Re: Difference between SkyWatcher Explorer 130 RA and 130M?
Posted: Sat Jul 25, 2020 4:53 am
Heh Heh, I would love to have looked through that scope! Never seen an 8" F/ 9 Newtonian before...
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It was a planet killer and great on globs. BTW Gary Seronik has built a 6" f9. At the time I made my own optics.
notFritzArgelander wrote: ↑Sat Jul 25, 2020 5:42 amIt was a planet killer and great on globs. BTW Gary Seronik has built a 6" f9. At the time I made my own optics.
https://garyseronik.com/the-big-red-one ... /#more-141
Thanks for the suggestion, but I ended up pulling the trigger on the Explorer 130M due to the BBC Sky at Night magazine and customer reviews. It's been cloudy across the pond but have tested it on some distant objects and the view through the 25mm (the "good" modified achromat) is nice and crisp. I think I might replace the 52° Super MA 10mm with a Svbony 62° "goldline" eyepiece, which I've heard have a nice price:quality ratio. The biggest hurdle for me has been the mounting: I am pretty proficient interrynak wrote: ↑Sat Jul 25, 2020 1:19 amAn 8" F9 Newt means a scope 1803mm long or almost 6 ft. in focal length!notFritzArgelander wrote: ↑Fri Jul 24, 2020 10:45 pm
The technical detail is whether a spherical mirror deviates from a parabolic mirror in a way that satisfies the Rayleigh criterion that a defect in the shape of a mirror should produce no more than a 1/4 wave error in the wave front.
In Jean Texereau's How to Make a Telescope the application of this is discussed in detail. On pp. 16-17 there are formulas and a table:
Measuring f and D in cm: f^3 = 34.9D^3.
Measuring in inches one gets: f^3 = 88.6D^3.
The table has f/D = 7.0 for a 4" and f/D = 8.2 for a 6".
Both the examples you cite are "close enough" to parabolic so that within the Rayleigh criterion I wouldn't care about the difference between parabola and sphere. If the difference is smaller than what is required to have perfect optics, there is no functional difference. I wouldn't care whether the Edmund Palomar Jr or RV-6 Dynascope was advertised as a parabola or a sphere. Rayleigh has them as being equivalent.
I built an 8" f9 with a mirror that was spherical with only slight parabolizing. The table gives f/D = 9.6 for a 10".... So a REALLY long tube.
I was lucky enough to find a 5"F/ 8 Newt (127/1020mm) - the Meade 127NT, which was available only in '97/'98 before it was discontinued. Fits the Rayleigh criterion for a spherical, but someone mentioned that it has a parabolic mirror as well. Came with a LXD 500A mount, with a payload capacity of 15 lbs (according to manual). Very sturdy mount for anOTA this size.
Bresser currently markets a 130/1000mm orF/ 7.7 Newt in Europe (Messier NT-130/1000 EXOS-1) - like the Meade above, it may even have a parabolic mirror as well (may not make a noticeable difference with a spherical at high magnifications, but still nice to have); the EXOS-1 mount also has a payload of 15 lbs.
Flord.lord, if you still want a long focal length 5" Newtonian, the Bresser scope is the one to get. Here's the link:
https://www.bresser.de/en/Astronomy/Tel ... scope.html
Congrats on your new scope F.L (despite all the comments you've seen on this thread)!flord.lord wrote: ↑Sat Jul 25, 2020 11:16 am Thanks for the suggestion, but I ended up pulling the trigger on the Explorer 130M due to the BBC Sky at Night magazine and customer reviews. It's been cloudy across the pond but have tested it on some distant objects and the view through the 25mm (the "good" modified achromat) is nice and crisp. I think I might replace the 52° Super MA 10mm with a Svbony 62° "goldline" eyepiece, which I've heard have a nice price:quality ratio. The biggest hurdle for me has been the mounting: I am pretty proficient in RA and Dec. axes but need to learn the slo mo controls. Balancing was a big one too.
I need to improve on polar alignment and moving the scope when you can't actually see the controls.
Another thing is the portability and the weight; I got a bit carried away while purchasing! Has to be completely assembled and reassembled when I move it between floors and is a challenge for a small sportswagon boot (trunk?)
I hope to target planets/DSOs tonight!
This basically summarises last night !
flord.lord wrote: ↑Sun Jul 26, 2020 8:02 amThis basically summarises last night !
Got rough polar alignment (I think) by puttingDec . at 90° and "sighting" down theRA axis at Polaris.
Let me just say (as I'm sure you're aware) going from a tabletop base to anEQ (skipping an altaz!) is daunting!
I was so near M13 when...the slow motion control cable fell off the tripod!! :flame: How do I stop that?
Some other bugbears include:
The telescope hitting the tripod
The telescope hitting the cables
Not being able to move it in *that* place in an altaz direction
The altaz base lock not staying and having to re-polar align
Anyway...it's obviously a learning curve. I'm interested in how long it took you/takes people to learn?
Apologies for the rant...