Pentax XW 5mm quick first light

[JOC]

Perhaps I am just not dedicated enough, but no matter how interesting I find what I see there is no fun in standing outside freezing my bits off just because its a rare clear night.  Hence, it has taken rather longer that I might have liked to try out some of the newer acquisitions in my, somewhat eclectic , EP collection.  One early evening over the recent holiday I had the time and darkness and a clear sky early in the evening and decent bit of the moon up.  I didn't get long before I froze and came back indoors, but I did get a fairly decent look at the moon through the Pentax XW 5mm and got a chance to share with my mum for a few minutes.  I don't know if perhaps the scope hadn't cooled sufficiently (though it did have about 20 mins and is kept in an unheated porch anyway), but there was a certain amount of 'movement' in the image.  However, that notwithstanding I had a play with all the interesting bits I'd bought and in particular got a look through the much anticipated Pentax XW 5mm.  Now it got us in lovely and close and image was nice and crisp across the whole width of the EP, but what I noticed was that in comparison with some of the other EP's the image seemed lacking in, well lets call lit, 'brightness'.  The moon is generally a lovely bright subject, but more so than any of the other EP's - and I do have an 8mm in the collection, the image seemed significantly darker using the XW 5mm.  the image was sharp, we could see into the bottom of the craters, but a def. amount of brightness/light call it what you will was missing from the image.  Has anyone else noticed that?

[John]

The 5mm XW is probably my favourite high powered eyepiece on the planets and the moon. It's better than the 6mm Ethos and the Nagler zoom in terms of sharpness and brightness. No complaints about any of the XW's from me. Their transmission rate is 96% which is pretty high. Radian's, for example, range from 88% to 93%

If it's not for you though, I'm sure you will have no problem moving it on.

 

[JOC]

Hi John

I just wondered if the apparent lack of brightness was a feature of the very high magnification, i.e does it follow with a high magnification EP that because it sees less of an object there is less area to gather light from and therefore the image could show less apparent brightness, i.e. am I looking for it to deliver light that just isn't available, i.i.e. is there a practical reason why a high mag. EP might give the impression of lower brightness?

[John]

As you increase the magnification the brightness does decrease as the image scale increases. Thats why observing the moon at high powers is mentioned when folks feel they need to use a moon filter.

Don't ask me about the maths behind this though, or I'll start to whimper

 

[JOC]

9 minutes ago, John said:

As you increase the magnification the brightness does decrease as the image scale increases. Thats why observing the moon at high powers is mentioned when folks feel they need to use a moon filter.

Then perhaps this is exactly what I was seeing.  The thought that it made the moon more comfortable on the eyes was a passing consideration and indeed if it is a factor that results in a suggestion that it might alleviate the need for a moon filter I can see why this might be the case from my own experience.  There was certainly nothing wrong with the view - lots of detail and a real close up look into the bottom of the craters - mum was thrilled with what she saw, just this apparent lack of brightness that made me wonder - I sort of the guessed the high mag. had something to do with it, but it is nice to get this confirmed and it means there is no intrinsic fault with the EP :-D  I shouldn't think this is such an issue with a star or planet as you are still fitting the whole of the brightness into the field of view, but with a big object like the moon getting in close means you are looking at significantly less of it and as we are discussing there seems to be less light available from this.  I will have to see what I think when I am able to try a planet or a star which will fit completely inside the FOV even at high magnification - so far my first target has only been the moon. 

Thanks for the input John, it has confirmed that there is a legitimate reason for what I experienced which is a good thing to know.

[JOC]

s'funny I've just been browsing for browsing's sake and the next thread I pick up about 'why so many mm incremental lengths' and a poster called Lowjiber made exactly the same observation as you did John.  to whit:

Quote

That pretty much sums it up, especially when galaxy hunting.  Because their surface brightness can have a wide range of variance, a small change in eyepiece length can bring out details.  However, too much of a change can make the details worse (dimmer).  As magnification increases, surface brightness decreases

 

[Ruud]

John is right, of course. When the magnification becomes n times as big, the illumination decreases by n^2.  (The image becomes n times as wide and n times as high, so it's surface is proportional to n squared, and the brightens inversely proportional to the same factor.)

