davidc135

On fluorite objectives. I once reworked an uncoated Tak FC100 doublet from the 80s that had become badly cloudy. In this Steinheil objective, the hot, moist Japanese climate and perhaps poor storage had, over decades, etched the rear surface of the front flint lens enough for it to need regrinding. Although the fluorite lens was separated by only 0.25mm it appeared completely unaffected and pristine. David

You might be able to both rough cut with a jigsaw and then clean it up with the cutter. David

I'll try to explain it better. All eyepieces are sold with the intention of being used with telescopes that are properly corrected for spherical aberration. However, traditional designs of eyepieces such as Huyghenian, Ramsden, Kellner and Plossl in roughly ascending order of capability will themselves cause aberrations, both at centre of field and off axis. These problems increase exponentially with the speed of the telescope. At f10 or so, scts (with their correctors) are so slow as to be useable with any kind of eyepiece. But with Faster Newtonians working at sometimes less than f4, modern sophisticated designs are needed for sharp images over a wider angle as well as at the centre of field, especially when low powers are used. I think Baader are just saying that their product is doing a good job, not that it will correct the deficiencies of any telescope, let alone the insurmountable aberration of a sct without its corrector. David

The Baader eyepiece won't correct anything in this situation. Cheaper and simpler eyepieces may themselves cause spherical aberration when used with faster telescopes, even if the scope itself is well corrected. The longer the focal length of the ep, the worse the effect. Better ones should be corrected for this shortcoming. However, slow systems like most scts are free of this anyway. David

That was a good price and outcome. At f10 or so compared to the f2.2 of the RASA, sct correctors/mirror sets need to be made to more exacting standards, so Celestron only offer to replace the entire optics set, AFAIK. But, like you, I wonder how much difference it would make to visual instruments if only the corrector was replaced. David

The SA of a C14 without its corrector would be of the order of 22 waves depending on the exact details of the mirrors. A lot worse than Hubble so you are wise to keep the corrector! David

Given your figures of radius of curvature of 2400mm and measured sag of 0.32mm on the 10'' mirror, I make the spherometer radius to be Ooops 39.19mm if just the simple equation: R = sqrt(2Hr) is used. Adding either or both of the two extra terms only adds a tiny error. Is 2400mm exact? David

I've a feeling that for the Schupmann surfaces, just r^2/2R may be close enough especially if the spherometer feet balls are very small. David

The formula for deriving R from a known concave surface is simply the earlier formula (in terms of r) rearranged. What is half the diameter of the ball supports? But you could do with the formula rearranged again to give the required Sag, or H. My Maths is poor as well, anyone? David

sqrt is square root.

I'm hardly in the best position, not having finished my 4.5'' version but I think the difficulty level is in the same league as an achromat. Definitely, you should be confident of success. I messed up the maths on my Mangin but the optical fabrication was straightforward, especially as you have the flat etc. David

Ah yes, I was using 4.25'' aperture. David

The three ball spherometer formula is: r = R^2/2H + H/2 +/-B (- for convex surfaces) where r = surface radius of curvature R=radial distance to centre of balls B = radius of balls H = measured sag Calibrating the spherometer against a concave surface of known curvature in order to find R, I get: R = sqrt(2Hr-H^2-2HB) if I have it right and the first formula with B negative to calculate r of a convex surface. David

I should scrub the Schupmann in my signature as after I'd made the Mangin I discovered that I'd made a basic mistake and so it's only half (or less) made. David

Great project! Hopefully we'll be able to follow your progress. I think that you skipped a 2 in the denominator. I got .0832'' for R1. Just to make things more complicated, looking up a 'sagitta of a chord' calculator, it gives the equation as the more precise: S = R-sqrt(R^2-r^2) where R = radius of curvature giving S1 as .0857ins, a difference of 63 microns. A lens shouldn't make any difference. David

Isn't your scope just badly out of collimation? David

The warm weather has brought some fine seeing over recent nights. I compared the view of Saturn at around 3.00 am through an 8.5'' f7.5 Dob and an FC100 Tak. This has an early 1980s non coated objective suffering from a touch of astigmatism after one corroded surface had been reground and polished. Whether this affects planetary performance I don't know but I wouldn't be surprised if it did. Both scopes around x200. A crisp view in the Tak with a shadow under the ring and a hint of belts just visible. The narrow gap between the rings and the planet wasn't seen but I think it made its presence felt by a slight local dimming of the rings. The Dob was able to go one stage further with the gap between rings and the planet sharply defined much of the time.

I too was out with my Tak FC100 looking first at Epsilon Lyrae and comparing full aperture with the scope stopped down to 50mm where I split Eps 2 successfully at x100, although I'm not sure about Eps 1. Fine seeing. Later I pointed it at Saturn at x200. Crisp view showing the shadow of the rings against the planet but I needed the 8.5'' f7.5 Dob to reveal the fine sliver of darkness that was the inner gap. No sign of Cassini's division, not surprisingly. David

Chaz2b, It looks like it's the coating on the rear surface that's patchy. What do you think? Can you see if the objective lenses are cemented or air-spaced. It may even be that it's the cement that is degrading. Shining a pen laser through the objective should give just the two reflections if the lenses are cemented but four if they are air-spaced. As well as Galvoptics, I suggest contacting Vacuum Coatings Scientific Instruments (VCSI) who I think also have wide experience in repairing optical gear. David

Scientific mirrors (VCSM) did a good job coating my 8.5'' Dob mirror. David

If the whisky doesn't remove the cement, I expect acetone will. eg nail varnish thinners. Bits of postage stamp or memo pad could also be used at this f#. David

They are beautiful. How long was the time lapse over the mill? I can see the clouds changing. David

Assuming the same optical quality I don't see the Skymax 90 showing greater lunar detail than an ED 80 refractor and so I'd say the bigger the better, within the limits of budget, mount capability and seeing. David

It may not be suitable for you but there is an ad on Astronomy Buy & Sell site for a 6'' f8 Newt going free in Hastings. Ad # is 214283 placed on 1st May. It has high quality optics by David Hinds. Interesting amateur tube and mount construction that could be replaced. I don't know if it's still available. Maybe a bit long and more awkward than some designs but a contender for the title of best planetary scope for the aperture. David