Archive for October 3rd, 2020


Unboxing fest, part 2: Celestron f/6.3 focal reducer-corrector

October 3, 2020

Even before I had decided to get the NexStar 8SE, I knew that if I got a big SCT, I’d want a focal reducer-corrector for it. SCTs and Maks have secondary mirrors, which partially obstruct incoming light. This implies a necessary tradeoff: make the central obstruction small, which results in a long focal ratio–typically f/10 to f/15 for commercially-available SCTs and Maks–or get a shorter focal ratio by using a larger secondary mirror, which blocks more light and degrades the contrast.

There is a third solution, which is to use a focal reducer to make a steeper light cone and a shorter focal ratio. Astrophotographers use these all the time to making their scopes optically ‘faster’ so they can get brighter images with shorter exposure times. For SCTs they are useful because they bring the focal length of the scope down to something more reasonable, and increase the true field of view. As shipped, the Celestron C8 OTA has a focal length of 2032mm, so even a 32mm Plossl gives 63.5x, and the included 25mm Plossl gives 81x. The max true field of view with 1.25″ eyepieces is about 0.7 degrees, which can be a little claustrophobic. You can juuust fit the Double Cluster in a 32mm Plossl, as long as you don’t mind clipping the edges of both clusters. You can get M81 and M82 at the same time, as long as you don’t mind parking them on the extreme edges of the FOV.

The Celestron f/6.3 reducer-corrector turns the C8 from an f/10 to an f/6.3, with an effective focal length of 1280mm, which is pretty close to the mid-sized commercial Dobsonians–the XT6, XT8, and XT10 and equivalent models from other companies are all 1200mm scopes. That pushes the max true field to something like 1.2 or 1.3 degrees, which is a big jump over the native 0.7 or so. That’s enough to put some of the larger celestial objects, like the Pleiades and M44, back in a single field of view, although the Pleiades will be cutting it mighty fine (I will test this and report back!).

As the name implies, this unit is not just a focal reducer, it’s also a corrector, which makes the field flatter for better image correction across the entire field of view. In other words, stars at the edge of the field should still look round with this thing in, as opposed to oblong. This isn’t a factor for me–I don’t photograph stars, and the stars at the edge of the field in the C8 haven’t bothered me so far–but it’s nice to have if either of those things change.

Here’s the gizmo, out of its (unsealed) bag, with its custom dust caps. I got mine from Astronomics (here), and evidently it was one of the last they had, because the availability listing there says “More on the way”. Amazon has it, too (link), I just like supporting brick-and-mortar telescope stores and specialized astro-gear dealers whenever I can.

Why am I bothering to do an elaborate unboxing post for such a trivial piece of gear? Mostly because it took a lot of digging for me to figure out how one was used. What I wanted was a photo essay that showed how “this part goes here, that part goes there”. When I couldn’t find one, I decided to create my own.

Here’s how the NexStar 8SE OTA comes as shipped, with a 1.25″ visual back on the rear port. When I was first getting into this, it took me forever to understand what a “visual back” was. Did it mean anything other than the bit where you stick the eyepiece or diagonal? And if not, doesn’t every telescope have one?

The answer is that not everyone sticks an eyepiece or diagonal at the back of a scope. Some people put a camera, or a spectrograph, or who knows what else. So if you are going to use the scope visually, you need a doohickey that holds the eyepiece or diagonal, and that’s the “visual back”. It was just new to me because non-SCTs aren’t generally described as having a “visual back” (some high-end astrograph refractors excluded), they just have focusers.

ANYWAY, the focal reducer-corrector threads onto the rear port of the scope, after the visual back has been removed.

Then the visual back screws into the focal reducer.

Then you put eyepieces or diagonals into the visual back, as usual. Everything is just scooted back 3/4 of an inch or so by having the focal reducer interposed between the scope and the visual back.

Incidentally, this is something to keep an eye on for NexStar users: when the scope is pointing high, the diagonal already comes pretty close to the base of the mount, depending on how far forward the OTA is scooted in the dovetail clamp. I’ll have to mess around with the system, but I might have to start mounting the scope a smidge farther forward to make sure the diagonal clears the mount when observing up high.

And here’s where the last post and this post come together: the Baader Hyperion 8-24mm clickstop zoom eyepiece, riding in an Astro-Tech dielectric diagonal, inserted in the stock 1.25″ visual back that came with the C8, screwed into the f/6.3 focal reducer-corrector. All saddled up and ready to go (er, minus the finders). Now all I need is for night to fall.

I have one more piece of gear to write about, but it hasn’t come in yet, so the next post might be an observing report–hopefully, a first-light report on the focal reducer and the Baader zoom. Stay tuned.


Unboxing fest, part 1: Baader Hyperion 8-24mm clickstop zoom eyepiece

October 3, 2020

It has been a long, long time since I got a new eyepiece. Three and a half years since I got my Edmund 28mm RKE, and just under five years since I got my Meade 5mm MWA, which was the last “premium” eyepiece I picked up (the 28mm RKE is one of my all-time favorites and most-used eyepieces, but at $85 I don’t think it counts as “premium”–for however much or little that is worth!).

I have been wanting to try this Baader zoom for years and years. It has an almost unbelievably positive reputation on Cloudy Nights, where at least some observers report selling off a lot of their premium fixed-focal-length eyepieces after acquiring the Baader zoom.

