Pentax XW7mm

[markse68]

I recently picked up a s/h but supposedly  unused XW7mm but it seems to have a problem. Or maybe not... 

when I observe Jupiter with it the image is excellent- I’m getting just about as much sharpness and detail as a 7mm ortho I think but it does this strange and quite irritating thing. It’s hard to describe but it’s like Jupiter is forming a beam of light that swishes across my eye as the disk moves across the fov and if I move my eye it kind of dances around. The 5 doesn’t do it nor the 14 or any of the others I have (I don’t have the 10 yet) I’m wondering what could be the cause? The eye lens is clean and like I say the image it produces is excellent. Anybody own the XW7 noticed something similar? I guess it could be an interaction with my scope 🤷‍♂️ Maybe it’s a reflection off my eyeball? It’s fine with less bright objects and I didn’t notice it on the moon when I first tried the ep.

[Louis D]

I've had one for some time now, but I can't say that I've experienced anything quite like that.  I am irritated by some chromatic aberration near the field stop that is not present in the XL 5.2mm or XW 3.5mm.  Do you see bright stars turning into prismatic rainbows near the edge in your XW 7mm?  If you get your eye exactly lined up with the star, the prismatic effect mostly goes away.  Perhaps this and your experience are related?  Try lining up your eye with the incoming light rays from the edge image to see if the effect changes.

[bomberbaz]

35 minutes ago, markse68 said:

I recently picked up a s/h but supposedly  unused XW7mm but it seems to have a problem. Or maybe not... 

when I observe Jupiter with it the image is excellent- I’m getting just about as much sharpness and detail as a 7mm ortho I think but it does this strange and quite irritating thing. It’s hard to describe but it’s like Jupiter is forming a beam of light that swishes across my eye as the disk moves across the fov and if I move my eye it kind of dances around. The 5 doesn’t do it nor the 14 or any of the others I have (I don’t have the 10 yet) I’m wondering what could be the cause? The eye lens is clean and like I say the image it produces is excellent. Anybody own the XW7 noticed something similar? I guess it could be an interaction with my scope 🤷‍♂️ Maybe it’s a reflection off my eyeball? It’s fine with less bright objects and I didn’t notice it on the moon when I first tried the ep.

strange you should say that and you have just prompted me. My session tonight I got the same thing on Jupiter using a nagler. What OTA were you using mark?

 

[markse68]

I’ve not noticed that Louis but I’ll check next time. Are you using it with a refractor? I’m using it with a longish newt. What I’m seeing is a bit like but not really the reflected double image I get with a Nagler 4.8 with something bright like Sirius. In that case though I think it’s an internal reflection- it moves the opposite way to the star image and doesn’t seem to interact with my eye- it’s contained within the ep. This though really feels like it’s projecting out of the ep and sweeping across my eye. All the XWs apart from the 2” ones seem to have identical upper optical assemblies so it must be some strange interaction with my scope I guess. 

[markse68]

9 hours ago, bomberbaz said:

strange you should say that and you have just prompted me. My session tonight I got the same thing on Jupiter using a nagler. What OTA were you using mark?

 

That’s interesting Steve- It’s an 8” f8 dob I’m using. I suppose it could be just a case of the beam size and concentration overwhelming the anti reflective coatings or something- I’ve not noticed it on other targets 

Edited June 25, 2020 by markse68

[John]

I own an XW 7mm.

I've not noticed this effect.

They all have slightly different optical designs though so I guess it's quite possible that some FL's will suit an individual better than others ?

 

[markse68]

That’s a nice pic John- id have that on my wall 😉

It must be that big curved lens that the 7, 10 and 14 have I reckon- high brightness objects cause internal reflection that beams out as secondary image moving faster through optical gearing in opposite direction. For some reason the 14 isn’t as noticeable but I bet the 10 will do the same or similar. That’s frustrating.

[Sunshine]

That's an interesting phenomenon, i have the 3.5 and the 14mm XW, I have yet to notice such an event, there are just so many variables involved. It may be very difficult to track down

the exact cause, i hope you solve it and, if you do, please share as the 7mm is next in my collection.

[John]

8 hours ago, markse68 said:

That’s a nice pic John- id have that on my wall 😉

It must be that big curved lens that the 7, 10 and 14 have I reckon- high brightness objects cause internal reflection that beams out as secondary image moving faster through optical gearing in opposite direction. For some reason the 14 isn’t as noticeable but I bet the 10 will do the same or similar. That’s frustrating.

I find the 10mm through to 3.5mm excellent even with some optical design differences. I've avoided the 14mm and 20mm because of their field curvature which is quite different to the shorter focal lengths and does not suit certain scope designs that well:

[markse68]

Perhaps it is an eyeball reflection but then I don’t know why I’d not see it with the others. Can you get multi coated contact lenses? 😁

[Louis D]

2 hours ago, markse68 said:

Perhaps it is an eyeball reflection but then I don’t know why I’d not see it with the others. Can you get multi coated contact lenses? 😁

Perhaps on hard contacts. 😉 Don't give Zeiss any ideas on how to charge $600 for a single pair.

[Don Pensack]

17 hours ago, markse68 said:

I recently picked up a s/h but supposedly  unused XW7mm but it seems to have a problem. Or maybe not... 

when I observe Jupiter with it the image is excellent- I’m getting just about as much sharpness and detail as a 7mm ortho I think but it does this strange and quite irritating thing. It’s hard to describe but it’s like Jupiter is forming a beam of light that swishes across my eye as the disk moves across the fov and if I move my eye it kind of dances around. The 5 doesn’t do it nor the 14 or any of the others I have (I don’t have the 10 yet) I’m wondering what could be the cause? The eye lens is clean and like I say the image it produces is excellent. Anybody own the XW7 noticed something similar? I guess it could be an interaction with my scope 🤷‍♂️ Maybe it’s a reflection off my eyeball? It’s fine with less bright objects and I didn’t notice it on the moon when I first tried the ep.

