1066 replies to this topic

[Jon Isaacs]

Jon,

 

Thank you for that detailed explanation. I have a few more questions though.

 

Do all eyepieces have off-axis astigmatism. Or is that only a problem with certain eyepiece designs or say less expensive brands/models?

 

I'm going to use a 31 Nagler and a 21 Ethos with the TSFLAT2 in my upcoming AT92 F5.5 scope for low power wide-field viewing. Am I going to see off-axis astigmatism with those eyepieces?

 

Thank you,

 

T.J.

 

T.J.:

 

The 31 mm Nagler and 21 mm Ethos are essentially perfect, zero off-axis astigmatism, they're as good as it gets at their respective focal lengths. 

 

You should see very little field curvature with the TSFLAT2 if you get the spacing reasonably close. Both eyepieces essentially parfocal so both require the same backfocus spacing, about 10 mm inward.

 

The right diagonal is important, the nosepiece must be threaded on and use 48 mm filter threads so it can be removed and replaced with the TSFLAT2.

 

Jon

[bobhen]

Wow, lots to unpack here.

 

Takeaways so far:

 

Slower refractors

 

Upsides:

Can use simpler, less expensive eyepieces.

Easier to achieve higher magnification

Easier to focus

Don't suffer field curvature to the extent faster ones do

 

Downsides:

Smaller field of view

Larger and harder to handle

 

Question:

Is it practical (for visual) to use a reducer to shorten focal length?

 

Faster refractors

 

Upsides:

Large field of view

Smaller and easier to handle

Can take high magnification without field curvature

 

Downsides:

Require more complex eyepieces to work optimally

Require higher power eyepieces or Barlows to reach high magnifications

Suffer field curvature

 

Questions:

Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.

With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?

 

Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?

 

Larry

I’ve had both fast triplets and slow triplets. My fastest triplet was a LZOS 102mm F6.25 in a SV tube with a 2-speed FT focuser using OK4 glass. It had a killer optic! My slowest triplet was a 1989 152mm F9 Astro-Physics triplet with a single speed focuser that used no ED glass but used special flint glass. What a wonderful scope!

 

IMO, when using either a fast or slower high quality triplet, most of the comments in this thread about disadvantages are over-blown.

 

I never had an issue finding exact focus with my fast LZOS F6.2 triplet with the 2-speed, FT focuser. If the optic is of high quality it will snap to focus. The 2-speed focuser was a help but you could tell when focus was achieved even without it. Color correction was superb. More color was introduced at the edges with wide field eyepieces and the atmosphere than anything from the main optic.

 

For the planets, field curvature at high power was not an issue. And to be honest, field curvature was never an issue on visual deep sky. I really don’t spend much time observing the “extreme edge” of the field but usually center the object of interest. And, of course, slower refractors might not have field curvature but they also have smaller fields to start with. 

 

Today’s top quality Powermates and Barlows are of very high quality and will not impact the quality of the optic and can be used for high power observations with fast triplets. The same goes for reducers and field flatteners. Eyepieces like TV Delites (which actually have  something like a Barlow built into the design) with their high ER and flat fields and “excellent ergonomics” are very comfortable to use with fast triplets, even more so than the short, simple eyepieces with low, uncomfortable ER used with long FL refractors of yesteryear. Astro-Physics would not produce a Barlow, reducer or flattener that compromised the quality of their main optic. How much “harder” is to use a Barlow anyway, I mean really.   

 

My wonderful, 1989 AP 152 F9 “slow” triplet produced superb, basically color-free visual images, but it was rather long. My later AP 155 F7 did the same but was surprisingly short and compact for a 6” class refractor (without the dew shield it was “shockingly” short), and of course was better for imaging. My 155 F7 had a single speed focuser that also presented zero issues finding focus. 

 

My current Takahashi TSA 120 at F7.5 has basically zero noticeable field curvature and basically zero-color, even in the star test. No need to go longer. Tak’s TOA lenses use 2 ED elements so there is also little need to go longer.