The sensitivity of the eye is logarithmic. That complicates matters a little.

Did you know that a telescope never shows a surface brighter that the naked eye? It will show a lot bigger surface, but not brighter. Maximum surface brightness is reached when the exit pupil matches the observer's pupil. That maximum brightness is the original brightness of the surface.

I've included a pdf which I made a while back. It shows surface dimming in magnitudes as a function of the exit pupil. What we're discussing here is shown in the left hand sides of the graphs. Small exit pupils give a lot of dimming. The right hand side deals with a different kind of dimming: that which occurs when the shadow of a central obstruction in the exit pupil begins to fill the observer's pupil, when the exit pupil becomes bigger than the observer's pupil.

The lowest point in each graph corresponds to 'least dimming'. The graphs assume 100% transmission.

A refractor has no central obstruction. For a refractor the image reaches maximum brightness when the exit pupil gets as big as the observer's, and stays as bright from then on.

With a reflector, the shadow of the central obstruction grows with the exit pupil, eventually filling the observer's pupil. The observer will notice it as a shadow. With extremely large exit pupils, the observer could move his eye sideways to place his pupil in the illuminated portion of the exit pupil. The graphs in the pdf assume that the observer keeps his eye in the centre of the exit pupil

The graphs show dimming in magnitudes: Dimming from Magnification and CentralObstruction.pdf

[Louis D]

3 minutes ago, Ruud said:

Did you know that a telescope never shows a surface brighter that the naked eye?

And that is why image intensification is growing so popular among high end amateur astronomers.  No matter how large the telescope, a dim nebula is still dim, just bigger.  So if a nebula is already large (think Barnard's Loop) and dim, you're never going to see it naked eye without some photonic integration/amplification/multiplication help.

[Ruud]

Congrats with the eyepiece, JOC. It has an enormous reputation for sharpness and comfort!

[Louis D]

@JOC Try your new 5mm XW on a large, bright globular cluster when one is well placed for you.  M13 or M15 would be a good choice.  M22 is also good for me, but I think it's always too far south for UK observers.  I love looking at these GCs through my 5mm XL.  It breaks them up into tiny sparkling diamonds on black velvet.  The smaller GCs remain fuzz patches for me, though.

[JOC]

3 minutes ago, Ruud said:

Congrats with the eyepiece, JOC. It has an enormous reputation for sharpness and comfort!

Thank you Ruud - I was very lucky to get it from an SGL'er that had a bill to pay!  Also, many thanks for the explanation above.  I am glad I raised the issue that I'd seen now.  To begin with I thought 'Nah, it's just me being silly - I mustn't appear to criticise the performance of what I know is an excellent EP - they will start shouting 'heresey''.  However, I actually tried it out twice and thought 'I dunno there's summat going on here that I need to know about.  In a way I actually see it as a good thing now for two reasons.  1) I was seeing what was really happening and had sufficient experience to now be aware of a subtle difference in what I could see, and 2) I've now learned about why it happens :-) 

I can't wait to get it zoned in on a planet!!

[Ricochet]

1 hour ago, JOC said:

I will have to see what I think when I am able to try a planet or a star which will fit completely inside the FOV

It has nothing to do with this. You can consider what is happening in two ways. Let us use both your 5mm and the 10mm XW for these examples. 

Firstly, consider that you are looking at the moon with the 10XW and you happen to see a square crater with an apparent width and height of 1°. This crater has an area of 1 square degree. Now you switch to your 5XW and look at the same crater. You have doubled the magnification so now the width and height of the crater must appear to be 2°. The apparent area is therefore 4 square degrees. 

The light from the sun that is reflecting off the crater, through your telescope and into your eye is the same in both examples, but in the 5XW that light is spread over 4x the area and so the image brightness is only 1/4 of the brightness using the 10XW. 

