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moon won't focus?


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tried to look at the moon last night with my 3 inch reflector, 10mm eyepiece and 2x barlow and no matter what i did it just wouldn't focus. it was perfectly clear so i can't blame clouds maybe its too much magnification for the scope? on a brighter note i did manage to separate mizar and alcor for the first time and pick out mizar's double :) every cloud and all that

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The answer probably depends on exactly what you saw. Was it blurry, or shimmering? If you couldn't focus because it was shimmering, this is caused by air turbulence, and is known as bad seeing. Too much magnification will just make this worse.

If blurry, assuming there aren't thumb prints on any of the glass surfaces(!), then I wonder which part of the moon you were trying to focus on? The moon was close to full last night, with only a small area showing shadows. It's much easier to focus on an area partly in shadow than an area fully lit by the sun.

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In general the 10mm eyepieces and 2x barlows supplied with scopes seem to be poor so putting both in could simply have been a case of compounding the problems.

What focal length is the scope?

The moon is pretty big so unless you want to centre a specific crater you don't need great magnification.

If you can borow a fair 6mm or 8mm eyepiece then give one a try. I guess that the difference would be significant.

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You should get 60x magnification fairly easily. I am back to guessing that the eyepiece and barlow don't work too well and you have in effect multiplied the problems.

Hope that the 6mm is a lot better.

I am assuming that the image is really blurred and out of focus, gets better then goes really blurred and out of focus again?

Just want to make sure that you are managing to pass through the focal plane even if it is not good.

Just read the figures:

300 mm FL and 75mm dia = f/4

Unless it is a good parabolic mirror then you will be getting optical problems all over the place.

I doubt a little that a 3 inch mirror is parabolic.

You will also need a pretty good eyepiece for an f/4 reflector.

Are you sure?

What is the make and model?

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Just checked the Skywatcher site and they say nothing regarding the mirror used. I would therefore suspect that it is a spherical mirror, if parabolic I would guess they would have advertised it.

To state it as apochromatic is a kind of stretching the truth, it is a reflector and therefore doesn't split white light as a refractor does so cannot suffer from chromatic abberations.

However at f/4 I suspect there is excessive coma and if spherical even worse.

Even if it is a parabolic then at f/4 it is pushing it. At f/5 many find they need some of the decent TV eyepieces at £170 and above to be any good. At f/4 the heritage would need better = more expensive.

Hope it is a case of a poor eyepiece and a poor barlow, they wouldn't put reasonable ones in the hertitage package, simply even a basic eyepiece is £15 it caomes with 2 and the heritage package cost is around £50.

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The Heritage 76 has a spherical mirror and that in itself causes some problems with getting clear images. You need to center the objectas this is where the focus is best. It drops off around the edges. I don't use the eyepieces that came with the scope- they are rubbish. Get yourself some decent ones and also look at the scope's collimation. You can only move the secondary on these scopes but by adjusting the screws you can get an improved image. Focus on a fairly bright star then adjust the screws one at a time with small movements until you get a more pinpoint image.I also used a cheshire eyepiece to fine tune the collimation. The results are amazing. i find this scope is fantastic for viewing clusters. On a good night I can use my 4mm eyepiece but usually use my 10mm with 2x barlow. Hope this helps.

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Hi wattsy, I also have one of these little gems, advise checking the collimation. I use a cheap 12mm plossl and the supplied ep's and achieve quality views. Never had much luck using a barlow in it though, to much mag maybe. Just love the design, impulse buy and glad i did. :icon_salut:

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There are many factors governing how well the scope performs. If it was a humid atmosphere you'll get a shimmering effect on the moon especially if it was low on the horizon - viewing higher is better. Ensure the mirrors/ep's weren't misted or dewed up. Put the scope outside for a while and let it cool to the ambient temperature (half an hour or so). If you used it indoors then hot air currents from the house will disturb the views, and viewing through a closed window is totally pointless. Finally good collimation as mentioned above is crucial for good views.

Hope that helps :icon_salut:

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