 

I don’t remember having any issues finding focus even with my original TV Genesis F5 either.

 

A slower triplet (like from CFF) will most likely cost less but will also be less conducive for imaging. But the actual quality of a faster triplet lens from CFF and a slower triplet lens from CFF will be “the same” when it leaves the factory. The easier to make argument does “not” mean you get a lesser quality lens if it’s “faster” but only applies to “price”.

 

Most F5 apos use more than three lenses to correct for aberrations and they are used mostly for imaging. With mid FL apo triplets, any disadvantages listed in this thread will be non-issues for the visual, planetary or high power observer.

 

Marcus Ludes of APM has been quoted as saying that their LZOS 130 F6 outsells their LZOS 130 F9.2 by about 10 to 1. The market moved to faster lenses when the new glasses allowed manufacturers to make faster lenses. And the market embraced the advantages of these faster apo refractors.

 

With a high quality triplet, get the FL that is better suited to your visual or imaging and portability goals/needs. The more important concern is the quality of the main optic, which will have a much greater impact on any visual or imaging pursuits. The portability of the scope and mount will also impact use. 

 

As far as why all apos are not Petzvals. The F5 TV Petzvals (and most all fast quadruplets) basically have a permanent reducer/flattener in the optical train whereas a triplet lets the user add the “additional expense” of a reducer or a flattener if so desired.

 

All IMO and based on my experience of course.

 

Bob

[alnitak22]

You need to provide a link. It's required by the TOS and in this case, it's important because I would like to read the context of Roland's comments.

 

It's clear he is not talking about modified Petzvals like the TeleVue NP series and the Takahashi FSQ series that use ED glasses in both doublets.

 

He's clearly not talking about eyepieces like the 31 mm Nagler, 41 mm Panoptic etc. These are not "normal" eyepieces and show essentially no aberrations in very fast scopes. The field curvature with the 31 mm Nagler of a 4 inch F/5 doublet is easy to see, in the NP-101, it's as perfect as can be.

 

I normally find Roland to be on the money. In this case, his comments seem to be addressing achromatic Petzvals used with Erfle's and the like rather than apochromatic "modified Petzvals" used with well corrected eyepieces. These days, one does not have to spend large sums to buy a well corrected ~30 mm 82° eyepiece. The $200 Astro-Tech 28 mm 82° provides excellent views in the NP-101.. 4.33° at 19x with a 5.2 mm exit pupil.. 

 

Do you have much experience with scopes like the NP series using eyepieces like the 31 mm Nagler?

 

Jon

Agree about a link and is why I asked how old these comments from Roland were as they’re clearly dated. 

[Jon Isaacs]

Most F5 apos use more than three lenses to correct for aberrations and they are used mostly for imaging. With mid FL apo triplets, any disadvantages listed in this thread will be non-issues for the visual, planetary or high power observer.

 

 

I know of only one F/5 triplet, the Esprit 80mm F/5, I know of no F/5 apo doublets.  Do you know of any others?

 

As far as why all apos are not Petzvals. The F5 TV Petzvals (and most all fast quadruplets) basically have a permanent reducer/flattener in the optical train whereas a triplet lets the user add the “additional expense” of a reducer or a flattener if so desired.

 

 

 

The Petzvals are something more than just a reducer/flattener.  In the case of the NP series, the front and rear sections are designed to work together and cannot be used alone. For astrophotography a flattener can be added but visually, there is only one flattener I know of that has enough back focus to be used (TSFLAT2) with a 2 inch diagonal.  And while it does a good job, it is not as perfect as the NP series.  

 

If you want a true flat field visually, the Petzval's are the way to go, triplets and doublets plus a flattener get you most of the way.  Also, with a Petzval, the spacing is always correct because the optics do not move with the focuser.  With a doublet or triplet, the flattener is attached to the focuser so the actual spacing depends on the eyepiece.  If you want an 4 inch ~F/5 apo refractor with a flat flat field for visual use, it is going to modified Petzval.  

