Posts Made By: John Cassella

Greetings,

O.K., here is/are the $64,000 question(s): How can I calculate the magnification render by a Mark V binoviewer when used in a Takahashi Mewlon 180?

To start: Without the binoviewer a 10mm eyepiece renders 216X (2160/10).

With the binoviewer, a 1.25X glasspath compensator and the Barcon (no extension) screwed into the Baader prism diagonal what would that 10mm eyepiece render and how do you calculate it?

The Barcon alone, screwed into the front of the prism diagonal is said to render 2.4X. So the above 216X becomes 518X. Do you then multiply that by 1.25X to get 648X?

Some help would be greatly appreciated,
john

Greetings all,

Well with all the talk about Mewlons I had to post some musings.

Nothing spectacular, but I was viewing Jupiter the other night and the view was pretty good but the seeing was not up to snuff because of the heat here in the desert. It did improve as the night when on but I can not stay up to late because I have work in the morning.

But some interesting notes:

1. I just got the Mewlon back from TNR where they gave it the once over. So I haven't use it in a while and, in fact, haven't used it too much since I got it. But I was struck by two things: 1. How black the sky background was 2. How small the stars were. This later point is of significant note as, for example, the stars in the double double were little orbs that fluctuated only slightly with the seeing, much better than Jupiter. While one would expect stars to hold up better it just stuck in my head how compact and well formed they were. I have never had a view like this in any other compound telescope I have looked through. I did own a TEC6 before, but it was a while ago so I can't really remember the views with any clarity.

2. Collimation: Interesting experience here. Having just been shipped back, it was a little out so I decided to go for perfection. A half hour later I had made a complete mess out of the star test. This is actually pretty unusual for me as I have collimated many scopes over the years and never considered it that difficult, but on this night it got away from me.

So what to do? Well I took the scope off the mount and brought it inside. I set it on the counter and removed the diagonal. Staring down the visual back I simply made the reflecting circle containing a reflection of my eye concentric with the circular opening of the visual back. By the way, in doing this you could immediately see how badly out it was. But in just a couple minutes everything was concentric. Took it back outside and guess what - DEAD ON!

Again, just some musing that may be helpful to some with some thought about the night's experiences. Others have said it and it clearly is true, for a folded optic with the central obstruction a Mewlon has the airy discs are very small (a function of aperture as well of course) but very clean. Really surprising.

And I think there is a lesson there. Many will cry, "with that kind of central obstruction it can't be any better than an SCT." But I think it's not that Mewlons are defying physics, it's that our expectations have been reduced by poor execution in some mainstream scopes. Clearly, with attention to detail and optical quality, even in a scope with a fairly large obstruction, one can achieve crisp clean images with superb contrast.

Take care,
john

Took some time with the bino & 180 and looked at Jupiter tonight. Images were very nice. Air was pretty steady but a good bit of high clouds that eventually did me in. Good detail in the bands and I picked up a fairly large festoon tonight between the equatorial belts.

Also looked at some double stars. Once again found myself surprised by the color intensity and also the lack of extraneous junk around the airy disk. I fully expect to see more diffraction rings around stars given the size of the central obstruction. But tonight I was essentially getting one around the airy disk, pretty unexpected.

Collimation is pretty good but I would not say it is dead on center. I will probably try tweaking it again this weekend. So far my tweaks have not be successful, but after doing some reading, and applying a little common sense I think I know why. Still, just looking up the visual back and eyeballing it gets me 1/2 a radius away in an 8.5 mm Pentax XF - not bad.

Hey Larry, how did you make out with your new Tak SCT?

Take care,
john

Hey Kevin,

Don't be discouraged by some of the responses you're getting. Personally, I think you're dead on 100% on the money. Your question is a good one, and your version of the truth is probably much more accurate then some of the responses you're seeing here.

Keeping thinking,
john

I don't know Charles, I think you need to use some magnification. 1000X with an 11", you're just coasting. And please remember, use of alcohol while viewing will reduce your visual acuity.

john

Hey Larry,

One night I tested an 8mm radian (mine), a 7mm Pentax XL, and a 7mmNT6 (in a 10" f4.5 newtonian). I looked at deep sky (galaxies, clusters) and Jupiter.

Quite frankly I couldn't seem much difference. Seeing wasn't good enough to push the eyepieces on Jupiter. I did pay attention to eye reflections (a problem I had with a 6mm Radian), contrast, and sharpness. No real standouts, although I was impressed by how far Al had come with the coatings on the Naglers.

Eye relief was a bit tight on the Nagler, but usable. The Radian and Pentax were both very comfortable to use.

I also noticed a weird optical illusion caused by the fields of view. For example, an open cluster appeared bigger in the Pentax because the field was smaller than in the Nagler.

Bottom line, I think differences in design (eye relief, field of view) will be a much bigger factor to the user than performance differences.

I have actually been contemplating the purchase of a 5mm Pentax XW for high power deep sky work (to replace my 5mm Tak LE).

But on planets I generally use U.O orthos.

Take care,
john

Parralax also makes very nice untis.

Take care,
john

I think you're right about the viewing. However, all horses are definitely not created equal.

Hey Floyd,

What do think of the new Starmaster your using?

Thanks,
john

Hey David,

I don't know about the tilting issue, usually a slight tilting of the primary can be accommodated. As long as the optical axis is not obstructed by the mechanical axis, it shouldn't be a problem.

Have you checked to see if the primary mirror is pinched?

Take care,
john