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GoScope 80 Eyepieces

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#1 Tony Flanders

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Posted 17 June 2019 - 06:00 AM

A week from tomorrow I fly to Argentina to view the July 2 solar eclipse. While I'm there, I also intend to make a full pass over all the bright objects in the far-southern sky with my Orion GoScope 80 TableTop Refractor. Instead of using the native tabletop mount, I will bring a Bogen 3001 tripod with a 484 RC2 ballhead, as shown in my post GoScope 80 vs. TeleVue Ranger. In fact I chose the GoScope precisely because it works extraordinarily well with that tripod and head -- which also work extraordinarily well with my camera. Using the same head for telescope and camera saves a lot of weight and hassle!

 

In case you're wondering, I didn't end up bringing the GoScope to Hawai'i as I originally discussed in that note. The weather forecast just before I flew looked very ominous, and I doubted I would make much if any use of the telescope. And in fact, I arrived on the Big Island just in time for Hawai'i to experience some of the wettest weather on record. It was very nice to wake up in the morning with a view of fresh snow on Mauna Kea -- but not conducive to astronomy. And Kaua'i, which we visited later, broke the all-time record for any U.S. weather station, with 47 inches of rain in one 24-hour period. I did have some opportunities to view the night sky regardless, but binoculars were just fine for the few clear hours that I got.

 

I am hoping for better in Argentina and Chile, which boast some of the driest and least cloudy areas in the world. But I'm still keeping my fingers crossed ...

 

Anyway, the biggest problem with the GoScope is eyepieces. Its ultra-fast f/4.4 focal ratio makes it really challenging to obtain reasonably high magnifications. And neither the GoScope's plastic diagonal nor the ultralight mount takes kindly to eyepieces that are big and/or heavy. My usual solution to fast scopes is to use a Barlow, but a Barlow plus any but the lightest eyepiece places an inordinate strain on the system.

 

Note that the native tabletop mount has much the same constraints. It works quite well within its limitations, but when the entire optical tube, complete with diagonal and focuser, weighs 2.5 pounds, it doesn't take much to get it wildly out of balance!

 

My final decision is as follows:

 

Explore Scientific 26-mm 62-degree

Vixen Lanthanum 8-24 zoom

Vixen Lanthanum 4-mm

 

The ES 26 turns the GoScope into a delightful 13.5x80 monocular with a 4.6-degree field of view -- perfect for sweeping the Milky Way. This somewhat overlaps the capabilities of my image-stabilized 10x30 binoculars, which I will also bring, but with a tad more magnification and vastly more aperture.

 

I settled on the ES 26 because of its long eye relief, together with the fact that it shows the entire field theoretically possible with a 1.25-inch aperture. Oh yes, and also because it's relatively small and light. The outer part of the field isn't terribly well corrected, but that doesn't really matter because the dominant aberration in this setup is field curvature. No matter what eyepiece I use, it's impossible to see the edge well when the center is in sharp focus.

 

I tried both the Baader Hyperion and Celestron 8-24 zooms as alternatives to my ancient Vixen Lanthanum zoom. Although the Hyperion is the clear winner both in field of view and optical quality, I reluctantly disqualified it because it requires an inordinate amount of in-focus. That wouldn't be an issue with a high-quality focuser, but it requires an inordinate number of twists to the GoScope's effective but peculiar thread-in focuser. And it makes switching from the zoom at 8 mm to the LV 4 pretty challenging, since the latter typically shows no stars whatsoever when it's grossly out of focus.

 

I waffled between the Celestron and LV zooms. Optically, I couldn't tell any difference between them; they must have nearly identical designs. Mechanically, the Celestron is better. But the LV is closer to parfocal with my other eyepieces, so it ultimately got the nod.

 

As for the Vixen Lanthanum 4, it's sat mostly unused in my eyepiece collection for many years. It's not a great eyepiece -- just a 45-degre apparent field of view, and perhaps a bit on the dark side. But it's almost perfectly parfocal with the LV zoom at 8 mm, and it has ample eye relief. So I can use all the eyepieces both with and without eyeglasses, which is great when you want to share views with other people. And unlike the overwhelming majority of long-eye-relief, short-focal-length eyepieces, it is small and light enough to work well in the GoScope.

 

I also borrowed a friend's Nagler 3-6 zoom, which in many ways would be even better than the LV 4. The zoom at 6 mm gives 58X with a 1.36-mm exit pupil, which is well-nigh ideal for deep-sky observing. And it gives 117X at 3 mm, which is approaching the telescope's top useful magnification. I can see a lot through the GoScope using the LV 4 for 88X, but 117X is definitely superior for viewing the planets and splitting tight doubles.

 

The downside of the 3-6 zoom is its short eye relief, which changes its effective field of view from a nominal 50 degrees down to more like 40 degrees for me even when I'm not wearing glasses. Even if the Nagler zoom at 6 mm does give better deep-sky views than my LV zoom at 8 mm, it does so with a much smaller true field of view. And although the optics of the GoScope hold up nicely to the Nagler zoom at 3 mm, the same cannot be said for the mount. It's pretty tricky to microtune the direction of the scope at 117X either with the Bogen 484 ballhead or with the native tabletop mount.

 

And, of course, the Nagler 3-6 zoom costs a small fortune -- more than the entire scope, tripod, and head combined with any one of the other eyepieces. And since I will be traveling around Argentina and Chile largely by bus, I'd just as soon not be carrying any more expensive equipment than necessary.

 

We'll see how it all goes; I definitely have high hopes!


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#2 Crow Haven

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Posted 17 June 2019 - 11:04 AM

Have a wonderful trip! waytogo.gif


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#3 Jon Isaacs

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Posted 17 June 2019 - 08:21 PM

Tony:

 

Interesting and thought provoking. It stresses the importance of everything working together. You didn't choose the best set of individual eyepieces but the set that works the best together.

 

Did you consider the possibility of parfocalizing them?

 

Have a good trip and be safe.

 

Jon


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#4 Tony Flanders

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Posted 18 June 2019 - 05:53 AM

Interesting and thought provoking. It stresses the importance of everything working together. You didn't choose the best set of individual eyepieces but the set that works the best together.


Yes, I have devoted pretty much all the clear hours over the last couple of months (which isn't many!) to trying different combinations, and seeing how they work together.
 

Did you consider the possibility of parfocalizing them?


No, I didn't. Excellent idea, but for now I think I'll leave well enough alone.

One hidden agenda in taking those eyepieces, by the way, is that if I find a deserving taker, I may give away the scope and at least one eyepiece before coming home. So I don't want to take anything that I will really regret giving away.


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#5 Jon Isaacs

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Posted 18 June 2019 - 07:11 AM

No, I didn't. Excellent idea, but for now I think I'll leave well enough alone.

 

One hidden agenda in taking those eyepieces, by the way, is that if I find a deserving taker, I may give away the scope and at least one eyepiece before coming home. So I don't want to take anything that I will really regret giving away.

 

 

Tony:

 

I like that... 

 

The taker could also be undeserving. (Thief). And there is also a real chance that something gets lost or damaged. But parfocalizing would have given you more latitude in your choices.  In any event, the die is cast and I am sure you have other last minute tasks to concentrate on.

 

Personally, I would probably have taken my decloaked 24m Meade SWA, the Celestron Zoom and a 2x Celestron/ Shorty barlow.  The Meade is a nice eyepiece but kind of thrashed and if I gave it to someone, I could justify replacing it.  :)

 

Jon 




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