 

Adding a flattener to a doublet or triplet for visual use is a compromise.

 

Jon

[vahe]

You need to provide a link. It's required by the TOS and in this case, it's important because I would like to read the context of Roland's comments.

 

 

 

Can't give you a link but if you know how to find it it was in Yahoo Refractor group Post #21377, April 11, 2007.

It can be argued that this particular post is somewhat dated, but I always find Rolands comments pretty much right on the mark, and if you can read in between the lines even in an old post like this there are things that do apply to all Petzvals, old and new.

.

Vahe

[alnitak22]

Can't give you a link but if you know how to find it it was in Yahoo Refractor group Post #21377, April 11, 2007.

It can be argued that this particular post is somewhat dated, but I always find Rolands comments pretty much right on the mark, and if you can read in between the lines even in an old post like this there are things that do apply to all Petzvals, old and new.

.

Vahe

Well, if this quote was actually from 2007, Roland surely knew that the f/5.2  TV Petzvals have much better color correction than an f/6.3 doublet achro. So it’s puzzling why he would say this. And shouldn’t be used in discussing current scopes. An f/5 Petzval would have the same correction as an f/6.3 achro ONLY if they used the same glass. That’s a pretty significant fact to leave out, especially in 2007 when the NP scopes were on the scene and when the SDF and TV101 had already been discontinued and their color correction was FAR better than an f/6.3 achro. 

Edited by alnitak22, 31 March 2022 - 09:07 AM.

[bobhen]

I know of only one F/5 triplet, the Esprit 80mm F/5, I know of no F/5 apo doublets.  Do you know of any others?

 

 

 

The Petzvals are something more than just a reducer/flattener.  In the case of the NP series, the front and rear sections are designed to work together and cannot be used alone. For astrophotography a flattener can be added but visually, there is only one flattener I know of that has enough back focus to be used (TSFLAT2) with a 2 inch diagonal.  And while it does a good job, it is not as perfect as the NP series.  

 

If you want a true flat field visually, the Petzval's are the way to go, triplets and doublets plus a flattener get you most of the way.  Also, with a Petzval, the spacing is always correct because the optics do not move with the focuser.  With a doublet or triplet, the flattener is attached to the focuser so the actual spacing depends on the eyepiece.  If you want an 4 inch ~F/5 apo refractor with a flat flat field for visual use, it is going to modified Petzval.  

 

Adding a flattener to a doublet or triplet for visual use is a compromise.

 

Jon

All correct Jon.

 

But, like the better color correction delivered by a triplet over a doublet, the flat field of the Petzval apo also comes at additional expense, which gets pretty hefty as size increases. Before purchasing, one must really want and list a wide, flat field for lowish power, visual, deep sky observing as a top priority on his or her list of refractor attributes. For example, that attribute would be an added/unneeded expense for the planetary or double star observer.

 

With telescopes, the name of the game is compromise.

 

Bob

[Jon Isaacs]

Can't give you a link but if you know how to find it it was in Yahoo Refractor group Post #21377, April 11, 2007.

It can be argued that this particular post is somewhat dated, but I always find Rolands comments pretty much right on the mark, and if you can read in between the lines even in an old post like this there are things that do apply to all Petzvals, old and new.

.

Vahe

 

I always find Roland's comments on the mark but reading between the lines, clearly this quote does not apply to telescopes like the NP and FSQ series used with premium eyepieces.

 

For example:

 

"Improved field curvature does nothing much for you visually. In a normal refractor the field curvature is not noticed in a typical eyepiece, and the edge aberrations in eyepieces usually dominate."

 

The 31mm Nagler is not a typical eyepiece and the edge aberrations one sees in a doublet or triplet are pure field curvature. 

