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DSC-60: Tackling Meissa with a 60mm SCT!

This is a DSC-60 Project observation –  for project details go here.

from Double Star Club list

[  ] 22 Lamda Orionis 05h 35m.1 +09° 56′ 3.6, 5.5 4.4″ 43°

from John’s post:

Meissa (Lambda [λ] Orionis)
RA: 5h 35.1m  Dec: +09° 56′
Magnitudes –  A: 3.5     B: 5.5     C: 10.7     D: 9.6
Separation –   AB: 4.9″      AC: 28.7″      AD: 78.0″
Postion Angle –   AB: 50°      AC: 185°       AD: 272°
Distance:  1056 LY
Spectral Classification –  A: O8  B: BO.5

So you didn’t know they made a 60mm Schmidt Cassegrain? Niether did I.  But if you have one of the ubiquitous 8-inch SCTs – hmmm, come to think of it, I have three –  you can make an off-axis mask for it and turn it instantly into a color-perfect, 60mm scope. The cost can be close to nothing, the time about half an hour, and, of course, you do no permanent injury to the scope – the mask is something you put on and take off like a lens cap. It just cuts the scope down to 60mm.  I’ll do a separate post on why and how, but the object here is to see how this performs on Meissa a really wonderful star in Orion’s head.

I just got out at 5 am this morning as astronomical twilight began and quickly swung my “parked” SCT  (it stays set-up in a little observatory) to Meissa. What a sight this is. I didn’t even try to split it at first. I just enjoyed the wide field view. And I did this without the mask on the SCT so I was picking up most of that wonderful pattern of stars that make’s up Orion’s head and is part of an obscure open cluster. (See John’s post for details and charts. )

I was using a 24mm Panoptic which yields 83X and roughly a  49-minute field of view.  I wanted more, but serious twilight was closing in and a wider field eyepiece was 100 feet away in the house, so I stuck with the 24mm Pan, absorbed the beauty of the scene, then popped in a 16mm Nagler. Yep. I could see some signs of a split, but the stars were pretty fiery. So I quickly put on the off-axis aperture mask. (The off-axis part is to avoid the central obstruction.)

Voila! I now had 60mm of clear, unobstructed, color-free aperture. And what a difference. Meissa and her brightest companion  – yes, we’re only going after the brightest companion because that’s what the DSC list calls for – and that’s what is easily in reach of 60mm of aperture.  (I stress “easily” because John is always pulling rabbits out of the hat with his 60mm scopes – secondary,  tertiary and whatever you call the fourth star in a multiple just tumble out of his scopes like clowns out of a tiny car in the circus.  Not me. I stick with the easy stuff, by and large. I don’t have John’s eyes,really dark skies, endless patience and observing skills.)

Anyway – these are the sights I love. I just wanted to sit there an absorb it.  A large, white dot tinged with blue, and next to it a fainter, smaller, violet dot. Simply lovely. I mean these were the kind of perfect, round, well-behaved 60mm stars that we thrive on. Such order has a special ascethic of its own. Wonder if I can split it witht he 24mm? Out came the 16mm (125X) and back in went the 24 Pan – and yes, sure enough,there they were. Absolutely exquisite in their delicacy.

And at higher power? Well with an 11mm Nagler (182X) we had a big old honking split.  Yes, skies were steady! And yes, this is too darned much power for a 60mm if you follow the rules, as I generally do, and limit yourself to 60X per inch (2.5X per millimeter)  then 144X should be tops. But the heck with the rules, what about the 9mm Nagler – 222X?  Yes! We certainly have lost light – and eye position becomes absolutely critical, but we still have a perfect pair of stars.

That eye position business is interesting. John and I both became acutely aware of it when we first experimented with masking. The reason for it is simple. The higher the power, the smaller the exit pupil – the cone of light exiting the scope – and so of your eye isn’t in perfect position you don’t see anything.  There’s a wonderful eyepiece calcualtor on the Televue web site which I use when I want data like this and going there I learned that the 16mm Nagler gave me an exit pupil of about half a millimeter with the scope masked  which is right at the limit of what any sensible person recommends. (Hey, that’s 125X and pushing real near to the 60X per inch limit. ) Televue recommends no more than 2.5X per mm which would put the top at 150X and the 13mm Nagler – next in my case – delivers 154X and that, for Televue, is too much. And keep in mind, these folks are in the business of selling us eyepieces – so i put a lot of stock in their recommendations wwhen they start telling me NOT to use certain eyepieces they make.