Does it really live up to the hype? Thanks to a bolt of consulting money coming in last week, I’m about to find out.

I found it in stock at Woodland Hills Camera and Telescope, and one arrived on my doorstep a couple of days later, extremely well-packed. Since I ordered mine, Woodland Hills seems to have run out of the standalone eyepiece, although they still offer it bundled with the matching 2.25x Baader barlow, which I skipped, for a little more dough (here). Amazon has it in stock, but for a little more than most other outlets–$325 right now, versus $289 or so most other places (here).

There’s no instruction book inside the box–the box itself is the parts inventory and instruction manual. I really dig this. I think every other eyepiece I’ve ever owned has come in an unadorned box, some of them quite fancy and nice enough to repurpose as gift boxes for birthdays and holidays (looking at you, Explore Scientific). This box is functional. Every surface is illustrated, and most of the sides show you how to use the eyepiece in various situations.

Even when you open the box, there are more instructions on the newly-revealed surface! Who does that? As a catalog-junkie who likes looking a photos of gear, this box ticks all my, er, boxes.

Now this is kinda awesome, and kinda hilarious. Instead of coming in a bolt case, like my two Bresser eyepieces; or clamshell foam, like all of my Explore Scientifics; or a sturdy plastic can with a screw-on lid, like the Celestron 8-24mm zoom; or just dust caps inside the cardboard box, like all of my Orion eyepieces, the Baader Hyperion 8-24mm clickstop zoom comes in a rugged nylon pouch with a velcro closure and–as you can see in the inventory photo on the side of the box–a belt strap. And it’s high-visability yellow so you won’t lose it.

This is pretty cool, and definitely functional, and also amuses the heck out of me. This is the kind of case I expect for a multi-tool, or a handheld GPS unit, or some other thing you might take hiking, not a telescope eyepiece. I guess if you ever wanted to bushwhack across Utah or Patagonia with a high-end telescope eyepiece on your hip, or clipped to the side of your pack, now you are prepared. I imagine I’ll keep mine in my eyepiece case, like any normal person.

Here are the eyepiece and the extra bits, out of their respective bags. The four things in the extra bag are (clockwise from top) the winged rubber eyecup, the fold-down eyecup, the spotting scope adapter, and the dust cap that goes with the fold-down eyecup. The eyepiece itself comes in the bag with the larger dust cap on top of the adjustable eyecup, which rotates up and down, and with the 2″ adapter and its dustcap on the bottom end.

I should mention right here that both plastic bags were open, not sealed, which is good, because it prevents dew formation. The one for the accessories had one of the little silica gel packs, which I always steal to throw in with my meteorites. (Note to self: blog about new meteorite storage system soon.)

Pull the dustcap off the 2″ adapter and you find inside the smaller dustcap on the 1.25″ adapter. You could use the eyepiece in a 2″ focuser or diagonal with both the 1.25″ and 2″ adapters in place, and I’m not sure if there’s any benefit to taking the 1.25″ adapter off.

Here’s the adjustable eyecup in the down position…

…and twisted up. I’m a fan of these things–I notice that civilians are much less likely to stab the eye lens with a greasy finger* if there’s something in the way. Plus it’s nice to be able to signal to people where they should put their eyes, something that can be surprisingly tricky with eyepieces that have long eye relief.

* I’m not mocking n00bs or people who come to public star parties. To astronomical optical surfaces, all fingers are greasy.

Here’s the Baader compared to my Celestron 8-24mm zoom. The Celestron has been in service for a few years and eventually the knurled rubber wrap around the middle of the eyepiece split and fell off, which doesn’t impact the function whatsoever.

I like the Celestron a lot. It’s been one of my most-used eyepieces, and along with a 32mm Plossl it is hands down the eyepiece that I’ve recommended the most. Long-time readers may recall that when I flew to Texas in 2017 for the Three Rivers Messier Marathon, the only eyepieces I took were the 28mm RKE and this Celestron zoom, and they proved to be all the eyepieces I needed.

In fact, it was my positive experiences with the Celestron zoom that convinced me to venture the money for the Baader zoom. I rarely splurge like that for a piece of gear in a class I’ve never tried. In normal times, the Celestron zoom runs $65-$75. With the temporary shortage of astronomy gear because of increased demand during the pandemic, the price has spiked a bit, at least at some retailers. I see it going for just over $90 at Amazon (link).

If I have one knock on the Celestron zoom, it’s that it is ever-so-slightly less sharp than the best of my fixed-focal-length eyepieces. The difference is subtle–I would not have been able to spot it in my first 3-4 years of observing, and even now it takes a determined effort for me to see it–but it’s there. (As always, your eyes, your experience level, and your sample of this piece of equipment might vary from mine, and probably do.) Which is another reason I wanted to try the Baader, with its sterling reputation for clarity and sharpness. If it lives up to that, I will be using it a lot.

I haven’t pulled out a ruler, but the eye lenses of the two zooms look about the same size.

My kitchen scale says the Baader zoom weighs 10.4 oz, or 296 grams.

And the Celestron zoom weighs about 3/4 as much, 7.7 oz or 218 grams. I was pleasantly surprised by the lightness of the Baader; at least to me, it looks like it might weigh a little more.

With any luck, I’ll have a first-light report along soon. And there’s more unboxing coming soon–stay tuned!