taking a toll (red 25 filter helped some.)



This set up is growing on me, mainly for its compact size for backyard observing in comfortable weather.  The 2" mirror isn't hurting the planetary on good nights.  While it is asking a lot to manually hop with only a 1.27 deg FOV in bright sky--aided only by an RDF--it works...and in so doing is a lot better than a 1.25" eyepiece's 0.81 deg max field in this scope.


The 55 Plossl and nebula filters give this scope some reasonable capability.  It isn't a great choice for large nebulae, but it works.  A refractor will thrash it of course, as will Dobs.


So about that vignetting with the max 46mm field stop...I really don't notice it while observing.  Oh, I can tell some is out there if I look for it, but the eye handles it well and I can track dim features field stop to field stop.  One of the things I like about the f/15 Mak is that field curvature is not an issue at low power.  The edge of the field has flaws, but not to the level of an f/10 SCT.  


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