 

"The 25% improvement in color correction means that an F5 Petzval will look identical to an F6.3 normal doublet color wise, so there is practically no gain"

 

These scopes are based on long focal length doublets, estimates are from F/9 to F/12, color correction is very good.

 

Have you ever actually looked through an NP-101 under dark skies with an eyepiece like the 31 mm Nagler?  

 

I am not saying that one cannot enjoy views in other fast scopes that are not corrected for field curvature but I am saying there is something very special about a low power wide field view that is completely aberration free.  When I bought my NP-101, I didn't know what I was buying into..

 

But I found out and as someone who spend a lot of time under dark skies, I realized soon enough that it was the right refractor for me.

 

Jon

 

Jon

[Jon Isaacs]

All correct Jon.

 

But, like the better color correction delivered by a triplet over a doublet, the flat field of the Petzval apo also comes at additional expense, which gets pretty hefty as size increases. Before purchasing, one must really want and list a wide, flat field for lowish power, visual, deep sky observing as a top priority on his or her list of refractor attributes. For example, that attribute would be an added/unneeded expense for the planetary or double star observer.

 

With telescopes, the name of the game is compromise.

 

Bob

 

I agree, the flat field and fast focal ratio is actually a burden if someone is only going to use a scope like the NP-101 only for planetary and double stars.  You really need to use eyepieces like the type 6 Naglers if you want more than just the center of the field to be sharp and contrasty.  F/7, F/9 are more forgiving.I said exactly that in the thread regarding the purchase of an NP-101. 

 

My point was that while a field flattener is an option, it does not duplicate what is possible with a Petzval. 

 

There is a lot to be said for a F/7 doublet, the field is not as wide but it is wide enough for most purposes, there is field curvature but it's not all that bad and it can be corrected.  It's easier on eyepieces.. If I were buying a 4 inch scope for my backyard only, it would be about F/7 with some pretty good glass.

 

Jon

[Space_Race_T.J.]

T.J.:

 

The 31 mm Nagler and 21 mm Ethos are essentially perfect, zero off-axis astigmatism, they're as good as it gets at their respective focal lengths. 

 

You should see very little field curvature with the TSFLAT2 if you get the spacing reasonably close. Both eyepieces essentially parfocal so both require the same backfocus spacing, about 10 mm inward.

 

The right diagonal is important, the nosepiece must be threaded on and use 48 mm filter threads so it can be removed and replaced with the TSFLAT2.

 

Jon

I've been researching diagonals and their light paths in order to get the correct spacing I'll need with the 31 Nagler & 21 Ethos. I was confused at first how the inward 9.6mm location of the eyepiece field stops added to the light path and the spacing requirements but it's clear to me now. 

 

I agree, the flat field and fast focal ratio is actually a burden if someone is only going to use a scope like the NP-101 only for planetary and double stars.  You really need to use eyepieces like the type 6 Naglers if you want more than just the center of the field to be sharp and contrasty.  F/7, F/9 are more forgiving.I said exactly that in the thread regarding the purchase of an NP-101. 

 

My point was that while a field flattener is an option, it does not duplicate what is possible with a Petzval. 

 

There is a lot to be said for a F/7 doublet, the field is not as wide but it is wide enough for most purposes, there is field curvature but it's not all that bad and it can be corrected.  It's easier on eyepieces.. If I were buying a 4 inch scope for my backyard only, it would be about F/7 with some pretty good glass.

 

Jon

Jon,

 

Thanks for your help and advice. I understand now that the AT92 + TSFLAT2 is a compromise compared to a real Petzval design like the NP-101.

 

Do you think I can get 90-95% of the performance/flat field with this combo versus the NP-101? The AT92 does provide a slightly wider field of view at 506mm FL, and with the TSFLAT2 is going to cost me less than half the money of a new NP101is.

 

I hope it's going to be a great combination out under the dark desert skies of the Southwest.

 

T.J.

