Just wondering what might be better, an apo (4-5") or mak-cas (6-7") or ???
scope preference for doubles
Posted 21 June 2019 - 11:21 AM
My preference is in the "or" category. I have used all of my scopes for doubles, but I love my 10 inch reflector... it is a double star magician... except for Sirius B... just can't get that one in the 10 inch. But I have split it ONCE with my 4 inch achro (retired this one to give to my granddaughter)... she loves doubles too...
- Nucleophile and flt158 like this
Posted 21 June 2019 - 11:57 AM
I have an ETX90RA and am actively shopping for a 102mm refractor (AT102ED) as a second scope.
The ETX works pretty well for doubles....it has limitations. The tightest split has been about 2.5 arc seconds at pretty high power. There are plenty of doubles out there for me to view, different magnitudes, separations, angles, etc. which makes any scope a potential "doubles scope."
Anything sub 4 arc seconds requires some work for me, particularly if there is a large difference in magnitudes.
- SeaBee1 likes this
Posted 21 June 2019 - 12:21 PM
I use my Stellarvue 105mm APO most of the time for doubles wider than 1" and when the seeing is only fair. It gives such nice images with no central obstruction.
If the seeing is above average I use the Intes 180mm Mak-Cass with its astro-sital 1/9 wave optical system on the tighter doubles, and planets.
I don't usually use the 10" LX 200 on doubles, but one night when the seeing was very good I was using the Baader 8-24 zoom on the double double in Lyra and zoomed all the way to 660x, the stars looked perfect and the separation was enormous.
I usually don't use my 18" Obsession for doubles, but once while doing a two star alignment on Antares with my 12.5mm cross-hair eyepiece, there it was a bright orange star with a little green orb next to it. I had to just stop and take a good long look, it was beautiful, and so was the seeing that night.
Edited by Astro-Master, 21 June 2019 - 05:57 PM.
- SeaBee1 likes this
Posted 21 June 2019 - 02:18 PM
My current favorite is the refractor. With reflecting telescopes like Newtonians I almost always have thermal issues (tube seeing). Especially with very tight double stars with overlapping diffraction disks a thermally stable telescope is very important. A four inch refractor can easily handle doubles of nearly equally bright components with a distance of 1 arcsec. Even with a 55mm refractor I have no problems observing doubles like 12 Lyn (5.4/6.0mag, 1.9'') or Alula Australis (4.3/4.8mag, 1.9''). But that depends on the observer. A Mak could be critical with large difference in temperature, for example in winter.
- barbie likes this
Posted 21 June 2019 - 08:34 PM
Between an 4-5 inch apo and a 6-7 inch Mak, my preference would be for the 5 inch apo but if the Mak is thermally stable with quality optics, it has the ability to split closer doubles.
A four inch refractor can easily handle doubles of nearly equally bright components with a distance of 1 arcsec.
The Dawes limit for a 4 inch refractor is 1.14 arc-seconds. A Dawes limit split is not an easy split, the disks are overlapping with slight drop in the contrast between the centers.
My own preference for close splits is a 10 inch or larger Newtonian. In a 10 inch Newtonian, a 1.0 arc-second double is a wide split, the disks are not overlapping. Newtonians require attention to detail, collimation and thermal management are critical but represent challenges rather than true obstacles.
The most important factor in splitting close doubles is the seeing and seeing is primarily a function of location. My San Diego backyard, generally has good seeing and can have exceptional seeing. The jet streams are normally to the north and the mild climate and close proximity of the ocean make for stable nights. On the other hand, Mr. Roberts is located in Illinois at 42 degrees north. The seeing across the northern lattitudes is generally not ideal, nights when a large aperture scope can be used successfully for doubles are probably few and far between. The climate to can be an issue, the indoor-outdoor temperature differential can be quite large, making thermal equilibrium more difficult.
I think when stable seeing is an issue, a refractor is a good choice, refractors are thermally stable and operate near peak efficiency nearly 100% of the time. You are trying to make the best of a less than ideal situation, half arc second doubles are rarely on the menu, a large aperture is rarely useful.
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Posted 22 June 2019 - 11:31 PM
My AP 130 f/8.35 is a great scope for doubles but, I am one of those Refractor nuts who likes the view of doubles with Maksutovs because of the central obstruction which throws extra light into the first diffraction ring. So, either a 5” apo or a 7” Maksutov would work for me. As Jon said the critical issue is the seeing plus acquiring and maintaining thermal equilibrium. A larger 7” Maksutov can have more trouble cooling down than a 5” Refractor in some areas.
When the seeing conditions are good here, there is little drop in temperature overnight so a larger Maksutov has no trouble with cool down.
Edited by JimP, 23 June 2019 - 09:51 AM.