BST 8mm something odd

[bendiddley]

Just bought a BST 8mm for my SW 200p dob and used it for the first time on friday night. When I move my eye to look at the the right or left side of the FOV, the opposite side (almost over a third) gets blacked out. I have got round this by trying to move my head to look at the sides if I need to which helps but by keeping my eye central and moving only my eye I get this effect. Can anyone tell me what this is and if it is particular to the BST? I am wondering whether to send it back and get a different make.

[Moonshane]

this is a common 'issue' (it's normal) and referred to as kidney beaning. all it means is that you have not got your eye positioning quite right. use the adjustable eye shield (it will screw up or down) to get the right spot for you and you should be fine then. it's not a fault so don't worry.

[gooseholla]

Happens on all eyepieces.

[bendiddley]

that's good to know, thanks

[bendiddley]

Just had a play with the adjustable eye shield and it seems better when it is at it's highest but the blacking out is still there, more shadowing rather than blacking out though. I had a look through my standard 25mm and 10mm and I don't get this at all with either of them. Are plossl's not affected so much by this kidney beaning effect?

[Moonshane]

simpler designs seem less affected but daylight is not the best way to check the position, much better in the scope at night. it will be fine once you get the right position. that said, my 32mm plossl has long eye relief and eye positioning is quite tricky for some.

[paul schofield]

Yes happens with most if not all eyepieces I own, funny thing is though and it might just be me, the more you use it, the less you notice it. I hardly notice kidney beaning on my BST 8mm any more at all. Be at ease, I don't think you have a faulty e.p.

[pvaz]

Same has happen to me when I switched from plossls to wide field EPs. As said above the trick is to use the eye guard. I also find it helps to position my head so that the side of my nose will be brushing the side of the eye guard. It becomes natural with time.

[astrofox]

Definatly kidney beaning. Only had one 25mm which showed this phenomena and then only in daylight when the eye's iris is closed up.

[bendiddley]

Definatly kidney beaning. Only had one 25mm which showed this phenomena and then only in daylight when the eye's iris is closed up.

I didn't realise the size of the iris plays a part in this effect happening, do you know why the iris can create this effect?

[michael.h.f.wilkinson]

Only my (now sold) Paragon did not show distinct kidney beaning. In that EP, the image just got slightly dimmer when the eye was not at exactly the right spot. All others suffer from it.

[cs1cjc]

I didn't realise the size of the iris plays a part in this effect happening, do you know why the iris can create this effect?

For eyepiece forcused to produce an image at infinity, as is normal, there is a "pencil" of parallel rays for each point in the sky. If these pencils do not cross at the same point in front of the eyepiece (spherical aberration of the exit pupil) then, as you move your eye, the iris will cut some pencils but not others causing some parts of the image to darken, "kidney beaning". With the iris contracted as in the daytime, the effect is more obvious. More information here:

http://www.telescope-optics.net/eyepiece_aberration_2.htm

[Foggymike]

I had this when I tried some BST EP's and couldn't settle with it. I now use Celestron X-Cel LX's and I don't get any kidney beaning with those and don't have to be fussy how I look into them. I guess everyone is different though so it's just a case of finding what works for you. Hopefully you'll just get used to looking in the best way to avoid it - it will save you a few quid if you can!

[bendiddley]

I had this when I tried some BST EP's and couldn't settle with it. I now use Celestron X-Cel LX's and I don't get any kidney beaning with those and don't have to be fussy how I look into them. I guess everyone is different though so it's just a case of finding what works for you. Hopefully you'll just get used to looking in the best way to avoid it - it will save you a few quid if you can!

Thanks for this. I read somewhere that somebody else also found the Celestron LX's better for the kidney beaning problem. I was going to send the BST back and get one but another person said the LX's were very bad for kidney beaning so i decided to stick with the BST, weird how 2 people can have such different experiences. Interesting to know it works for you though. Out of interest how do you find using the 7mm? Is it not a bit too high mag for an average night? I have been told that 8mm is about as high as you would usefully go 'generally' which is why i went for the BST 8mm. If I go for the LX's at the moment I would want either a 7mm or a 9mm but not sure which (don't want to buy the 7mm and find I don't use it very much). What do you think? I have the same set up as you by the way.

[Foggymike]

Just replying via PM