 

  

Attached Thumbnails

• 

[Echolight]

Availability and price was the reason I chose a (fast) f/8 150mm achromat.

 

Of course it would be nice to have 3 degrees or somewhere close with a faster f/5, f/5.9, or f6.5 focal ratio 6 inch achromat. And the shorter tube would mean less tripod adjustment would be needed for looking from horizon to zenith.

But the f/8 has provided some crisp high power lunar and planetary views that the shorter scopes would have trouble matching.

 

Same holds true for my f/7.5 80mm fpl-53 apo.

A shorter, lighter scope with equal apochromatic properties would be nice. But much less available and more expensive at the time.

Edited by Echolight, 31 March 2022 - 10:56 AM.

[alnitak22]

From what I've seen the answer is usually no. Flatteners will often work just fine visually and if the manufacturer's own model doesn't lend itself to visual use then it's not hard to find a third party product that will.

 

Reducers significantly cut the amount of back focus available and in most cases leave you with too little to permit the use of even a 1.25" diagonal so unless you fancy observing straight through they're not really an option.

 

That's a huge advantage for Petzval scopes which are designed from the outset to have enough back focus for imaging and visual.

 

 

If he's talking about f/5 scopes and achromats then he must be referring to the original Genesis. I also suspect that a lot of observers would take issue with the suggestion that field curvature isn't a problem for visual astronomy.

 

 

That tends to be a problem at very low focal ratios - f/4 and below so it's unlikely to be an issue with most refractors. If you were using an FSQ-130 with its reducer to give you an f/3 scope then it's something you'd need to consider, but anyone who can afford gear like that can afford to spend a bit extra on filters!

“he must be referring to the original Genesis.” Well, this post is reportedly from 2007, so the question remains WHY he would be using the long discontinued Genesis as a reference standard? By then, even the successors to the Genesis had been discontinued and the NP scopes were the flagship designs. Tak also had high performing quads by then as well as Pentax and perhaps Vixen. Even his comments about vignetting with quads were dated by then. Seems Roland was in a mood to throw shade at the quad design that particular day. I can’t think of another reason he would post this.

[Jon Isaacs]

I've been researching diagonals and their light paths in order to get the correct spacing I'll need with the 31 Nagler & 21 Ethos. I was confused at first how the inward 9.6mm location of the eyepiece field stops added to the light path and the spacing requirements but it's clear to me now. 

 

Jon,

 

Thanks for your help and advice. I understand now that the AT92 + TSFLAT2 is a compromise compared to a real Petzval design like the NP-101.

 

Do you think I can get 90-95% of the performance/flat field with this combo versus the NP-101? The AT92 does provide a slightly wider field of view at 506mm FL, and with the TSFLAT2 is going to cost me less than half the money of a new NP101is.

 

I hope it's going to be a great combination out under the dark desert skies of the Southwest.

 

T.J.

 

T.J.:

 

It's hard to put numbers on a view but I have an 80 mm F/6 ED doublet and use it with the TSFLAT2. I don't feel like I'm giving up much to the NP-101. 

 

Do you already have the AT-92?  Used NP-101's are around $2000-$2500 though I haven't seen one recently.

 

Jon

[Jon Isaacs]

Seems Roland was in a mood to throw shade at the quad design that particular day. I can’t think of another reason he would post this.

 

 

I suspect he was answering a specific question and that the quote Vale used was taken out of context. He really needs to provide a link, that's his responsibility. 

 

Jon

[Jon Isaacs]

I just wanted to add this:

 

This thread has spent a lot of time discussing the NP-101, I guess because it's an example of a very good fast apo/Ed.  And in discussing the NP-101, implicit in the discussion are it's disadvantages which, in turn, can be advantages for other, longer focal length scopes.

 

There are lots of fine refractors available, finding the one(s) that suit ones needs is where it's at. 

 

Jon

[Space_Race_T.J.]

