31 replies to this topic

[Ric Whatley]

When you measure an eyepiece that way and you calculate apparent field from a vertex point of the triangle, the starting point is at the exit pupil of the eyepiece,

so the distance from the top surface of the eyepiece to the exit pupil has to be subtracted from the eyepiece-to-wall distance.

So if you got 70.84° from the top of the eyepiece, that means the actual apparent field was wider because the point of the triangle was several mm out from the eyepiece's eye lens.

You could find that point by holding a surface up to the light by the eyepiece and moving it back until the light was the smallest diameter.

That is the point you measure from.

This illustrates what I mean:

https://www.cloudyni...view/?p=7958975

Hi Don, I had  seen that topic quite some time ago. You'll be happy to know that I did measure from the exit pupil to the screen. It was actually about a half inch from the eyecup to the exit pupil. (I wasn't concerned about eye relief so I didn't measure that) Since my calculation was less than 1° off, I'm confident that it's actually a 70° eyepiece. I did cheat a little by using this online calculator.  I'm actually a sheet metal layout specialist. I use geometry every day at work. I always get a kick out it when I see someone post a comment about how they made it through another day without using pi R squared and I have to respond "I used it yesterday." Even so, I like to use calculators when they're handy.

 

https://www.omnicalc.../triangle-angle

 

On another note, did you see the picture of lens stack? I looked at several eyepiece design charts and didn't see one that matched the configuration of this eyepiece. I've seen a chart that I can't find again which shows about every eyepiece configuration you can imagine. I think I remember that it had a blue background. I thought I'd saved it on one of these computers somewhere but I can't find it. I'm not sure if you would describe this one as a 1-2-3 or a 3-2-1 but it is a 6 element in 3 groups.

[Starman1]

The design chart you saw was one of these:

https://www.cloudyni...tion/?p=8421832

You might find this useful, too:

https://www.telescop...ce_raytrace.htm

It is a VERY long page.

[Ric Whatley]

The design chart you saw was one of these:

https://www.cloudyni...tion/?p=8421832

You might find this useful, too:

https://www.telescop...ce_raytrace.htm

It is a VERY long page.

That one with the blue background was the one I was looking for, it didn't help. the closest design is the Takahashi SW aspheric (like the Meade SWAs) but it's still not quite the same. I had even looked through the whole page of designs on that telescope-optics.net pageand still no match.

Edited by Ric Whatley, 08 February 2025 - 04:43 PM.

[Ric Whatley]

Collimated my scope last night. Got up at 4:00 this morning to catch some dark sky with stars and look through the eyepiece.......stupid clouds. 3 nights in a row now, cloudy, moon creepin', stupid job interfering. It's a conspiracy.

Edited by Ric Whatley, 08 February 2025 - 04:54 PM.

[AJW]

What is the significance of a "Moonfish"?

[Ric Whatley]

What is the significance of a "Moonfish"

Hi, is this Al? Moonfish is a brand name. They have other eyepieces that are well documented but this one is obscure and there's very little informatation about it. The only reason I believe it's a Moonfish is because of a you tube video review done years ago that has an eyepiece that looks just like it. Measurements of the eyepiece have so far ruled out speculations about it being a rebranded eyepiece from another line. It might well end up being a copy of another eyepiece but so far no one's suggested an eyepiece that's just like it. At a glance it looks very much like a Meade 4000 32mm SWA but the optical design and dimentions are different.

[Ric Whatley]

Everyone following this discussion will be glad to know that I was finally able to test the eyepiece in a telescope. I tested it in an f4.6, 10" Meade Starfinder. The closest eyepieces that I had to compare it to are my Meade 4000 SWAs. I had the 32mm with eyecups and a smoothie. I also had my Explore Scientific 100° 25mm and a couple of QX EPs (30 & 36mm). I decided to focus on the Pleiades since at 35x it would probably make a good target.  The moonfish did show a slight increase in apparent FOV and true FOV  but the coma was very close to the same ammount shown by the Meade SWAs. Even though they have different optical designs I could tell very little difference in the point at which the stars started to elongate. Because of the incresed apparent FOV (I think 71° to 68°) the Moonfish may have had a slightly larger usable field over the Meade but if so it was very slight. The QX EPs were not close and have no business being in a fast scope. The ES 100° 25mm was like comparing apples to watermelons and I quickly just put it back in the case. I also observed Orion with and without filters and came to the same conclusion that it's performance is very close to the Meade 4000 SWA. I hope to make it to the next observing session at our clubs observatory. I'll see if I can get some more comparisons from my club members against some of their other eyepieces and in some different scopes. I'm sure the SCT guys will love it. I'll probably end up combining it with the EPs I have in a case just for slower scopes. For now I'm just going to say it was at least worth the $37.50 + shipping that it cost me. I'd still be interested in finding out if there's an eyepiece out there just like it or if it was designed and produced all by it's self.