Tele Vue NP101 no longer produced?! ?

[Cleetus]

Pretty innocent comment really but if you insist on throwing a tantrum then be my guest. My last comment in this thread. Have a nice day!

[Joves]

Haha... sounding like most Cloudy Nights threads. No offence taken, no tantrum thrown. Now, where is my dummy?!

[MNJay]

11 hours ago, Stu said:

An APM 105mm LZOS f6.2 is another option of course, just a ‘little’ more pricey and not as fast; even then I don’t know if it suffers from field curvature at all.

I think technically all scopes have field curvature, it just depends how apparent it is. The relatively short FL of 650mm and fast FR of f/6.2 means you will notice some defocused stars at the edge of the field. However, I had mine out last weekend at a dark site, first time since November, and was so enamored with seeing the Veil without a filter (and then with a UHC, WOW!) that I honestly didn't notice any FC. I used my Ethos/Nagler lineup, 31T5/21E/13E/8E/6E. Throughout the night I enjoyed many views and never once noticed objectionable defocused stars. The Borg 90FL 90/500 f/5.6 that I had for a bit, though, showed significant amounts of defocused stars even starting 50% out from center. It was enough that, combined with the slight CA, I ended up returning the scope. I wanted to love it but the FC/star blur plus CA turned me off. The 105/650 on the other hand does the exact opposite. ?

[Stu]

11 minutes ago, MNJay said:

I think technically all scopes have field curvature, it just depends how apparent it is. The relatively short FL of 650mm and fast FR of f/6.2 means you will notice some defocused stars at the edge of the field. However, I had mine out last weekend at a dark site, first time since November, and was so enamored with seeing the Veil without a filter (and then with a UHC, WOW!) that I honestly didn't notice any FC. I used my Ethos/Nagler lineup, 31T5/21E/13E/8E/6E. Throughout the night I enjoyed many views and never once noticed objectionable defocused stars. The Borg 90FL 90/500 f/5.6 that I had for a bit, though, showed significant amounts of defocused stars even starting 50% out from center. It was enough that, combined with the slight CA, I ended up returning the scope. I wanted to love it but the FC/star blur plus CA turned me off. The 105/650 on the other hand does the exact opposite. ?

Oh great, another one I CAN’T cross off my list

Thanks for the info, very useful. Is there any truth in a theory of mine that a fast triplet can be designed with a flatter field than a similar focal ratio doublet?

[MNJay]

2 minutes ago, Stu said:

Oh great, another one I CAN’T cross off my list

Thanks for the info, very useful. Is there any truth in a theory of mine that a fast triplet can be designed with a flatter field than a similar focal ratio doublet?

No problem! Unfortunately that's one question I haven't been able to answer for myself, either. I've seen some people say that triplets can have flatter fields than equivalent FL/FR doublets, but I have yet to see anything that would explain such a thing. The field curvature is a product of the focal length and regardless of whether you use two lens elements or three lens elements, you're going to have curvature. It is possible that some premium triplets in popular FL/FR configurations (like 80mm f/6) exhibit better edge correction than more cheaply-made doublets and that this reduction in edge aberrations appears to some people like improved field flatness. But from what I know, a triplet and doublet with the same focal length have the same amount of field curvature. I still think I'm missing information because how else do field flatteners work? Could a triplet have an element that has a field-flattening effect? I don't know. Good question...

[Stu]

1 minute ago, MNJay said:

No problem! Unfortunately that's one question I haven't been able to answer for myself, either. I've seen some people say that triplets can have flatter fields than equivalent FL/FR doublets, but I have yet to see anything that would explain such a thing. The field curvature is a product of the focal length and regardless of whether you use two lens elements or three lens elements, you're going to have curvature. It is possible that some premium triplets in popular FL/FR configurations (like 80mm f/6) exhibit better edge correction than more cheaply-made doublets and that this reduction in edge aberrations appears to some people like improved field flatness. But from what I know, a triplet and doublet with the same focal length have the same amount of field curvature. I still think I'm missing information because how else do field flatteners work? Could a triplet have an element that has a field-flattening effect? I don't know. Good question...

Thanks for your thoughtful answer.

My experiences are based on seeing more F.C. with say a TV76 at f6.3 than the only Triplet I’ve owned which was a 106mm Astrotech at f6.5. Even with a 21Ethos or 31mm Nagler I never had a problem with the triplet but found it noticeable in the 76. 