Different eye lens shapes reflect back at the eye bright images that reflect from your eye.  You're seeing the reflection of Jupiter from your eye, reflected back at you from the eyepiece.  It will go all over the place if your head moves back and forth.

Other eyepieces, with different curves, may not have the same effect.

[markse68]

17 minutes ago, Don Pensack said:

Different eye lens shapes reflect back at the eye bright images that reflect from your eye.  You're seeing the reflection of Jupiter from your eye, reflected back at you from the eyepiece.  It will go all over the place if your head moves back and forth.

Other eyepieces, with different curves, may not have the same effect.

is there a solution Don? would it be scope specific?

[bomberbaz]

1 hour ago, Don Pensack said:

Different eye lens shapes reflect back at the eye bright images that reflect from your eye.  You're seeing the reflection of Jupiter from your eye, reflected back at you from the eyepiece.  It will go all over the place if your head moves back and forth.

Other eyepieces, with different curves, may not have the same effect.

I got it last night too Don, in my 26mm Nagler. It was a strange thing for sure.

[Don Pensack]

57 minutes ago, markse68 said:

is there a solution Don? would it be scope specific?

Well, better coatings on eyepieces might help.  But with a specific eyepiece, a drier eye will be less reflective.

It could help to look at the planet slightly off axis, as the reflection angle from the eye might put the light of the reflection somewhere outside your pupil.

[jetstream]

Delos...

[markse68]

5 hours ago, jetstream said:

Delos...

yes. it happens with those too apparently 😂🤪

”Yes, it happens to me too and last night I was really thinking about it.  I was using Delos eyepieces but it happens with others as well.  I think it might depend on the curvature or lack of curvature of the eyepiece top lens.  It may also matter if your eyeball is too dry or too wet.  I forget which. “ from an interesting thread on it on CN.

several people suggested moving eye closer to eye lens than normal er helped- seems to be common issue with long er  eps with large eye lenses more so than eps with smaller lenses.

actually reading that thread made me feel a bit better- i had been thinking there must be something wrong with my particular xw7 and that i should have  bought a new one but now i’m reassured that it happens with a LOT of eps from all makers and seems to be fairly inherent in long er widefield eps so i’ll just tolerate it... or get fitted with some expensive multi-coated contact lenses which surely must be the next big  thing for visual astronomy! 🤔

 

Edited June 26, 2020 by markse68

[John]

Eyepieces are all about trade offs I think. To achieve something outstanding in one area, something else needs to be compromised a little. The trick is to learn what you as an individual prefer and then to research and trial to find what suits you and what does not.

What one person feels is excellent for them will not always find favour with another person at all.

 

 

 

[JeremyS]

45 minutes ago, John said:

Eyepieces are all about trade offs I think. To achieve something outstanding in one area, something else needs to be compromised a little. The trick is to learn what you as an individual prefer and then to research and trial to find what suits you and what does not.

What one person feels is excellent for them will not always find favour with another person at all.

Well said, John! This should be a sticky somewhere!

However, it's great fun discussing the minutiae of those (personal) trade offs 🙂

 

[John]

6 minutes ago, JeremyS said:

Well said, John! This should be a sticky somewhere!

However, it's great fun discussing the minutiae of those (personal) trade offs 🙂

 

Yes it is as long as we all realize that one persons take on something won't be in any way a definitive one, it';s just one "data point" as they say.

That's one of the main reasons why I've stopped doing reviews now - I realised that what floats my boat is quite probably not going to do the same for others !

 

 

[bomberbaz]

9 minutes ago, John said:

Yes it is as long as we all realize that one persons take on something won't be in any way a definitive one, it';s just one "data point" as they say.

That's one of the main reasons why I've stopped doing reviews now - I realised that what floats my boat is quite probably not going to do the same for others !

 

 

Good point well made John. When I see people cooing over ortho's I am thinking, noooh, don't like that, couldn't be doing with the tight er and small fov. Some of those same people might think 100 degrees fov, so what. It's all very subjective.

[markse68]

16 minutes ago, bomberbaz said:

Good point well made John. When I see people cooing over ortho's I am thinking, noooh, don't like that, couldn't be doing with the tight er and small fov. Some of those same people might think 100 degrees fov, so what. It's all very subjective.

well my 7mm ortho didn’t have this issue 😉 Nice to have the extra fov and er though for sure.

[John]

3 minutes ago, markse68 said:

well my 7mm ortho didn’t have this issue 😉 Nice to have the extra fov and er though for sure.

So orthos might be better choices for you than wide fields with long eye relief and large curved optical elements ?

 

 

[markse68]

11 minutes ago, John said:

So orthos might be better choices for you than wide fields with long eye relief and large curved optical elements ?

 

 

like i said John, i don’t have this issue with my other xws so no- not generally but yes perhaps for the 7 when looking at Jupiter with this scope😉

Edited June 26, 2020 by markse68

[John]

1 minute ago, markse68 said:

like i said John, i don’t have this issue with my other xws so no- not generally but yes perhaps for the 7 when looking at Jupiter 😉

Another reason why cherry picking from ranges is better than going for the "complete set"

I had the 12mm and the 17mm ES 92's for a while. The ergonomics and eye relief of the 12 just didn't work for me but the 17mm, after some practice, I am getting used to and I know like. The 12mm went to a new home and its still a superb eyepiece. Just not for me

I've avoided the 14mm and 20mm XW's up to now because of the reported field curvature but it might be that they are absolute gems for me if I do try them one day

Only one way to find out ......