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Adjusting an eyepiece focal plane

perdrix

By perdrix
in Discussions - Eyepieces

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perdrix

perdrix

Posted
Posted

I have a Skywatcher 6mm UWA eyepiece which isn't parfocal with my other eyepieces.   I have to adjust the focuser to move the EP inward by almost 1/4 inch to get focus as compared the my other eyepieces.

Unscrewing the eye end, I can see that is constructed almost like a mini telescope with an "objective lens" and an eyepiece lens (which is the bit that unscrews).

There is a disc inside that holds the "objective" lens of the EP.   This is externally threaded, engaging on the casing and has two holes to allow it to be turned.  The actual ring that holds the elements of the "objective" is threaded into this disc.   Is this how one adjusts the focus point, or does this change the focal length (or both)?

Yes, I could by parfocal rings for ALL my other eyepieces (which are pretty close to parfocal) but that doesn't seem the right approach somehow.

Thanks

Dave

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John

John

Posted
Posted

Dave,

I'd not go fiddling with the eyepiece internals. The spacing of the lenses and the field stop in relation to each other is critical to making it work so they are not really adjustable. The internal ring you mention holds the lenses in place in the eyepiece barrel. It's not designed to adjust their position and that of the focal point.

I have quite a few eyepieces that have diffrent focal points to each other, sometimes by quite a lot more than 1/4 inch !. It can be mildly annoying but I'm not going to start taking them apart to make adjustments !.

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ronin

ronin

Posted
Posted

It will change the focal length of the eyepiece.

The first lens is likely present to collect over a wider angle then deliver the image that it creates to the correct location for the remainder of the eyepiece.

You will alter the focal length and possibly the angle that the eyepiece collects light from and almost certainly prevent the remainder of the lens in the eyepiece from producing an image.

This is why I tend to suggest eyepieces from one set as they are normally parfocal eg all the BST's, all the X_Cels etc. If you mix eyepieces then this lacjk of being parfocal is a consequence. It has to be within a set as to best I know TV Delos are not parfocal with TV Ethos and other TV sets are the same. Parfocal within a set but not with anything outside of that set.

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NGC 1502

NGC 1502

Posted
Posted

Hi Dave. I'd go carefully if modifying an eyepiece. Not saying it cannot be done, but you will need to keep the distances between the lens elements exactly the same.

Even if using very expensive eyepieces from the same manufacturer, they are not always parfocal. If it were me, I'd either live with it, or buy a set of parfocal rings.

Regards, Ed.

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John

John

Posted
Posted

You could add a 1/4" spacer to the barrels of you other EP's so they all have the same focal point as the UWA, what to use as a spacer is another ball game... :)

Thats what parfocalisation rings are for.

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alan potts

alan potts

Posted
Posted

Isn't everything astornomical expensive when it comes to little bits and pieces. I have just been having a sort out and I paid 25 Euros for what is a 1.25 inch tube with a thread on the end, I don't even remember what it is for.

I guess the parfocal rings are the same, for what they are very dear.

Alan.

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Tinker1947

Tinker1947

Posted
Posted

I guess a thick but shortish elastic band around the eyepiece barrel might do the same job ?

The E-Bay rings with the locking screws won't get with in a mile of my EP's, i was thinking plastic waste pipe sliced off at the appropriate length then cut so there spring would grip the barrel, but i don't recollect having to move the focuser much with my brands of EP so the pipe can stay in the roof void of the workshop for another 10 years..:)

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