Posts Made By: Alan French

I found that my astigmatism was a problem with my old 14x70 Fujinons, and they didn't have enough eye relief to allow me to see the entire field with my glasses on. If it weren't for that, $350 is a very decent price for a good pair of large binoculars.

Clear skies, Alan

David Simper said:

I have 24h before shipping out my new reflector to WSP and found that the mirror is not marked. There is a center ca. 2 inch protective film for the "foot" of the mirror cover but no center cutout in the film nor a marked spot. A web instruction would ne nice.

Thanks,

David

David,

You've probably done it by now, but here is how I center "dot" my mirrors....

I make a plexiglass disk exactly the same o.d. as the mirror. I use a homemade router table to turn the disk, and can get it very accurate. Because I use a pivot to it, the disk winds up with a 1/4" hole in the center.

I put a 1/4" drill bit just through the hole, push a fiber washer onto the protruding bit, put a little rubber cement on the washer side away from the disk, and then carefully center the disk on the mirror. A little downward pressure on the center of the disk will push the washer against the mirror. While still pressing the disk downward, I pull the drill bit out. Releasing the pressure leaves the washer nicely centered on the mirror.

I also use the disk when I send the mirror out for coating, or if I store the mirror for a while. I lay it on top of the mirror and put a few pieces of tape around the edge.

Clear skies, Alan

Tim LeVane said:

Are scopes with ED glass apos or just really good achromats?

The scope market seems to be flooded with terms from semi APO to semi ED.

Newcomers such as myself are over whelmed when trying to make an informed choice on what to buy.

It is confusing, isn't it? Ideally, the company should provide you with a graph showing the variation of focus with wavelength (chromatic focal shift). A typical achromat has a focal shift between green and the common blue and red focus of about 1 part in 2000 or so. If a maker or seller is claiming "ED" or "semi-APO" (a rather loosely used term that would be better buried) the chromatic focal shift should be considerably less.

Sometimes they will say they are using a fluor-crown or true fluorite. If so, you are probably getting better color correction, especially if they specify the fluor-crown is FPL53. Of course, manufacturers can buy these substrates at different quality levels, and the certified ones with high homogenity cost more.

A set of narrow band interference filters could be used to check the focal shift. I have not seen any reviews that do this. This check would easily show the lenses that are simply achromats, but would be difficult to do with the small focal shifts of true APOs.

I'd stick with someone who is willing to provide some measure of the chromatic focal shift, whether it is a graph or simply a number measuring the shift over some specified part of the spectrum. If they give you a song and dance and no data, walk away.

Unfortunately, consumers are often swayed by the promise of a low price and something better, and don't insist on the information they need to make an informed decision.

Clear skies, Alan

Terry Sweet said:
[SNIP] Aluminum has thermal issues and they say fiberglass is better. Can anyone help? :S

Most refractors use aluminum, and they seem to work very well.

Clear skies, Alan

A 4" refractor would be an excellent second scope. I recommend getting one with a relatively short focal length - somewhere around 600mm - so you can enjoy low power, wide field views that are simply unavailable through your Portaball. If you also want the best performance at high powers, an APO would be the best choice. For low power viewing, a good achromat would be fine.

You might also give some consideration to somewhat smaller refractors, especially if your main interest is wide field views.

Clear skies, Alan

Danny Vettoretti said:

Just thought I would shoot my laser collimater through the lens of my TV102. Looks like the outer lense is crown glass while the inner is SD glass. There is no light path visible throught the inner glass - could it be a flourite type glass element?

Thoughts?

You really need to do it with a brighter green laser. If that doesn't show, then the element is fluorite (not fluor-crown). However, I understand there are some, perhaps lesser, varieties of fluorite that show a faint path.

Clear skies, Alan

Dean Rouncville said:

Hello!
I'm new and enjoying my 4.5 inch reflector,but its been cloudy of late, so I'm researching my next scope. I may have a bit of aperture fever but I'd really like to know the answers to these:
1. Does a lens have to be glass? In a reflector it gets covered with aluminum so why not use something cheaper?
2. Why not use aluminum shape it and then polish it?
3. Pyrex seems to be the glass of choice. Why not permanently borrow Mom or the Mrs. pie plate and grind that?
I'm sure there are reasons or people would be doing it, but I'm new(and cheap.. er frugal) so I had to ask. Thanks, Dean.

Glass is easy to work and takes a good polish. If you use Pyrex, it has a low coefficient of expansion, and so changes shape very little with temperature changes. Before glass, mirrors were made from a mix of copper and tin ("speculum" mirrors). These mirrors tarnished fairly quickly and had to be redone often.

Folks have made mirrors of aluminum. I believe it is more work, and aluminum has a very high coefficient of expansion.

Someone did make a mirror from a Pyrex pie plate. A possible issue is incomplete annealing. Good Pyrex blanks are precision annealed to remove stress from the blank. This prevents it from distoring with temperature changes. Pie plates are unlikely to be precision annealed, although any Pyrex material has been annealed to prevent the internal stress from breaking the dish with tempertature changes.

Folks have even made mirrors from Obsidian.

Clear skies, Alan

William Chang said:

I have M13 through a Royce 10" f/6 indelibly etched on my retina -- wondering if there's a chance I might be disappointed by a (much more convenient) f/5. Not worried about collimation, but rather field curvature. Any thoughts? Thanks,

-- William

p.s. I do have a Paracorr but it has trade-offs; prefer to not need it.

I am rather partial to an f/6, but even there certainly recommend a Paracorr. I don't know if it will help, but attached is a comparison of the RMS spot size versus field angle for up to 0.25 degrees off axis. Neither scope has a very wide field where the RMS spot size is less than the Airy disk, but the f/6 is an improvement.

Clear skies, Alan

jon roewer said:

This link may be worth perusing..

http://ao.osa.org/upcoming_pdf.cfm?id=66926

So we get a 70mm f/7 with five elements, large airspaces, and a total length of 1000mm - more than twice the effective focal length. Or am I missing something?

A tolerance analysis would be interesting and useful.

(A 70mm f/14 achromat should be nice and color free visually.)

Clear skies, Alan

Jim Miller said:

The 80mm was supposed to be available in March, but it's
still not even on Meades webpage. Are they getting out
to consumers? I'm really anxious to hear some reviews/
anecdotes on these! 8O

Celestron is also coming out with a "premium 80mm refractor," the Onyx 80 EDF. It is a fluor-crown/crown doublet.

I wonder which will actually start showing up first.

Clear skies, Alan