APM UFF 30mm

[25585]

What does RD stand for?

I can understand weight issues but only use my 30+mm eps as finder unless a big FOV is needed. 

The big Panoptics at F5 do benefit from a Paracorr. Not only for coma but almost as though it's the "missing lens" in their design AFA Newtonians for perfecting what you see. More weight there too using one.

Tempting to try the APM but as has been said their uses are limited at that FL, and my Panoptics need to give their money's worth. 

[Piero]

RD stands for rectilinear distortion.

 

57 minutes ago, Littleguy80 said:

I certainly want to get the 13mm APM before anything else. Perhaps, as you suggest, a 30mm eyepiece is going to be a lot more specialised than I think. I kept thinking about the larger exit pupil (6.35mm) with filters and fainter large nebulae. It’s a 4.23mm exit pupil with the 20mm though. That feels like it should be large enough for most targets. 

It all depends on your target preference and how dark your sky is. 

For me a 30mm is handy because I like F6 ish telescopes. The other thing is that I tend to use 2 eyepieces per session:

- 20 Lunt, Zoom + barlow;

- 42mm LVW, docter +/- barlow.

The 2 eyepieces in each pair have very similar weight and size.

The 30 would replace the 42. It wiould also work well on my f6 Dobson.

[Louis D]

1 hour ago, 25585 said:

What does RD stand for?

Rectilinear distortion, I think.  It's the flip side of AMD, angular magnification distortion:

To quote https://www.handprint.com/ASTRO/ae4.html#distortion:

Distortion is characterized in two ways. In positive or rectilinear distortion, the magnification increases toward the edge of the field: this causes straight lines to appear curved outward, even when the image is stationary. In negative or angular magnification distortion, the magnification decreases toward the edge of the field: this causes straight lines to appear projected onto a spherical surface curved toward the viewer, which is especially noticeable when the optical axis is moved — the image then appears to "roll away" near the field edge.

[Piero]

To add an example, upon RD the Moon appears like an oval near the edge, whereas upon AMD, it appears more spherical albeit smaller. 

[25585]

Presumably a scope's optics and design play a part in what and how much distortion? If an eyepiece has the same it would make it worse, but opposite type help reduce the distortion?

Also perhaps, coma correctors, barlow/extenders, and prisms can make a difference?

Why reviews should always say what the whole optical arrangement specifications are, a component is being tested as part of.  

[Piero]

Richard, as far as I know RD and AMD are eyepiece distortions, and therefore are not related to the telescope. In astronomy, TeleVue (primarily) and other brands tend to minimise AMD and other aberrations such as astigmatism within a certain threshold, accepting a substantial increase in RD (RD and AMD are inversely related as Louis mentioned). The idea is to preserve distances between stars in a star field when these move from on to off axis. As a side effect, large objects like the Sun, Moon or planets can appear elongated near the edge. Conversely, many spotting scope eyepieces (terrestrial observation) favour a certain amount of AMD. Interestingly, some of terrestrial eyepieces have been used in astronomy with excellent results (e.g. Leica ASPH zoom, Zeiss D Vario zoom, Docter UWA, ...). Personally, I like eyepieces that accept a compromise between RD and AMD, as the view seems more natural to my eye. Other members find AMD intolerable instead.

 

Coma is an aberration of a telescope, particularly fast newtons, and eyepieces do not correct it. Hence one needs a coma corrector for this. 

Astigmatism can be introduced by both the eyepiece and the telescope optics.

Barlow / extenders change the focal length of the telescope. They don't affect RD or AMD.

A prism can affect chromatic aberration.

[jetstream]

2 hours ago, Piero said:

Richard, as far as I know RD and AMD are eyepiece distortions

 

3 hours ago, 25585 said:

Presumably a scope's optics and design play a part in what and how much distortion?

http://www.holgermerlitz.de/globe/distortion.html

 

Good ol' Holger

[jetstream]

Just now, jetstream said:

 

http://www.holgermerlitz.de/globe/distortion.html

 

Good ol' Holger

ps- how about our eyes...

[25585]

Thanks guys, some good reading there for me to study ?

I know some on SGL use microscope eps, especially for bino viewing. How do those suit astronomy viewing? 

[25585]

7 hours ago, Louis D said:

Rectilinear distortion, I think.  It's the flip side of AMD, angular magnification distortion:

To quote https://www.handprint.com/ASTRO/ae4.html#distortion:

Distortion is characterized in two ways. In positive or rectilinear distortion, the magnification increases toward the edge of the field: this causes straight lines to appear curved outward, even when the image is stationary. In negative or angular magnification distortion, the magnification decreases toward the edge of the field: this causes straight lines to appear projected onto a spherical surface curved toward the viewer, which is especially noticeable when the optical axis is moved — the image then appears to "roll away" near the field edge.

Of those 2, RD seems less bad. This may be because I am used to that from my glasses lenses edges..... and why I am not bothered by looking through my Panoptic 35mm. ?

[Piero]

On 30/06/2018 at 00:20, 25585 said:

Of those 2, RD seems less bad. 

 

in my opinion it depends on what you are looking at. For double stars, less AMD is welcome as this makes the view more consistent across the field (the separation between stars doesn't change much). For Lunar/Solar and even planets, less RD is welcome unless you like to see those targets with an elongated shape.

[Piero]

Okay, my APM-UFF 30mm arrived today. 

Oh man, if it's tiny! Okay, not really tiny, but I expected it to be a bit bigger. I will report when I get a chance.