But . .. who can resist trying? So the 11mm delivered an exit pupil of .33mm and the 9mm of .27mm – ridiculously small cone’s of light. No wonder eye position was absolutely critical.  But this is of more than academic interest. I learned something from it this morning. Eye position is pretty darned critical when you are using full aperture, too!

Now it’s interesting, because you don’t hit the wall – half a millimeter exit pupil – until you put in a 5mm eyepiece into your  8-inch scope – I’m talking unmasked now. WIth 200mm of aperture the numbers change. Using the 2.5X per mm rule that Televue applies you should be able to use 500X. But Televue has another rule that cuts in before then – they recommend 350X as the maximum “regardless of aperture.”  Of course they’ll sell you eyepieces that will take you higher – at least on the typical 8-inch SCT –  but they won’t recommend you use them to deliver such power.

But here’s what I found as I went back to viewing Meissa unmasked: I could see perfect stars  in the 8-inch SCT this morning – conditions were very good – but only if I was very careful to get my eye centered perfectly over the eyepiece. I’m sure someone will tell me that’s because I need to be on the axis of the light cone, or something like that – and this is hardly an entirely new revelation. But quite honestly, before fooling around with masking I was more likely to assume that  when stars  misbehaved it was because:

a – I didn’t have the focus correct

b – seeing was too poor to deliver a sharp image

To these two obvious thing I now have to add eye position, which with my Naglers, at least , is critical. I could really get quite an attarctive  split this morning with quiet , well-behaved stars, at several different powers and full aperture IF I was very careful about eye position.  Not all that easy to do.  I always observe sitting down, of course, but to position your eye correctly you need to really be just at the right height above the eyepiece and  in this case I used a hand to form a brige between the eyepiece  and my face to help steady it. (OK – I’m 70 -maybe younger folks would find this easier.)

If I don’t do this, the view unmasked is pretty if you like dancing, flaming stars.  In between the flaring you do see the split – and the additional aperture does show you the fainter companions John describes – and with the wide field provided by the 24 Pan and bright stars provided by eight inches of light grasp, Meissa really is beautiful. But I am also sure that if I didn’t have such great seeing conditions I would not have seen it that way – the 60mm would have provided an improvement for seeing Meissa A and B as clean, steady, dots. (One  other possibility – the scope I was using is a Meade LT-8 ACF – the Advanced Coma Free variety.  Perhaps that contributed to the view – I just don’t know. It would have been fun to have it go head to head against one of my older SCTs without the advanced design.)

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One Response

  1. I always thought a mask was something you wore on Halloween or on the way to a bank robbery, but once I became interested in astronomy, I found there were other uses for those little articles of clothing.

    And they do work, too — although some would argue otherwise, but that’s a debate for a later time. But in the case of a masked eight inch SCT, it’s downright uncanny how similar the views are to an f/16.7 60mm refractor. In fact, Greg and I did the math, and the masked version of the SCT at full focal length (normally 2000mm) works out to f/33.3. I was using a focal reducer on my C8, which reduced the focal length to 1260mm, giving an f/21 view. And that was plenty long enough for my eyes — I didn’t try it at f/33.3 like Greg did. I’ve been on the lookout for an SPI 60mm f/20 refractor, which are rather rare, but after this experience, I think I’ll just stop with the f/16.7 60mm I’ve got.

    At any rate, the views in my masked black bandit were a real treat. Just as Greg describes it, all the flaring and dancing of brighter stars is totally trimmed from the view. I sighted mine in on Polaris the first night, and I was treated to a lovely dot of round light with very sharply defined edges — and that beady little secondary was there, too, but I had to look very closely to see it, which duplicates my experience in 60mm refractors.

    I never would have done it if Greg hadn’t put me up to it — and I’m glad he did. Give it a try! It’s a real eye-opener — even with a miserly exit pupil of .27mm.

    Oh, and for future reference, the fourth star in a multiple star system is called a Fourple-ary — that comes from an obscure source entitled John’s Little Book of Deservedly Obscure Star Splitting Terminology, which is no longer in print. In fact, it never even made it into print. 8)

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