Eyepiece eye relief

[Joe G]

Hi all

Ive tried getting information on this query online but with no luck so I’m hoping someone with more practical experience can help me out. Without going into the reason for now why I’m asking this, I’ve just got a near new Pentax XW14 which has the twist up eyecup to help with eye placement & Pentax has stated it gives 20mm eye relief. My query is does 20mm eye relief remain constant as the eyecup is raised from the lowest point to the highest point. Hopefully that makes sense & thanks in advance of any feedback Joe

[John]

The eye relief is measured from the centre of the top surface of the top lens in the eyepiece. That does not change whatever the position of the eye cup is. What does change is the amount of that eye relief that is actually useable because the eyecup positioning eats into the 20mm more and more as it is raised.

So those who wear glasses and need more eye relief will find having the eye cup at or close to, the lowest position works best. Those who don't wear glasses will generally find that the eyepiece works best with the eyecup at, or close to, the highest position. Pentax's own instructions for the XW eyepieces actually state this.

 

 

Edited May 7 by John

[Bivanus]

Like in the atached pictures. The numerical value of ER does not change , however , the cup positioning matters.

Edited May 7 by Bivanus
Text added

[Joe G]

Hi John & Bivanus

Thanks for the quick responses. I guessed that the eye relief would vary but actually in the opposite direction 😵‍💫However, as long as it works some way it’ll help resolve the present dilemma I’m going through. I have a small collection of eyepieces ( see attached photo) I also had a Pentax XF8.5 which I just sold to help fund the XW14 one & a Baader Hyperion 17mm which I mainly use in my 8” dob which I’m hoping to replace with the Morpheus 17.5mm. I also have a couple of decent x2 barlows 

I’m looking for a premium x2.5 accessory that would my 25, 17.5 & 14 approximately 10, 7 & 5.5 respectively. My two are the TV 2.5 PowerMate or the Baader VIP Barlow +extension to achieve this. The TV doesn’t affect eye relief but the VIP does but the adjustable eye cup will compensate for that as I don’t need glasses. I will mainly use this accessory in a 4” refractor F9 & getting to 70 years old I just enjoy the views & now I know both will work. Again apologies for the long email & I’m now going to check out the online reviews before I decide thanks Joe 

 

[John]

I've owned both the 2.5x Powermate and the Baader VIP. Both excellent. 

I have the XW's in 3.5, 5, 7 and 10mm but went for a TV Delos at 14mm and above because of the reports of field curvature with the 14mm and 20mm XW's.

Not so much of an issue in an F/9 refractor though.

[Mr Spock]

I use a x2.5 Powermate. Apart from the increased magnification you can't tell it's there 

[Bivanus]

I am currently in my TV phase of Nagler flavor  😅 so I would steer you towards the PowerMate which is actually a Tele-Extender (this being the reason why it doesn't affect ER) . Funny enough , I don't own a PowerMate but the ES equivalent at 3x in 2" format 😄 

[Joe G]

Hi all 

I have plenty of time before the planet season arrives to do my research. Now I know I’ll be happy with either I’ll keep an eye out on the “ for sale “ sites for a nice used sample. Again thanks to everyone for the knowledgeable feedback 

[Joe G]

Things have moved along nicely since my last post. The final two “as new” upgrades arrived this week _ a Morpheus 17.5mm & a x2.5 PowerMate. It’s a tall combination but they’ll be mainly used for lunar & planetary viewing, which thankfully are high in the night sky from now onwards. Anyway, again thanks for the advice and feedback Joe 

[Bivanus]

That would be a 'Long Eye Relief' 7mm combo , perfect for planetary and some Moon riding 😄

I use a similar 'Long Eye Relief' 10mm ( APM UFF 30mm + ES 3x / 2in) and it's swell on both planetary and the Moon.

Edited May 24 by Bivanus