T.J.:

 

It's hard to put numbers on a view but I have an 80 mm F/6 ED doublet and use it with the TSFLAT2. I don't feel like I'm giving up much to the NP-101. 

 

Do you already have the AT-92?  Used NP-101's are around $2000-$2500 though I haven't seen one recently.

 

Jon

Jon,

 

Yeah that makes sense that its hard to put a actual number on it. It's a good sign for me that you feel like your 80 mm F/6 + TSFLAT2 doesn't give up too much to the NP-101. I hope the AT92 + TSFLAT2 will deliver a similar viewing experience for me. 

 

No not yet, I have it on backorder. The next delivery isn't expected till late 2022. I've always heard great things about it, and I like the "larger" aperture in such a short package. Although the tube is actually 105mm so it's more like a short tank apparently. I feel like at $1,795 it's such a great deal when the other options from Astro-Physics, CFF, and Televue cost so much more or are basically impossible to buy new anymore.

 

Different choices of f/ratio is part of the fun for me. I read such great reviews of everyone with the 102mm F11 scopes as well. F5.5, F7, F11 seems like a cool combination to own someday. I wish more manufactures would make more of the shorter and longer F/ratio scopes, there sure seems like there's a market for them but very few options.

 

Clear skies,

 

T.J. 

[25585]

Actually 1824. That is the Dorpat refractor, built by Joseph von Fraunhofer, and at the time the largest refractor in the world (9" aperture).

Tyson M (where he?) has a modern 9" refractor. 

[CHASLX200]

I like F/8 or slower. FC bothers me very much like coma does in a Newt.

[Wildetelescope]

Jon,

 

Yeah that makes sense that its hard to put a actual number on it. It's a good sign for me that you feel like your 80 mm F/6 + TSFLAT2 doesn't give up too much to the NP-101. I hope the AT92 + TSFLAT2 will deliver a similar viewing experience for me. 

 

No not yet, I have it on backorder. The next delivery isn't expected till late 2022. I've always heard great things about it, and I like the "larger" aperture in such a short package. Although the tube is actually 105mm so it's more like a short tank apparently. I feel like at $1,795 it's such a great deal when the other options from Astro-Physics, CFF, and Televue cost so much more or are basically impossible to buy new anymore.

 

Different choices of f/ratio is part of the fun for me. I read such great reviews of everyone with the 102mm F11 scopes as well. F5.5, F7, F11 seems like a cool combination to own someday. I wish more manufactures would make more of the shorter and longer F/ratio scopes, there sure seems like there's a market for them but very few options.

 

Clear skies,

 

T.J. 

There are several threads on the AT92 that give a very detailed discussion that I am sure you have seen.   I had my friend's AT92 up to ~280X with a 4.5mm Delos and a 2.5X Luminos Barlow looking at Jupiter.  The image held up quite  well.  The planet disk stayed sharp and there was no false color that my 53 year old eyes could detect, only atmospheric dispersion.  Overall the sweet spot was at about 180X for magnification and above that I did not see any more detail, but I also did not see much in the way of image degradation beyond some dimming and I did get the benefit of increased image scale(and floaters:-).  At 130X and above we could clearly see the red spot coming into view on the edge of the planet, barges and some texture in the cloud bands during moments of calm.   It was a reasonably stable night for our area and I was pretty impressed.  Of the mass produced refractors that I have owned or looked through, that one was certainly the best.   I am sure there was field curvature, but if the TS flat performs as Jon describes, I think you probably will have a lot of fun!   

 

cheers!

 

JMD

[mikeDnight]

I think the long focus achromat has its place even today. They can and do deliver outstanding views of everything in their aperture range, but they do suffer from chromatic abberation, which I personally feel is often blown out of all proportion. For example, my Genesis SDF would by many be considered acceptable in relation to CA. But my 6" F10 achromatic refractor would very likely be condemned before being given a chance to prove itself. Yet in reality the visible CA in the 6" Edmund objective is almost indistinguishable from that shown by the SDF, and only noticeable on the brightest stars and as a very thin slither around the lunar limb. The SDF however is much friendlier to use.