The second way to think of it is in terms of the exit pupil provided by each scope/eyepiece combination. The exit pupil using the 5XW is only 1/2 the of the exit pupil using the 10XW so the area of the 5XW's exit pupil is only 1/4 of that of the 10XW and again the image is only 1/4 as bright. 

This only applies to extended objects like the moon, planets, nebulae and the sky background. Stars (except the sun) appear as point sources and so their brightness depends on the aperture of your telescope. Increasing the magnification reduces the brightness of background but not the stars, increasing contrast and allowing you to see fainter stars. The limit for this is the point at which the airy disk becomes apparent. Once light appears to be spread between the star and its airy disk the star will appear to dim with increasing magnification. The point at which this happens is when you have an exit pupil of approximately 1mm. 

 

Edit. I am really too slow at typing. Looks like this was already covered. 

[Timebandit]

 

 

The moon always seems pretty bright to me whatever eyepiece  I use, within reason ?

The 5mm XW I have always seems to have good light transmission through it when looking at the lunar surface. And the Pentax 3.5XW when conditions allow also has very good light transmission for a short focal length. Having put a few other eyepieces against the Pentax XW in the past to try and compare light transmission as well as sharpness. Then the Pentax XW always is up there with the best. 

 

  

[25585]

Perhaps a Pentax zoom would be good for lunar.?

[Alan White]

The 5mm was at 240x magnification too JOC in your scope, the seeing may not have been able to support such a magnification when you gave it a run out.

The XW you have was the brother / sister to my 10mm when we bought them, the XW is highly rated, all 5mm images will be darker than a 8mm or 10mm one.
If you ever wish to bring the family back together and reunite the siblings, keep me in mind please.

If you don't like the cold, you can always buy multilayers of thermals and decent boots.
If you are not wrapped up like Scott of the Antarctic, you are under dressed.

[JOC]

19 hours ago, Alan White said:

If you ever wish to bring the family back together and reunite the siblings, keep me in mind please

Well if we ever do some observing together I'm sure we can easily share :-D , but {Smeagol mode enabled} it's mine......all mine!! {/Smeagol mode enabled} LOL

My experience with dressing like Scott of the Antarctic usually goes something like:  Clear outside - yes!  Scope out to cool, me inside - half hour of wriggling into suitable quantities of layers and jumpsuits, grab a set of EP's dash outside - cloud cover as far as the eye can see - come inside decide to see if it goes away, roast for the next 40 minutes (deeming it not worth getting undressed), dash back outside, clouds still there, bring everything back in and get undressed and find the TV guide!

"The 5mm was at 240x magnification too JOC in your scope, the seeing may not have been able to support such a magnification when you gave it a run out"

^^^ This is a good point, I'm certainly not giving up on it by any means - the high magnification is too tempting for that, but I am given to understand that it should be within the limit of my telescope, but as you correctly note that is only part of the equation.  In the UK decent seeing is def. the other - I bet I end up having some good sessions with it with conditions allow.

[JOC]

On 10/01/2018 at 22:21, 25585 said:

Perhaps a Pentax zoom would be good for lunar

Not that I'd go down that route, but in the vein of this thread would a zoom pushed from from minimum to maximum zoom also suffer a light loss as the magnification changed?

[25585]

45 minutes ago, JOC said:

Not that I'd go down that route, but in the vein of this thread would a zoom pushed from from minimum to maximum zoom also suffer a light loss as the magnification changed?

AFOV increases so that might make a difference ?

[Ricochet]

49 minutes ago, JOC said:

Not that I'd go down that route, but in the vein of this thread would a zoom pushed from from minimum to maximum zoom also suffer a light loss as the magnification changed?

Of course. The brightness depends on the exit pupil. As you change the focal length of the zoom you change the exit pupil and brightness. 

[Louis D]

2 hours ago, Ricochet said:

Of course. The brightness depends on the exit pupil. As you change the focal length of the zoom you change the exit pupil and brightness. 

The only way it wouldn't would be if it was an actively amplified eyepiece using image intensification.