Puzzling problem

[g30rg313]

My sister in law got the same scope as I for Christmas (except I got the goto handset and she didn't, hers is just a motorised mount).  The thing is, she has yet to be able to see the moon or stars or anything at night time for that matter.  I have helped her line the finderscope with the main scope and theoretically when she points the finderscope at any star or planet she should be able to see it through the eyepiece.  She has used the scope in daylight and inside the house to point to things and it seems to work fine.  However,  at night,  when she takes it outside all she seems to see is blackness.  I've asked her if she has made sure to leave the scope out to "cool" checked she has dark adapted herself and checked she has limited light around her.   I'm a bit flummoxed as to how she can see through the scope in the daytime but not at night.  I guess I would have to physically go down to her house and try her scope myself to try and figure out if there is a problem we're just not seeing. 

[michael.h.f.wilkinson]

At night some people (like my missus) have great difficulty finding the exit pupil. Maybe that is a problem

[Dr_Ju_ju]

leaving the caps on..... ??

has she aligned the finder to the scope ??

 

[g30rg313]

I've thought it might be something silly like leaving the caps on so I asked her that lol......I aligned the finder for her and I've checked that she hasn't touched that.  I was wondering if it might be the exit pupil for her Michael.  Is the exit pupil better on a 40mm than on a 25mm?  What's the formula to work it out again?  Is it f/l of scope divided by f/l of ep divided by focal ratio?  I was thinking of letting her try my 40mm to at least get the hang of "finding" something.

[brantuk]

Lining up the finder with the scope/eyepiece inside the house won't work on the night sky. Everything is too close indoors and I'm surprised she can focus it at such short distances. She needs to take it outside and line up on a distant small object like the tip of a church spire or pylon a mile or two away.

The finder has a wide field of view and the scope has a narrower fov. So you need to start with a low power eyepiece like a 20mm or 25mm. Get the scope pointing at the pylon tip (say) and center accurately. Then adjust the finder so the cross hairs are on exactly the same spot. Now change the eyepiece for a higher power (12mm or 10mm).

You'll see the scope is a bit off what the finder sees - certainly wont be centered. So reposition the scope again to get the object dead center of the eyepiece using fine motion controls - then adjust the finder cross hairs to match the centered object. Repeat with an 8mm or 5mm eyepiece for more accuracy.

Now it should work fine at night time. Hth

[michael.h.f.wilkinson]

The exit pupil diameter isn't my main concern, it is the position. A 25mm Plossl will have a 5mm exit pupil in an F/5 scope (exit pupil diameter is the focal length of the EP divided by the focal ratio of the scope). The exit pupil position of a Plossl is about 75% of the focal length of the EP, or 18mm. For a 40mm Plossl it is a huge 30mm, which is very uncomfortable, because the eye must hover at some distance from the EP. Even 18mm can be hassle without glasses if you do not have a good rubber eyecup.  Some EPs are very sensitive to positioning of the eye, blacking out quickly.

[g30rg313]

Brant I lined up the scope for her on a distant object.  I used the same process as I used to line up my own scope and my scope seems to work just fine.  That is why I can't figure out where she is going wrong.  I think that maybe I will have to go down to her house one evening when she is planning on getting her scope out and see if we can work out the problem together.  The only thing I can think is that it is either as Michael has suggested exit pupil or something really stupid as she is a total newbie.

[brantuk]

Ahh - in that case then yes I agree - check out the exit pupil next. Some eyepieces require your eye to be in exactly the right on-axis position to view effectively. I get too accustomed to very wide angle eyepieces - but always get caught out when I go back to something like a radian. Tell her to bring it up to your place lol

[Starwiz]

I agree on exit pupil. I have no trouble at the telescope eyepiece, but often have trouble finding it when using the polar scope due to the position I'm in, making me think I've left on the lens cap or something.

John

[Linda]

+1 for the exit pupil. Sometimes when I look into the eyepiece, I don't see a thing. And then I need to move my eye and look from another angle into the eyepiece. Other times, I need to remove the cap from the scope.

She could also have moved the position of the finder, by touching it by accident. Next time you see her, tell her how to realign it herself.

She also needs to use her longest eyepiece (40mm or 25mm). She could be using 4mm...

[g30rg313]

2 hours ago, Linda said:

+1 for the exit pupil. Sometimes when I look into the eyepiece, I don't see a thing. And then I need to move my eye and look from another angle into the eyepiece. Other times, I need to remove the cap from the scope.

She could also have moved the position of the finder, by touching it by accident. Next time you see her, tell her how to realign it herself.

She also needs to use her longest eyepiece (40mm or 25mm). She could be using 4mm...

Haha Linda at cap on scope I've done that a few times almost forget about the important thing.  Take off the lens cap haha.

[Marky1973]

My 8 year old has a habit of shutting one eye when looking through the telescope....unfortunately, it is often the eye she is holding up to the eyepiece...

[Torin]

9 hours ago, g30rg313 said:

Brant I lined up the scope for her on a distant object.  I used the same process as I used to line up my own scope and my scope seems to work just fine.  That is why I can't figure out where she is going wrong.  I think that maybe I will have to go down to her house one evening when she is planning on getting her scope out and see if we can work out the problem together.  The only thing I can think is that it is either as Michael has suggested exit pupil or something really stupid as she is a total newbie.

Don't forget to take your scope with you and put them side-by-side, that way you can test all the eyepieces and compare.

[Jimtheslim]

if she's looking at terrestrial targets in the daytime it might be that if she's then taking it out at night she is massively out of focus?