December 6, 2014: Upgrading the Mak, Part 2 – finderscope, high-powered eyepieces

The Mak comes with what’s called a red-dot finder.  This is a small – and inexpensive – device that projects a small red dot onto a clear flat surface through which you can see the sky behind it.  You use the RDF to align what you see in the sky with what’s seen in the telescope.  It’s not hard to do.  You line up a faraway object – in my case, the top floors of the Chrysler Building – in the RDF.  Then you look through the main scope with your lowest power EP and get the building in the exact same location by moving the OTA slightly.  Finally, you adjust the RDF slightly with little controls on it that move the RDF slightly up or down, right or left, to get the RDF to match the view in the OTA.

You use the RDF when you start the evening’s observing, to get the scope pointing to the stars that you’re going to align the computer with.  After that, you basically don’t need it any more, and you can switch off the red dot.

There are two problems with the RDF.  One is that it’s what’s called a “unity finder” – this means that when you look through it, it only shows the sky just as you would see it, no magnification, no increase in light gathering.  In fact, unfortunately, because it’s cheap, it actually dims the sky slightly.  Depending on the atmosphere conditions, you might have some trouble seeing even the brightest first magnitude stars through it.  As a result, it is sometimes difficult to use, and although the alignment can be pretty accurate, the lack of magnification puts a limit on how exact that alignment can be,

The second is that you have to look straight through it.  When you look through the OTA, you have a diagonal that changes the light path so that you’re basically looking down to look into the telescope.  However, when the telescope tube is raised up at the stars, so is the RDF.  That means you have to crane your neck upwards at an uncomfortable angle to look up through it.  In my case, since the scope is low to the ground in the first place, that means I literally have to get down on my hands and knees to look through.  To misquote Chris Rock (NSFW), there’s finders and and there’s RDFs, and RDFs have got to go!

The alternative is a finderscope.  This is a little telescope that mounts onto the main OTA, right where the RDF was.  Not only does it gives you a magnified view, typically 6, 8, or 9 times, but it also collects more light than an RDF (which collects none), allowing you to see dimmer stars so as to be able to see more of what’s going on in the sky as you move around to point the scope to the area of the object you’re looking for.

To a large extent, a finder isn’t strictly necessary on my scope, because the goto puts me on target.  The only reason for getting a finder would be convenience in terms of not having to get down and strain myself every time I align (or realign) the scope.  It would be nice not to have the annoyance.  But, like I said, you use it for a couple of minutes at the beginning of the night, and then that’s it.  Then again, I’m middle-aged, and these knees aren’t getting any younger.

Normally, the finder is either an 8×50, like this one, which costs about $60, or a 9×50, like this one, which costs about $76.  Just like with binoculars, the first number refers to the magnification, and the second represents the aperture in millimeters.  If I were to get one, I’d rather get the 9×50, because there’s something called the exit pupil – that’s the size of the shaft of light that comes out of the eyepiece, whether it’s the eyepiece of binoculars, finderscope, or a telescope.  You get the size of that exit pupil by dividing the aperture by the magnification.

This exit pupil matters because the pupil of your eye can only open so far.  When you’re young, it opens up to 7mm; but as you age, the muscles in the eye get less flexible.  In your 20s and 30s, it opens to about 6mm, and in your 40s and 50s, it opens only to 5mm.  Doing the math, the exit pupil in the 8×50 is 6.25mm wide; but my middle-aged pupils won’t open nearly that wide.  That means that the light from the finder EP is spilling out into my eye’s iris – the colored part of the eye surrounding the pupil.  That light is wasted, so that what I’m seeing in the finder is dimmer than it ought to be.

However, with a 9×50 finder, the exit pupil is 5.6mm, and that’s a better fit to my eyes.  But $90!  That’s a lotta bread, especially for something I only use for the first couple of minutes a night.

