Orthoscopic Low powered eps

[Rogering1]

Hi guys,

On the weekend at a star party I used some of my low powered GSO plossls, they gave great views of the moon with the 25mm & 15mm.

Recently I've been on a mission to replace the Plossls, because I wanted to do more DSO galaxy hunting, of course you need low power good light transmission (Plossl) and good contrast.

I've given thought to Panoptics, Nagler 17,20 & 22 & Pentax xw 20 and all the problems associated with low powered wide field observing with a fast Newtowian (f5) Weight, Coma, pincushioning, field curvature (pentax 20mm) .......

Anyway I was speaking with an experienced astro guy and I was asking about Plossl light throughput (must be good eh!) he suggested that Ortho's were better Pin sharp views he had some second hand Carton Orthoscopic 20mm & 25mm he did say they were 57 FOV.

He has offered the 25mm & 20mm for $50 each

Question is what's the eye relief of low powered orthos & what do you think?

[nightfisher]

I have a 25mm circle T ortho, and find it just a sweet thing to look through, very sharp and clear,the eye relief is quite long for an ortho

[faulksy]

bgo eye relief is roughly the same as the focal length

[Spec-Chum]

57 deg ortho?

I thought abbe's design was 40-45?

[Rogering1]

He said it was a Carton Ortho, maybe he meant to say 45

[E621Keith]

Be careful, if the field was 57deg, it's probably a plossl. Orthoscopic is a performance specification, and any design that meet these specifications can be called an ortho. In some cases, firm would label Plossl that meet these specs ortho. You need to check if they are Abbe or other types of orthos.

Vixen used to label some circle V plossl ortho, and Pentax XO is barlowed plossl rather than a Abbe.

Abbe's eye relief is usually 80% of it's focal length, so the 25mm should be quite comfortable to use.

[michael.h.f.wilkinson]

My Circle-T 25mm was the only ortho I could work with (20mm eye relief). It was fine for DSO work in my old 6"F/8 Newtonian.

Regarding light throughput: Plossls and Abbe Orthos have the same number of glass/air interfaces, and so have the same degree of light loss for the same quality of coatings. Note however that most modern coatings have reduced the problems of throughput hugely, and a Pentax XO, XF, or XW has a much higher throughput than an old (pre 1935), uncoated monocentric. Do not forget that we are talking about tiny fractions of a magnitude loss (less than 1%, or less than 0.01 mag). The number of groups in the design is not nearly as important as it used to be.

I find FOV much more important in DSO hunting. A large apparent FOV means I can see the same part of the sky at a smaller exit pupil, giving a darker sky background. This makes finding extended sources far more easy. I would never exchange my two galaxy hunting workhorses (the 22T4 and 17T4) with corresponding Abbé orthos

[John]

I saw some light throughput tests done on many eyepieces types a couple of years back. Some of the premium quality complex types (eg: Naglers) actually had higher throughput figures than simpler types such as the classic circle-T orthoscopics. As Michael says modern coating technology and glass types have revolutionized what is possible.