Pentax XW 30mm and 40mm Return

[Louis D]

3 hours ago, rl said:

One minor minus...I have short sight and this meant the eyepiece worked with the focal plane almost on the front surface of the field lens...any dust particles were all too obvious if it was used on the moon, as occasionally happened with a f/15 scope. A total non-issue on deep-sky though..or if I used specs. 

Doesn't this also lead to a soft field stop since you have moved the focal plane away from the physical field stop location?

[rl]

Yes, but I guess I'm used to that....

Strangely enough not all eyepieces do that. Don't know why.

RL

[John]

44 minutes ago, rl said:

Yes, but I guess I'm used to that....

Strangely enough not all eyepieces do that. Don't know why.

RL

With the 40mm Pentax XW, there is no lower lens set within the 2 inch barrel wheras with all the other XW's there is. So the field lens of the 40mm is a large heavily concave lens wheras with the 30mm (say) it's a convex lens further down the barrel. The field lens surface with the 40mm is also closer to the focal plane of the eyepiece / field stop position. This design difference may well explain why you can see these dust particles on the field lens of the 40mm XW when observing an extended illuminated target. With the other XW's that large lens surface is an internal one so not prone to picking up dust.

 

[rl]

Thanks John..

My comment was directed more to Louis' point about the field stop being in focus. The field stop is generally set up to be sharp for an eye looking to infinity...for a short-sighted person such as myself the correct position would be further in towards the eye...hence I see the dust in-focus and the field stop slightly blurred. 

But eyepieces like the Ethos, Delos, ES17/92, all seem to have a sharp field stop regardless! Maybe it's something to do with the Smyth lens. Or a little piece of magic from Unk Al. 

The view in a few eyepieces completely falls apart when used with uncorrected vision...ES25/100 is by far the worst example in my experience

But we're going off topic....apologies

[John]

It's a very interesting topic I think. We all have different eyesight prescriptions and these will affect how eyepieces work with our eyes on a person to person level. The eyepiece designer has to produce a generic design that works with a wide range of eyes but in some cases falls short, unfortunately. This probably accounts for why eyepiece choices and preferences are very personal.

Tele Vue recognised this and has produced accessories such as the Dioptrx to help and those can be used with brands other than Tele Vue in some cases. I'm not aware of other eyepiece brands having developed such things but maybe they have ?

 

[rl]

I do wonder whether designers ray-trace their designs for a wide range of eyeball prescriptions. Maybe the acceptable range should be in the spec.

[Don Pensack]

On 20/12/2019 at 12:53, Louis D said:

I'm good with my 30mm APM UFF at 30mm.  I've found at least one comparison that liked it better than the XW except in comfort.  Since I find the UFF comfortable enough, I'll stick with it.

 

+1 for the 30mm APM UFF in my f/5 scope.  I have to use glasses at this exit pupil, but correction is excellent and glasses were usable with the eyecup folded down.

It was just a little sharper and had slightly better contrast than the 30mm XW in the outer 10° of field.  I rated it equal to the 31mm Nagler except for apparent field.

It is NOT compatible with a TeleVue DioptRx, so if you need astigmatism correction, you'll need to use glasses or contacts.

[Louis D]

1 minute ago, Don Pensack said:

+1 for the 30mm APM UFF in my f/5 scope.  I have to use glasses at this exit pupil, but correction is excellent and glasses were usable with the eyecup folded down.

It was just a little sharper and had slightly better contrast than the 30mm XW in the outer 10° of field.  I rated it equal to the 31mm Nagler except for apparent field.

It is NOT compatible with a TeleVue DioptRx, so if you need astigmatism correction, you'll need to use glasses or contacts.

For me, the more relevant question is would the $400 40mm Pentax XW be noticeable better than my decloaked 40mm Meade 5000 SWA that I picked up for $125 when they were closing them out.  It's not quite astigmatism free across the field, but it's way better than the sub-$100 modified Plossls, SWAs and Erfles I had been using up to that time.  See the last four eyepiece AFOV images below to see what I mean.

 

Is the 40mm Pentax XW sharp across the field like the 17mm ES-92?  Since they're similarly priced, I would hope so.

[BGazing]

On 09/12/2019 at 17:27, John said:

Personally I'd be interested to know how the 40mm XW-R compares with my Aero ED 40mm which cost me £50 used.

+1

And then there's APM UFF 30mm for half the price of Pentax XW 30mm.

Edited March 3, 2020 by BGazing

[Louis D]

7 hours ago, BGazing said:

+1

And then there's APM UFF 30mm for half the price of Pentax XW 30mm.

Which, per Don Pensack below, is slightly better than the XW.  It's not surprising given that the design is over 15 years newer.

7 hours ago, Don Pensack said:

+1 for the 30mm APM UFF in my f/5 scope.  I have to use glasses at this exit pupil, but correction is excellent and glasses were usable with the eyecup folded down.

It was just a little sharper and had slightly better contrast than the 30mm XW in the outer 10° of field.  I rated it equal to the 31mm Nagler except for apparent field.