What if I could turn this to an advantage?  The Mak is notorious for having a limited field of view – mine has a field of view of just over one degree; although there is a six-inch Mak that has a 2-inch focuser that has a maximum field of view of about 1.6 degrees.  As I mentioned in my December 4 post, there are only a few astronomical objects that won’t fit in my field of view.  There’s a list of the largest objects here, and there are about a dozen or so that won’t fit.  But I wouldn’t be seeing most of those nebula from my rooftop in Manhattan anyway.

But there are a couple that I would like to see all at once, like the Pleiades (M45), the Lagoon Nebula (M8), and the Ptolemy Cluster (M7).  The Pleiades are shaped similar to a dipper, but I can only just get the bowl in one view.  I haven’t looked at the other two, as they’re summer objects, and I only got the scope in November.  They’re also in the southern sky, which is the most light polluted area for me.

Instead of getting just a finder, it might be fun to get an actual small refractor and mount that onto the Mak.  But where would I find such a refractor?   Why am I asking you?

In my internet travels, I came across the Celestron Travel Scope 70.  A “regular” 70mm refractor which was selling for just 50 bucks delivered to my door.  This little scope would be a lot of fun to have.  It uses the same 1 1/4 inch EPs, it has an incredibly wide field of view because it’s got an extremely short focal length of just 400mm, so it would fit on my scope pretty easily, although it would hang off the back some.  It has gotten decent reviews.  And with these mounting rings, I would be able to easily attach it to my scope for another $54.

But that sticks in my craw, that I’m paying more for the stinking metal rings than a finely tuned optical instrument.  And actually, the TS 70 isn’t such a finely tuned optical instrument after all.  I said it got decent reviews.  But it’s a cheap scope.  It’s okay for what it’s supposed to do, which is to be a scope you can throw in a bag and take with you.  The optics aren’t great – they’re not fully multi-coated, and they start to “pant” when they go past 30x.  That’s okay, but I wanted a scope that I could easily take to at least 50x, which is about the lowest my Mak will go, and even higher would be a nicer.

Enter the Orion Short Tube 80.  Now this is a quality scope, not some toy plastic piece of junk.  This one not only has decent reviews, but is well-reviewed.  Fully multi-coated.  This one doesn’t pant at 30x or even 50x.  And the increase in aperture increases not only the amount of light collected, allowing you to see more, but also the detail, also allowing you to see more.  It’s $120 new for just the OTA and nothing else, or $200 with the OTA, a diagonal, a finder, and a couple of EPs.  But it does come up for sale every other month or so used in the Cloudy Night Classifieds for about 90 bucks or so – sometimes less.  And then I wouldn’t feel so bad about spending $54 to mount a $120 scope.

But obviously, although this is one heckuva good idea, I don’t have money to do this right now.  Or do I?

This gets us back to eyepieces.   I have my low-power and mid-power EPs locked down with the 32mm and the 15mm.  I don’t need anything else in that range.  What about high-power?  Yes, I have both of the 9s, as I refer to my 9.7mm and 9mm eyepieces.  But they seem to sometimes be a bit soft in the focus.  Oh, sometimes they’re nice and sharp, especially on the moon.  But I want to upgrade from a mere run-of-the-mill Plossl, the Meade, to something better.

When what should catch my eye on eBay but an 8mm TeleVue Plossl.  TeleVue, for those who don’t know, is the optical top of the heap, the cream of the crop, the pinnacle of perfection in EPs.  The green lettering on the side of a TeleVue EP is lusted after, and is the object of envy.  And it had a bid of only $49, almost half off the new price!  Oh, hells yeah, I’m putting in a bid on this.  Well, I won the bid for $52 a few days ago, and the EP is on its way.  The 8mm will give me 193x, a nice high power that is still within the limits of the atmosphere on all but the worst “seeing” nights.  And the TeleVue will let me see the difference between a good Plossl and an outstanding one.

Just a few more days!  I can’t wait!  Unfortunately, the Astronomer’s Curse will be fully active when the TeleVue comes in.  It is expected to be cloudy until the middle of next week.

This only leaves ridiculously high-powered EPs, which I’ll cover in Part 3, after I have a chance to test out the TeleVue.


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