Posts Made By: Mike Hosea

How do you determine whether their position is really depending on the condition "provided someone else goes..."? I mean, I agree with you as far as you went, but unless they admit that they're only for it as long as they don't have to go, otherwise they would be against it, how do you know enough to be annoyed at them?
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Mike

The biggest con is that you have to be able to achieve precise collimation once without it in order to use it. The biggest pro is that you can then recollimate very quickly, when and where you choose.

For a backyard scope, I don't see the point. If you often transport over bumpy roads or something and arrive in the field in miscollimated condition, I think it might be a great help.
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Mike

The eye relief on a 1.25" 40mm Plossl is too long. That's what causes the problem. This is not really a 1.25" versus 2" problem, but I think with long focal lengths and long eye relief, smaller apparent fields of view are that much harder to use. I.e., for ease of use, it is better to have 32mm focal length and a 50 degree apparent field than a 40mm focal length and a smaller apparent field. I do think that in the 1.25" size, 32mm is as long a focal length as I would want. Many people seem to like the 30mm Ultima better than 32mm Plossls.
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Mike

Never heard that one before. However, eye strain can result from focusing error. To prevent that, Avoid "rocking" the focus back and forth when looking for the sharpest focus. Try to home in on the best focus from the outside, i.e. rack the focuser out and then slowly rack it in until the image is in sharp focus. This prevents your eye from snapping the image into focus when the telescope is, in fact, not focused, in which case it is like looking at an object close to your face--hence the strain. Hope that helps.
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Mike

A Barlow has a focal length, and because the lens has negative power, the focal length is a negative number. The magnification factor given by a Barlow depends on the distance from the Barlow lenses to the field stop of the eyepiece. The magnification factor is 1-D/L where L is the focal length of the Barlow and D is the distance from the Barlow lenses to the field stop of the eyepiece. This actually varies a little from eyepiece to eyepiece because there is no standard to which manufacturers must conform, but somehow the Barlow manufacturer decided on the dimensions of the Barlow assembly to hold the eyepiece a certain distance away. If you increase that distance with an extension tube, and a diagonal mirror is just a fancy extension tube for present purposes, the magnification factor will be increased (remember L is a negative number, so 1-D/L is the same as 1 plus something positive). Usually a 2x Barlow is about 3x when used ahead of the diagonal, but it depends on the Barlow and somewhat on the diagonal because what matters is how much extra spacing there is compared to the focal length of the Barlow.

The easiest analogy I can think of is a slide projector. When you look through a negative lens it makes things look farther away, right? Well, when you insert the Barlow into the light path of a telescope it is analogous to moving a slide projector farther from the screen (of course you must refocus). Increasing the distance between the negative lens and the eyepiece field stop (which is analogous to the projector screen) and refocusing has the same effect.


Like any refractive device, a Barlow can potentially affect chromatic aberration, but usually it only adds to the problem. Cancelling or significantly reducing the problem so near to the focal plane is far beyond a simple Barlow's capabilities. But if you notice it less, then that's a good thing.
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Mike

I think Gary Russell sells some nice eyepieces, but the type isn't particularly good for such a fast scope. An f/4 light cone is just brutal on eyepieces. With Konigs and such, expect sharpness centrally with a lot of softness as you leave the central area and approach the edges. Siebert standards (of which I've tried the 7mm only) will probably fare no better. Perhaps there are some good choices for an f/4 scope in his higher tiers--I wouldn't know. Generally I would caution you against bargain-hunting for an f/4 scope. You might find a few bargains in many trials, but more often you will be wasting your time and money on eyepieces that won't make you happy.

If you can avoid being bitten by the ultrawide/Nagler bug for awhile, you can do quite well with a few Tele Vue Plossls and a 2.5x Powermate. For an f/4 scope a Tele Vue Paracorr is recommended, but you can probably live without it very easily if you stick to eyepieces that have 50 degree apparent fields as Plossls do. Since you have no eyepieces, my suggestion would be to start with 32mm and 20mm Tele Vue Plossls and the 2.5x Powermate. Then later try a Siebert or a Russell or (heaven help you) a Nagler and follow your nose. Take your time. I'm here to tell you that no matter how much information you try to gather, you won't get this eyepiece thing totally figured out from the start, if for no other reason because like Al Nagler says, eyepieces are like shoes, you've got to try them on.
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Mike

Well, you'll need enough magnification that the exit pupil fits into your eyeball. Let's say that's 6mm. Then the 280mm objective will require magnification of 47x to get the exit pupil down that small. Now assuming you will have an 82 degree apparent field, that implies a true field less than 1.75 degrees for the benefit of the full aperture. That goes down to 1.38 degrees with a 65 degree apparent field, 1.1 degrees with a 50 degree apparent field. Something to think about, anyway.
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Mike

In addition to the 19mm, I tried a 15.5mm that was quite sharp (and a 42mm that I didn't care for).
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Mike

I use a single-beam laser for the secondary, a Rigel Aline tool for the primary, and a sight tube of my own make to re-check the secondary positioning.
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Mike

And all this time we should have been going for THICKER spider vanes. Doh!
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Mike