New 4" f/12

[jimcurry]

Jun 20, 2011 9:27:21 GMT -5 mikey cee said:

Great job there Jim!! I too like the longer focal length combined with my 2-3/8" diameter focusing knobs. Really makes focusing easier to me anyway. Nice proportions. Who made the mount? Mike

Yeah, Mike, those larger knobs combined with the silky smooth R&P is just as good as the dualies like on my Moonlite.

Jim

[Mike]

Yes, but Celestron calls it the "Celestron Advances Series GT" which is a little classier than "CG-5". Don't you think? I use one with my Lunt 60 Ha scope.

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[jimcurry]

Jun 20, 2011 22:43:59 GMT -5 Mike said:

Yes, but Celestron calls it the "Celestron Advances Series GT" which is a little classier than "CG-5". Don't you think? I use one with my Lunt 60 Ha scope.

That's good to know. Now I know I can charge more for it when I go to sell it.

Jim

[jimcurry]

Mike's been asking me to put together some thoughts about the scope but I was holding off until our club optician, we'll call him C, had a chance to critique the lens. I make no claim to optical testing expertise.

I've now been out 4 times with the scope over the past couple of weeks. Last night at our monthly star party I set up in the company of a C8 and a couple of smaller reflectors. The Istar 4" drew comments from everyone as to the quality of the view. I kept a variety of objects from M57, M51, M8, epsilon Lyrae, Saturn and a few other showpieces on view for club members and the public. Comments included that the views of Saturn were the best that night. Sky conditions: "orange" skies, humid, dewie, transparency 3.5 to start and by midnight probably 2.5. I was especially interested in C's opinion. He has some professional, sophisticated grinding and testing equipment and is the go to guy for unusual designs and builds. His primary instruments are a 16" gem mounted newtonian and 8" f/6 that have exquisite optics. C viewed most everything I had the scope on and during the course of the evening had quiet praise after his turn. While looking at M51 he commented rather disdainfully that he read O'Meara's Messier book and thought the drawings we're exaggerated for a 4" scope. His comment after a few minutes at this scope, "You really can see that level of detail in a 4". After the public had left I asked him to do a star test and explain to me what we were looking at. I had my turret installed with a suite of Brandons from 32mm to 6mm. The 6 gives a 200x view or 50X/inch of aperture. We looked at delta Cygni. First C racked it inside and outside of focus commenting on color correction then showed me what to look for. The CA was barely there and it took concentration to pick up the subtle changes. Next he examined the diffraction rings and explained to me what I was seeing. Naturally I've seen them on my own but I'm not in a position to qualify my observations. When C was done he summed things up by saying that the CA for an 4" f/12 was about as good as it gets. The rings were concentric and perfectly shaped with one comment. On one side of focus he thought the shape went subtley oblong due to collimation. No-one else could see this and he said to not worry about it. Jokingly, C said this looks lens like it was turned on a lathe, perfect star test images and as good as a 4" achromat can be. He was even speculating that maybe the glass was special and that this wasn't a Fraunhofer design to get such good correction.

One thing I can state is I can take this scope to 50X/inch (200x) on any given night and the views of stars remain perfect. Using a 5mm Pentax ortho pushes things to 240x and there is a slight deterioration. She's a keeper!!

Jim

[Mike]

Jim, I don't know why I sweat until I hear the report from the customer about a new Istar lens. They always turn out well. I get nervous evertime I star test a new scope. Until I see that perfect airy disk I can't breathe. So glad to hear such a fine report. Kudos to your personal optician also IMO, which I try not to do but can't help it sometimes, a 4" unobstructed refractor is equal to a 6" or 7" reflector or SCT in terms of what you can see. Certainly has more contrast and a darker sky and I believe this is where the refractor shines through, literally. That said, if your sky is dark it's not surprising what a 4" will "see". Is it OK if I post this little "review" on our site with your pictures?

[jimcurry]

>Is it OK if I post this little "review" on our site with your pictures? <

Go for it.

I sent Ales several photos as well.


>a 4" unobstructed refractor is equal to a 6" or 7" reflector or SCT in terms of what you can see.<

Are you kidding?? Between tube currents, dew problems on the corrector plate and collimation issues I'll take 7x50's over an SCT. When the SCT owners are lined up at your refractor and quizzing you about your ep's you know your scope is putting on a good show.