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WO Megrez 90 - First Light


John
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At last I have managed to actually look at the stars with this thing - for all of 45 mins before the clouds rolled back in !.

First observation to make is that the Megrez 90 needs a little cool down time. Its nice enough straight out of the house but progressively the images get sharper - it had just about peaked I reckon when the clouds intervened :-( I wonder if there is more glass (thicker lenses ?) than in the ED80 and perhaps also the thicker tube walls slow down the temperature equalisation a bit.

First object - the moon at around half phase. Not great seeing conditions but the views were very reminiscant of my ED80 and ED100. I could not detect any false colour at the lunar limb and at 120x (5mm plossl) the limb was sharply etched against black space. The terminator was a jumble of finely etched detail - the jagged shadows of the mountains seemed really sharp on the landscape and the shadows were pitch black. I had a wonderful view of the Alpine Valley and with more magnification I'm sure that I could have made out details inside it. I was also more aware that I have been before of the colour variations on the surface of the moon. The fine focusser wheel is really great - so precise.

Next - Vega to do some star testing. I did not use enough power to do this properly - 120x is really not enough but it was enough to show nice concentric diffraction rings, a little fainter outside focus than inside - I don't know what that means. Even on Vega there seemed to be little or no false colour - at focus I could see an airey disc surrounded by a 2/3 fainter diffraction rings when the seeing allowed. Have to try this again using more magnification when the seeing is better.

Then on to the Double Double - clearly resolved at 100x (13mm Widescan + Ultima 2x Barlow). A very nice view.

A quick look at Polaris to spot it's mag 9 companion and that was it - the clouds rolled over and it was all gone. It's chucking it down now so I'm forced to open a bottle of Californian white to celebrate 1st light :-D

This evening proved that I have to fit a finder to this scope PDQ - alignment by line of sight is just not good enough - it's awkward even on really bright objects.

An interesting thing about this scope (and other WO scopes maybe) is that the objective lens, although air spaced, does not have prominent foil spacers like the ED100 had. They did not affect visual viewing but I have heard that they could show up when taking photos. The Megrez 90 must use another method of separating the lens elements. The coatings on the lens must be pretty effective - from some angles the objective seems to dissapear completely.

All in all pretty satisfactory - moderate seeing conditions and only a short observing "window" but the performance seems very good and promises more. The overall build and finish quality is fantastic.

Must get a finder sorted though.....

John.

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I thought there was something wrong with my TV76 the first time I took it out and it was still showing distorted Airy discs after 20 minutes. But it needs a full half hour to cool down, perhaps because of the very heavier build. My much bigger and heavier Broadhurst Clarkson 89mm f13 refractor is raring to go after barely 10 minutes. I believe some of the very fast Taks and Pentaxes take up to an hour to cool down - something that isn't meantioned in all the "grab and go" publicity. More like"grab and go and have a few jars while the scope calms down."

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I thought there was something wrong with my TV76 the first time I took it out and it was still showing distorted Airy discs after 20 minutes. But it needs a full half hour to cool down, perhaps because of the very heavier build. My much bigger and heavier Broadhurst Clarkson 89mm f13 refractor is raring to go after barely 10 minutes. I believe some of the very fast Taks and Pentaxes take up to an hour to cool down - something that isn't meantioned in all the "grab and go" publicity. More like"grab and go and have a few jars while the scope calms down."

Mines cooling as we speak and I'm having a few jars as the clouds have invaded again.

Captain Chaos

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Cheers - I was once that one reason why the old English refractors cooled down so quickly was the high conductivity of the copper in the brass tubing. I once had a very old and heavy 4inch f16 Dollond that hardly needed any cooling down. I sometimes put an ancient Ramsden eyepiece in the TV76 and you can almost hear it yelling in pain. Everything has a lurid orange fringe.

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