 

 

[alnitak22]

I think the long focus achromat has its place even today. They can and do deliver outstanding views of everything in their aperture range, but they do suffer from chromatic abberation, which I personally feel is often blown out of all proportion. For example, my Genesis SDF would by many be considered acceptable in relation to CA. But my 6" F10 achromatic refractor would very likely be condemned before being given a chance to prove itself. Yet in reality the visible CA in the 6" Edmund objective is almost indistinguishable from that shown by the SDF, and only noticeable on the brightest stars and as a very thin slither around the lunar limb. The SDF however is much friendlier to use.

 

1648122234593_image0.jpeg

 

1646429931049_image0.jpeg

Those sure are 2 mighty purty scopes!!

[Wildetelescope]

I think the long focus achromat has its place even today. They can and do deliver outstanding views of everything in their aperture range, but they do suffer from chromatic abberation, which I personally feel is often blown out of all proportion. For example, my Genesis SDF would by many be considered acceptable in relation to CA. But my 6" F10 achromatic refractor would very likely be condemned before being given a chance to prove itself. Yet in reality the visible CA in the 6" Edmund objective is almost indistinguishable from that shown by the SDF, and only noticeable on the brightest stars and as a very thin slither around the lunar limb. The SDF however is much friendlier to use.

 

1648122234593_image0.jpeg

 

1646429931049_image0.jpeg

Got an 80mm F15 Towa that for strictly lateral color correction might be my best refractor lol:-) Definitely a nice set of achromatic optics, although as you say, perhaps not the most convenient.  However, as you say, there is more to an optic than lateral color correction.  I will not be trading in my AP scopes anytime soon, even though there is not a lick of modern ED glass present for either of them:-)  

 

[Rollo]

I agree with Chas,,, F/8 or slower for me also.   100% visual for me.   Sold all my fast refractors.   

[Lagrange]

I know of only one F/5 triplet, the Esprit 80mm F/5, I know of no F/5 apo doublets.  Do you know of any others?

 

 

 

The Petzvals are something more than just a reducer/flattener.  In the case of the NP series, the front and rear sections are designed to work together and cannot be used alone. For astrophotography a flattener can be added but visually, there is only one flattener I know of that has enough back focus to be used (TSFLAT2) with a 2 inch diagonal.  And while it does a good job, it is not as perfect as the NP series.  

 

If you want a true flat field visually, the Petzval's are the way to go, triplets and doublets plus a flattener get you most of the way.  Also, with a Petzval, the spacing is always correct because the optics do not move with the focuser.  With a doublet or triplet, the flattener is attached to the focuser so the actual spacing depends on the eyepiece.  If you want an 4 inch ~F/5 apo refractor with a flat flat field for visual use, it is going to modified Petzval.  

 

Adding a flattener to a doublet or triplet for visual use is a compromise.

 

Jon

Nearest I can think of would be a couple of the Borg fluorite doublets: the 55FL and 67FL are both f/4.5. The nearest you'd get in a larger aperture are probably the 72FL. 90FL, and 107FL which are a bit slower at f/5.6.

 

I think I read that the front doublet of an NP-101would work by itself but it wouldn't be as good as well as an objective that was designed as a doublet from the outset and like you said, the key to its performance is having those two lens groups designed to work in concert rather than relying on adding a reducer/flattener as an afterthought.

[Lagrange]

A rear-mounted h-alpha filter improves with higher F-ratios going up to F/50. With a very fast scope, you'd have to stop it down quite a bit to get there.

 

Beyond filters, ADCs have similar requirements.

Good point, solar H-alpha filters are a bit of a special case although I think it's still possible to use them with a reasonably fast scope without stopping down too much provided a suitable telecentric optic set is used ahead of the filter.

 

I'd imagine a similar approach could be used with an ADC.