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Symptoms of exit pupil exceeding pupil dilation?


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When using my (admitedly not very good) 26mm plossl, I've noticed that I see a lot of shadow areas. Bear with me, this is hard to explain. It's like a large dark cloud taking up approx a third of the of eyepiece. I can see through it, but it dims the area affected. I move my eye around the eyepiece and the dim area moves but not always where I'm looking.

Anyway, is this a symptom of the exit pupil being exceeded for my eye?

I have pupils that don't dilate that much (doctor did tell me what the harmless condition is called, but I've forgotten), so perhaps 26mm in an F/5 reflector is the limit of my eye?

Or perhaps this is some other problem with this eyepiece.

I need to work this out before I invest in some new eyepieces.

Thanks

Boz

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As I understand it - probably yes (that EP yields a 5.2mm exit pupil on your scope, is that larger than your max pupil dilation?), indirectly. It is actually the size of the secondary shadow in the light cone that is getting close to your pupil size, due to the size of the exit pupil exceeding your pupil size.

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Alternatively it might just be that you are having problems finding the correct distance your eye needs to be from the top of the eyepiece. This is called the eye relief and, for a 26mm plossl will be around 20mm (+ or minus a bit). If you have your eye too close to the eye lens then you will see blackouts and other effects. Eyepieces with eyecups that help the eye to be placed at the correct distance make this easier but many older eyepieces don't have eyecups so you need to find the correct position by "hovering" over the top of the eyepiece - it takes experimentation to find the correct eye placing.

I've found eyepieces up to 32mm work generally fine in F/5 scopes - it's when the eyepiece focal lengths exceed this that the problems can start.

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When using my (admitedly not very good) 26mm plossl, I've noticed that I see a lot of shadow areas. Bear with me, this is hard to explain. It's like a large dark cloud taking up approx a third of the of eyepiece. I can see through it, but it dims the area affected. I move my eye around the eyepiece and the dim area moves but not always where I'm looking.

Anyway, is this a symptom of the exit pupil being exceeded for my eye?

I have pupils that don't dilate that much (doctor did tell me what the harmless condition is called, but I've forgotten), so perhaps 26mm in an F/5 reflector is the limit of my eye?

Or perhaps this is some other problem with this eyepiece.

I need to work this out before I invest in some new eyepieces.

Thanks

Boz

a 26mm EP will give you an exit pupil of 5.2mm, bearing in mind that a youngster with good eyesite will max out at 7mm, id say that going to a wide field ~20mm EP (4mm exit pupil) would suit you better...always good to have an excuse for buying that ethos :)

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It is quite likely that you are seeing a shadow caused by the secondary mirror. It will be very visible in daylight with th 25mm. As your eye becomes fully dark adapted and the pupil fully dilated, the shadow should disappear.

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Alternatively it might just be that you are having problems finding the correct distance your eye needs to be from the top of the eyepiece. This is called the eye relief and, for a 26mm plossl will be around 20mm (+ or minus a bit). If you have your eye too close to the eye lens then you will see blackouts and other effects. Eyepieces with eyecups that help the eye to be placed at the correct distance make this easier but many older eyepieces don't have eyecups so you need to find the correct position by "hovering" over the top of the eyepiece - it takes experimentation to find the correct eye placing.

I've found eyepieces up to 32mm work generally fine in F/5 scopes - it's when the eyepiece focal lengths exceed this that the problems can start.

I completely agree with this. A secondary shadow is nothing like you described plus it's very doubtful you would see one with an exit pupil as small as 5.2mm. I am 42 and use a 31mm which gives me a 6.9mm exit pupil and I have never seen a secondary shadow through it.

Regards Steve

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I can see a large secondary shadow if I use my 40mm Meade 5000 SWA ep in my 200p dob, what causes this? Is it because the ep is too large? I don't notice it with my 32mm Meade 4000 ep.:)

It's because a 40mm eyepiece with an F/6 scope creates an exit pupil of nearly 7mm which is too large. With the 32mm it's a more reasonable 5.4mm.

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Is there a way to determine whether an ep will be too large? When I bought the 5000 I just thought that since me eye dilates to about 7mm it should be ok. I didn't realise that there was other things to consider...

At least the 5000 makes a nice ornament.:)

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Alternatively it might just be that you are having problems finding the correct distance your eye needs to be from the top of the eyepiece. This is called the eye relief and, for a 26mm plossl will be around 20mm (+ or minus a bit). If you have your eye too close to the eye lens then you will see blackouts and other effects. Eyepieces with eyecups that help the eye to be placed at the correct distance make this easier but many older eyepieces don't have eyecups so you need to find the correct position by "hovering" over the top of the eyepiece - it takes experimentation to find the correct eye placing.

I've found eyepieces up to 32mm work generally fine in F/5 scopes - it's when the eyepiece focal lengths exceed this that the problems can start.

Hi!

Regarding the comments above about eyecups I have found the standard rubber eyecup available at Sherwoods a very comfortable viewing aid to keep my eye a constant distance from the eyepiece.

Telescope Observing Aids

I did find however that on a cold night there was a tendency for the eyepiece to mist up with the warmth from the eye but this was cured by cutting a small piece out of the eyecup on either side of the cup to allow a free flow of cooling air.

They are used on standard Skywatcher 1.25 eyepieces and I am about to place an order for a couple more.

I did try the "winged" one but id didn't work for me.

I hope this helps further the discussion.

Robin

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Ok, last night after midnight was excellent viewing and I did everything possible to get good results. Collimated, left scope out all evening to cool down, polar aligned, sat outside for an hour watching the stars to get my night vision spot on and used the eyepiece cups.

Still the 26mm in my F/5 1000mm x 200mm reflector gave me this dimming/shadowing as I moved my eye position around.

I don't see any shadow while collimating, so assume that secondary mirror shadow isn't the cause.

Are my pupils really maxing out at 5.2mm?

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