I suspect you are probably right though and that a triplet has the same level of F.C. at the same focal ratio.

[MNJay]

2 minutes ago, Stu said:

Thanks for your thoughtful answer.

My experiences are based on seeing more F.C. with say a TV76 at f6.3 than the only Triplet I’ve owned which was a 106mm Astrotech at f6.5. Even with a 21Ethos or 31mm Nagler I never had a problem with the triplet but found it noticeable in the 76. 

I suspect you are probably right though and that a triplet has the same level of F.C. at the same focal ratio.

I can imagine seeing some with the TV-76. I have an AT72EDII 72mm f/6 that I haven't used much recently, but don't recall seeing too much FC... then again I haven't done much deep sky viewing with it. On a sidenote, the focal length is what determines field curvature, but the focal ratio determines the size of the focal plane. Shallow focal planes with fast focal ratios will show larger star blurs, like bokeh on a fast prime photo lens when shooting wide open at f/1.4 or something. The further out of focus it is, the larger the blur of the light source. The same thing happens with scopes, we just don't get to change the focal ratio on demand with mechanical aperture masks so we can't see the size of the star blur change. So, field curvature may be more apparent with fast focal ratios, but technically, it is only determined by the focal length. I struggled with this concept for a long time until Jon Isaacs explained it this way. (If you're reading this, thanks Jon!)

[Ricochet]

11 minutes ago, Stu said:

Thanks for your thoughtful answer.

My experiences are based on seeing more F.C. with say a TV76 at f6.3 than the only Triplet I’ve owned which was a 106mm Astrotech at f6.5. Even with a 21Ethos or 31mm Nagler I never had a problem with the triplet but found it noticeable in the 76. 

I suspect you are probably right though and that a triplet has the same level of F.C. at the same focal ratio.

Field curvature is not related to focal ratio but inversely related to focal length. A 102mm f6.5 doublet would also have less field curvature than the tv76 because it has a longer focal length. 

[Stu]

22 minutes ago, Ricochet said:

Field curvature is not related to focal ratio but inversely related to focal length. A 102mm f6.5 doublet would also have less field curvature than the tv76 because it has a longer focal length. 

Thanks. Yes, I was forgetting that point and it makes total sense.

[25585]

7 hours ago, Stu said:

Then I humbly suggest you carry on doing just that. Nothing has changed.

The Genesis is even faster than the NP101 so one is not a direct replacement for the other anyway.

Congrats on 19000 likes Stu ☺

If I saw another Genesis, I would buy it.

TV scopes are sort of cult in a way, as are Astro Physics. But a Tak TOA 120 costs less than a new NP101, which puts it in perspective.

BTW with my APS CI roof prism, min focusing distance for the Genesis is roughly 30 yards.  

[25585]

2 hours ago, MNJay said:

No problem! Unfortunately that's one question I haven't been able to answer for myself, either. I've seen some people say that triplets can have flatter fields than equivalent FL/FR doublets, but I have yet to see anything that would explain such a thing. The field curvature is a product of the focal length and regardless of whether you use two lens elements or three lens elements, you're going to have curvature. It is possible that some premium triplets in popular FL/FR configurations (like 80mm f/6) exhibit better edge correction than more cheaply-made doublets and that this reduction in edge aberrations appears to some people like improved field flatness. But from what I know, a triplet and doublet with the same focal length have the same amount of field curvature. I still think I'm missing information because how else do field flatteners work? Could a triplet have an element that has a field-flattening effect? I don't know. Good question...

Flat fields are the big feature of Petzvals as I understand. Why TV use them, making them expensive presumably. Vixen's 140 SSF achro and some Taks I believe are also Petzvals.   

[MNJay]

2 minutes ago, 25585 said:

Flat fields are the big feature of Petzvals as I understand. Why TV use them, making them expensive presumably. Vixen's 140 SSF achro and some Taks I believe.  

Petzvals are special designs that use a long achromat primary objective and a 0.5x reducer/flattener in the back, though. The Nagler-Petzvals operate this way. It's not just like the main objective has a special figure, it actually is using a dedicated fixed reducer/flattener. So I exclude this type of lens configuration from my thoughts about triplets being able to display a flatter field than a doublet of equivalent focal length.