It is NOT compatible with a TeleVue DioptRx, so if you need astigmatism correction, you'll need to use glasses or contacts.

Here's an excellent comparison of the edge correction of each.  I've linked the images below:

30mm Pentax XW:

30mm APM Ultra Flat Field:

Not only does the UFF have negligible chromatic aberration at the edge, it has negligible distortion as well.  This agrees well with my own observations and in my test photos below:

[Piero]

As I said before.. I am going to stick with my trusty APM UFF 30mm.. 

[Don Pensack]

Re: a comparison of distortion characteristics between the 30mm APM UFF and the 30mm Pentax XW:

The XW has a very small amount of positive angular magnification distortion, so is like most astronomical eyepieces.

The APM UFF has a very small amount of negative angular magnification distortion, so is a little more unusual (far from unique).

Both, however, have been designed to control AMD tightly, so both show pincushion rectilinear distortion in daylight use.

It's obvious both were designed for astronomy use, as opposed to daytime use in spotting scopes.

[Piero]

On 03/03/2020 at 19:24, Don Pensack said:

+1 for the 30mm APM UFF in my f/5 scope.  I have to use glasses at this exit pupil, but correction is excellent and glasses were usable with the eyecup folded down.

It was just a little sharper and had slightly better contrast than the 30mm XW in the outer 10° of field.  I rated it equal to the 31mm Nagler except for apparent field.

It is NOT compatible with a TeleVue DioptRx, so if you need astigmatism correction, you'll need to use glasses or contacts.

Thanks for your information, Don. 

May I ask you what is the TV Paracorr 2 setting for the 30mm APM UFF, please?

[jetstream]

59 minutes ago, Piero said:

May I ask you what is the TV Paracorr 2 setting for the 30mm APM UFF, please?

Where did the 30mm UFF focus in comparison to the 20mm Lunt HDC?

[Piero]

3 minutes ago, jetstream said:

Where did the 30mm UFF focus in comparison to the 20mm Lunt HDC?

I don't know and won't be able to test this as I sold my 20mm Lunt HDC as 100 deg is too much for my likes and I tend to jump from 30mm to 12.5mm.

The APM UFF 30mm is quite close to the Docter if I remember correctly, but I've never measured the distance. I'd be interested in knowing the TV paracorr2 setting for the 12.5mm Docter UWA too.

[jetstream]

3 minutes ago, Piero said:

I don't know and won't be able to test this as I sold my 20mm Lunt HDC as 100 deg is too much for my likes and I tend to jump from 30mm to 12.5mm.

The APM UFF 30mm is quite close to the Docter if I remember correctly, but I've never measured the distance. I'd be interested in knowing the TV paracorr2 setting for the 12.5mm Docter UWA too.

Next time the clouds disappear I'll check- we had 7" of snow the other day.

[John]

I'm going to stick with my Nagler 31mm and Aero ED 40mm in these focal lengths I think. I don't use 40mm eyepieces that much but the Aero ED seems good when it's been used, even in my F/5.3 dob. The best one in the Aero ED series I think.

I don't actually use the 31mm Nagler that much but I know I'd miss it if I parted with it. Most of my low power observing is done with the 21mm Ethos.

 

[Ships and Stars]

45 minutes ago, John said:

Most of my low power observing is done with the 21mm Ethos.

I checked astronomy tools FOV calculator as I like the APM EPs and was thinking about selling my 31mm Baader for the 30mm UFF after reading a bit about it, but noticed the 21E's working FOV is identical to the 30mm, so not really any gain for me, though perhaps nicer sometimes to have everything framed in a straight on  70deg view vs 100deg with a bit of head rolling to take it all in. 

I love the 21E but just picked up the 20mm Lunt UWA and can't decide which one to part with!

[John]

I go for the wider field every time when using my 12 inch dob, personally. I don't find that I need to roll my head to see the 100 degree field but thats not the same for all I realise.

I've had galaxy spotting sessions where the 21mm Ethos is the only eyepiece I've needed. I had the ES 20mm 100 before it but could not resist acquiring the Ethos 21 despite the ES being pretty good itself.

I have some light pollution to contend with which is why the 21 Ethos gets more use than the 31 Nagler. Darker background sky.

 

 

[Ships and Stars]

3 hours ago, Piero said:

Thanks for your information, Don. 

May I ask you what is the TV Paracorr 2 setting for the 30mm APM UFF, please?

Have you tried the permatrace drafting film trick for finding the right CC setting? I don't have the Paracorr, but with my ES HR CC instructions, you set the CC in the 'neutral' position with the helical mechanism rotated out at 13.5mm, tape drafting film (or frosted tape, but prefer drafting film) across the CC's EP opening, then use the focuser to move the CC in or out on the moon or a particularly bright star until sharp focus is reached on the film, kind of like a projector screen. Then lock focuser and don't move it. Next insert your (30mm, 12.5 EP etc) and screw helical top in or out until sharp focus is reached through the EP and note settings. That is the correct CC setting for that particular eyepiece.  Sounds a bit complicated, but only needs to be done once.  

