Eyepiece Quality

[billbrownlie]

Is there any way to check an eyepiece, I recently bought a couple of eyepieces. A 5mm eyepiece for £70 and a £21 17mm Possil.

The 5mm does not give a clear view. The best focus still looks pretty fuzzy. I get a much better view with the 17mm possil on a 2x barlow, this snaps into focus clearly.

How do you tell if its just the level of magification or if it is a duff buy.

all help appreciated

[Moonshane]

what scope are you using? It may be that the magnification is too much? if using a diagonal then you could put the barlow into the focuser then the diagonal into the barlow and then the eyepiece. this will give 3x and the 17mm becomes 5.6 so close enough to check if the scope can handle such mags.

could be seeing too of course.

[rwilkey]

What eyepiece is the 5mm, what make or model?

[acey]

The way to test it would be to compare it with an identical one or use it in another scope whose performance with a 5mm eyepiece is known to be good.

Most likely you've pushed the magnification too high for your scope and seeing conditions. The 5mm is giving you 70% higher magnification than the 17mm plus barlow, and that's a big difference.

[cyborg421]

A 5mm eyepiece is only able to be used on scopes very occasionally under ideal seeing conditions. It will give you an extremely high magnification.

Simon

[Moonshane]

A 5mm eyepiece is only able to be used on scopes very occasionally under ideal seeing conditions. It will give you an extremely high magnification.

Simon

unless you have a short focal length - 500mm focal length = 100x.

[cyborg421]

Fair point.

[billbrownlie]

Thanks for the replies.

It is a Celestron x-cel 5mm

Telescope is a skywatcher 200pds with a FL of 1000mm

So this should be 200x against 118x with the barlow and 17mm possl

I pointed the scope on the tip of a pylon about a 1/4 mile away and I could clearly make out the rivets with the Possl but the 17mm would not come into any sort of clear focus. I was at the focus point as it could clearly been seen coming in and out of focus either side but just would not give a clear image.

On the few occasions since I got it I've only just managed to see venus for a few minutes and again the possl/barlow was much better, thats why I tried it in daylight as I wasn't sure what to expect from it and the seeing has been rubbish for weeks to be sure of anything further away.

Is the pylon test any good at 1/4 mile?

[billbrownlie]

Correction "could clearly make out the riviets with the possl but the 5mm etc

[rwilkey]

Very suprising, I cannot understand why it will not come to focus.

[acey]

Anyone know the diameter of a rivet on a pylon? Then we can work out its angular size at a distance of 1/4 mile and compare this with typical daytime seeing conditions and the resolution of an 8" scope.

[billbrownlie]

That sounds like a riveting exercise but what will that prove. The image on the 5mm was there but no clear definition. It was percieved in the eye as much bigger than the 17mm, i.e. the higher mag. The comparison with the 17mm/barlow was within seconds on a clear sunny day, which just as it got dark completely clouded over, (I think I'm jinxed).

I just feel the definitition was out of focus all other things being equal. Have I wasted my money and do I need to go and hunt out receipts?

The best solution so far would be to have another telescope and another 5mm EP and swap them around and compare but alas my wife thinks I'm an anorak as it is and doesn't allow me to meet with other anoraks, yet????

[jimmyjamjoejoe]

200x for terrestrial viewing seems a bit much, and is most likely the cause of the mushy image. Bare in mind you're looking through the thickest, murkiest area of the atmosphere, and during a time where heat currents are rife.

You can only barely push 200x on most nights, near zenith, if your scope is very well cooled AND if the atmosphere plays ball.

[acey]

That sounds like a riveting exercise but what will that prove.QUOTE]

It'll determine whether you should have expected a clear image. I tried a bit of googling on your behalf for "rivet size" but don't have more time to deal with the problem now.