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General queries EP's Focal lengths etc

casemonster

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casemonster

casemonster

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Hi All, now that the mist has desended over Essex and tonights viewing has been given up on (was sooo happy when I watched the forecast this morning with clear nights all night):) I have a couple of queries.for those unlucky enough not to be out in the cold

1) Just bought a 6mm Celestron Omni Plossl for my scope, a 4" refractor, I was a little disapointed to see that the image when focused was very blurry and difficult to focus enough for a sharp view ( still don't have it perfectly focused). It is almost as though my scope is at the very end of it's capabilitiies and the lens doubles the size but not the clarity and detail. Is this normal ? I was using a K12mm which focused quite well and pretty sharp.

2) My starter for this new lens was Jupiter and even last night, when it was quite clear, I could still not make out any bands on the planet. Is this an "eyes can't make out the colours at night" thing and if so would I not be able to see the bands anyway even in black and white?

I have a 9.7 mm Plossl on it's way soon.

3) Not sure what "f " the scope is but when focused the scope length is 1650mm objective lens to EP .

4)All things being equal am I expecting too much from a 4" refractor?

5) What difference would a dark site make to this ( more light more detail?

Sorry for all the questions but wanted to see if I was doing something wrong.

Many thanks

Chris

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spaceboy

spaceboy

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Seeing conditions usually allow for high end magnification between x150-250 with very rare nights of x300+ (dependent on scope). The 6mm EP will give you x275 which I'm sorry to say is both beyond the capabilities of the scope and average UK seeing. The 9.7mm will give you better results and will be around the maximum usable magnification in your scope.

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AstroMadBloke

AstroMadBloke

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I total agree with Spaceboy, I have a 10" Newtonian and only once have I ever managed to get it above 300x in the 5 years I had it. This was on a night with very good seeing no wind and a relatively dark sky. Some times less is more!

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John

John

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Yep, as per the other posters, I think that's too much power for a 4" refractor. The max I use with my Vixen 4" is 220x and thats only on binary stars and the moon under excellent seeing conditions. For planets 170x - 200x is as far as it's worth pushing it.

Interesting refractor you have though with that focal length (it's focal ratio is F/16 !) - mounting it steadily must be quite a challenge.

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spaceboy

spaceboy

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Achromatic refractors often suffer CA Chromatic aberration - Wikipedia, the free encyclopedia causing purple halos (false colour) to become apparent around bright objects at high magnification. As you have a long focal length refractor this shouldn't be too bad. Regards seeing any colour on planets you may get a hint of brown on Jupiter, red on Mars, straw on Saturn, green on Uranus, blue on neptune and rust on Venus. It will only be vaguely distinguishable colouration and for the best part look B&W if your lucky enough to have a scope good enough to resolve some details.

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John

John

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It is an achromat ( i.e. two lenses at the front as I understand it) Is that a good thing for colour?

Achromat refractors generally show some false colour, more as their focal ratio (focal length divided by diameter of objective lens) gets shorter. If the specs of your scope are as you posted, ie: a 4" objective lens with a focal length of 1650mm then the focal ratio is F/16.1 which should, if it's a decent quality objective lens, show very little false colour. Your scope is what is described as a "slow" scope.

What brand / model of scope do you have ?,

Edit: spaceboy got their 1st !

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spaceboy

spaceboy

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I would check collimation if you picked it up off a car boot. John is a big refractor guy so I'm sure he will be able to tell you what it is. I think I remember you posting this in another thread. Looks a nice old skool frak and you can't complain for £30.

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casemonster

casemonster

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Hurray!

My new EP has arrived , the 9.7mm meade series 4000 which I am hoping will give me better views that the 12mm but will not be too much of a magnification like the 6mm which was showing blurry Saturn last night!

Clear skies but still misty and very windy so won't bother going out tonight as the big scope gets blown about in the wind a bit too much!!

Will let you know when I get better conditions.

Cheers

Chris

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spaceboy

spaceboy

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I'm sure you will be much more happier with the views in the 9.7mm Chris. Just remember to let the scope cool down out side for 30 mins or so before hand. It may pay to also make some sort of dew sheild for the scope as well while there are clouds in the sky :)

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