Having said all that, the P2 might be a totally different creature and only have click settings and not fine adjustment? Never used one, but thought the Paracorr and HR CCs were quite similar overall, as ES seems to copy TV a bit.

Might work if there's no reliable setting info on the UFF/P2 combo?

23b024e8c3e70b64385a1713de38290d.pdf

[Piero]

22 minutes ago, Ships and Stars said:

Have you tried the permatrace drafting film trick for finding the right CC setting? I don't have the Paracorr, but with my ES HR CC instructions, you set the CC in the 'neutral' position with the helical mechanism rotated out at 13.5mm, tape drafting film (or frosted tape, but prefer drafting film) across the CC's EP opening, then use the focuser to move the CC in or out on the moon or a particularly bright star until sharp focus is reached on the film, kind of like a projector screen. Then lock focuser and don't move it. Next insert your (30mm, 12.5 EP etc) and screw helical top in or out until sharp focus is reached through the EP and note settings. That is the correct CC setting for that particular eyepiece.  Sounds a bit complicated, but only needs to be done once.  

Having said all that, the P2 might be a totally different creature and only have click settings and not fine adjustment? Never used one, but thought the Paracorr and HR CCs were quite similar overall, as ES seems to copy TV a bit.

Might work if there's no reliable setting info on the UFF/P2 combo?

23b024e8c3e70b64385a1713de38290d.pdf 1.66 MB · 1 download

Hi Robert, I don't have the TV paracorr2, but would like to assess whether my current equipment has problems with it or not. Thanks for the links regarding the ES HR CC.

Edited March 7, 2020 by Piero

[Ships and Stars]

54 minutes ago, Piero said:

Hi Robert, I don't have the TV paracorr2, but would like to assess whether my current equipment has problems with it or not. Thanks for the links regarding the ES HR CC.

Ah ok, not sure what scope issues you had, sorry. Going off piste a bit (more) here, but I think I've reached the point with careful collimation, mirror(s) very close to or at ambient temperature and a well-adjusted coma corrector with decent eyepieces for fast scopes, that any optical performance shortcomings are most likely due to my astigmatism, which I've read becomes more noticeable as exit pupil increases, i.e. longer focal length EPs. I never wear eyeglasses when observing, but will try contact lenses the next time I have a clear night to see if things are any sharper, but they're fairly good now. I should take a closer look (no pun intended) at dioptrx, but am in the process of switching to mainly Lunt/APM EPs so perhaps a bit late on that.

Of course this is assuming mechanical things like the mirror cell, primary and secondary are all happy. I'm sure you know more about star tests and airy discs than I do, so won't bother mentioning that.  

Edited March 7, 2020 by Ships and Stars

[Piero]

6 minutes ago, Ships and Stars said:

[..] any optical performance shortcomings are most likely due to my astigmatism, which I've read becomes more noticeable as exit pupil increases, i.e. longer focal length EPs. I never wear eyeglasses when observing, but will try contact lenses the next time [...]

I don't want to hijack this thread, so I will answer this, but I'd suggest to move this conversation to a separate thread as it is clearly off-topic. 

To test whether astigmatism is due to your eye or the telescope, a simple tool is the magnification. Assuming that the eyepiece in use is astigmatism-free, an increase in astigmatism at low power is more likely due to one's eye, whereas an increase in astigmatism at high power is more likely due to the telescope.

Life can be hard for the amateur astronomer though.... issues with the mirror cell, for instance related to mirror lateral supports or triangles can show astigmatism, but also spherical aberration. When minor, the signature of the latter can be difficult to analyse whether due to the supports or the mirror changing temperature.   

[Ships and Stars]

Thanks for that, I shall try that test next time I'm out. True, there are many overlapping optical/mechanical issues that can mask one another. Gently steering back onto Pentax, I was always a big Pentax camera/lens fan so kind of surprised that when I became interested in astronomy, I never picked up any XWs despite the great reviews overall. It was always a matter of what was on the second-hand market, and more TV Ethos, etc came up than XWs when I was doing most of my buying. I guess that says a lot for Pentax. I'd love to get my hands on the 30mm, but at this point will wait until I'm observing with someone who has one, or less likely, a used one comes up for a good price. As my eyes worsen, and presumably they will, I may have to give up the non-TV 100deg EPs some day and go for enough eye relief to accommodate eyeglasses, so Pentax will come back onto my radar then.

Edited March 7, 2020 by Ships and Stars

[John]

My Pentax XW story is a little different. FLO loaned me the 10mm XW and then the 30mm XW a few years back to compare them with the Naglers that I was using back then. Rather to my surprise I found that I slightly preferred the 10mm XW performance to the 9mm Nagler T6 that I had despite the reduction in AFoV. 

What I wanted though, being still a "wide field junkie", was the neutral tone, sharpness and lack of light scatter of the XW's combined with a very large field of view. Unfortunately for my bank balance the Ethos range came along ......