152 replies to this topic

[balcon3]

I am aware of the arguments for why medium focal ratio refractors are preferable to longer focal ratios in today's world. With ED glass you can get excellent correction of chromatic aberration at shorter focal ratios. Shorter focal length scopes are easier to mount. They have wider maximal fields. You can get to the same magnifications as with slower focal ratios by using different eyepieces and/or Barlows. I get it. I have a  couple medium focal ratio refractors (f/7 to f/7.4) and I like them very much.

 

But here's the thing. Adding a Barlow to a faster scope will not correct the aberrations that come with faster focal ratios, such as astigmatism and coma. Even chromatic aberration, which can be nicely corrected at medium focal ratio ED scopes, will be even better corrected at slower focal ratios. This point is driven home to me when I use my Masuyama 5-element 85 degree AFOV eyepieces. I love these eyepieces. Even though they have significant outer field aberrations at f/7.4, I still prefer them at that focal ratio over my Morpheus's. The Morpheus's have a much better-corrected field over the whole field of view. But the view is much more alive in the Masuyamas. 

 

So this got me wondering, would it be worth trying to find a slower refractor where the Masuyamas would be better corrected across the field ? This is the first time I am looking for a telescope to match an eyepiece, but I like the Masuyamas so much I am considering it. What do people think of this idea?  I know that many here will not understand my quest. But I find that I prefer low-element eyepieces, and Masuyamas allow for wide AFOV provided you are at slower focal rations. What are good slower refractors (say f/11 or above) ? Probably on the used market since the better ones are probably not made anymore. But not expensive collectors items. I would be interested in achromats as well as ED scopes. 

Edited by balcon3, 17 June 2025 - 05:16 AM.

[Lookitup]

After searching for a long FL scope like the TakFC100DL or a AOM 105FLT I went for a Altair Planeta 101 F/11. With binoviewers the CA control is even better than with my Tak100DF. The optics and mechanics are top particularly for 1K. Last night I compared the double Iota Cancri with 7mm Delite's in the Tak100DF (X243) with Pentax XW10 (X255) in the Planeta. The FOV was similar and both scopes showed intricate details. Even X370 was easy with the F/11. Views of Jupiter and the Moon are excellent too. I use both scope's equally. CS Pete 

 

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[Eddgie]

 

 

This point is driven home to me when I use my Masuyama 5-element 85 degree AFOV eyepieces. 

It isn't astigmatism or coma from the faster scopes that you are seeing, it is astigmatism and coma from the eyepiece, possibly with some field curvature thrown in. The reason is that using 5 elements, it would be virtually impossible to correct all of these errors over such a wide apparent field. It is well known that the Masuyama 5-element 85 degree eyepieces have very poor off axis performance.

 

Going to a slower scope will help, but of course at the cost of losing true field, but don't be surprised to see that you have to get to maybe f/10 for them to give reasonable off axis performance. 

 

It is a questionable tradeoff in my mind. You get to use an eyepiece you like, but you have to sacrifice considerable true field, which works against the purpose of the wide field design of the eyepiece, which is often to give a bigger true field of view than a standard eyepiece. 

 

In the end though, what you propose will indeed work. Slowing down the telescope to f/10 will give you much better off axis performance, but it is not because the telescopes have coma and astigmatism, it is because the eyepiece is incapable of providing aberration free stars over such a wide apparent field in scopes faster than maybe f/8, and even that I think would be pushing it.  For such a wide apparent field in a 5 element eyepiece, I think you should consider going to f/10. 

 

Here is a great page that shows how different designs perform well off axis. Note that lenses with 5 elements tend to have far more aberration off axis than eyepeices using 6, 7, or even 8 elements. Bottom line though is that the problem is not the aberrations with the scopes, it is the aberrations inherent in the eyepiece itself. Using a slower refractor will indeed help, but not for the reasons you mention.  These charts will show you that the better solution for getting sharp stars and wide fields is an eyepiece with better off axis correction, but I realize that this is probably not acceptable to you. 

 

https://www.telescop...ce_raytrace.htm

Edited by Eddgie, 17 June 2025 - 08:08 AM.

[balcon3]

It isn't astigmatism or coma from the faster scopes that you are seeing, it is astigmatism and coma from the eyepiece, possibly with some field curvature thrown in. The reason is that using 5 elements, it would be virtually impossible to correct all of these errors over such a wide apparent field. It is well known that the Masuyama 5-element 85 degree eyepieces have very poor off axis performance.

 

Going to a slower scope will help, but of course at the cost of losing true field, but don't be surprised to see that you have to get to maybe f/10 for them to give reasonable off axis performance. 

 

It is a questionable tradeoff in my mind. You get to use an eyepiece you like, but you have to sacrifice considerable true field, which works against the purpose of the wide field design of the eyepiece, which is often to give a bigger true field of view than a standard eyepiece. 

 

In the end though, what you propose will indeed work. Slowing down the telescope to f/10 will give you much better off axis performance, but it is not because the telescopes have coma and astigmatism, it is because the eyepiece is incapable of providing aberration free stars over such a wide apparent field in scopes faster than maybe f/8, and even that I think would be pushing it.  For such a wide apparent field in a 5 element eyepiece, I think you should consider going to f/10. 

 

Here is a great page that shows how different designs perform well off axis. Note that lenses with 5 elements tend to have far more aberration off axis than eyepeices using 6, 7, or even 8 elements. Bottom line though is that the problem is not the aberrations with the scopes, it is the aberrations inherent in the eyepiece itself. Using a slower refractor will indeed help, but not for the reasons you mention.  These charts will show you that the better solution for getting sharp stars and wide fields is an eyepiece with better off axis correction, but I realize that this is probably not acceptable to you. 

 

https://www.telescop...ce_raytrace.htm

Yes. I know that the Masuyama 85's work very well in my Mewlon 180c f/12. Just a little aberration at the most outer edge, but since the AFOV is so wide one does not notice it unless one seeks it out. At f7.4 one does not need to seek it out.

[Jethro7]

Hello CS Pete,

Here is my take on the slower scopes. My Altair Starwave Accent 102ED F/11 became my best refractor and It replaced my AT102ED F/7 as my go to scope. I enjoyed the Altair 102ED for several years. Then I made the jump to premium Takahashi and TEC optics with medium focal ratios of F/7 to F/8 and the Altair 102ED F/11 is still sitting  exactly where I put it five years ago. So I guess that the theme of my story is, if you want better optical performance, it will come with better optics. longer focal lengths will only take you so far. With that being said. TS optics sell a version of my Altair 102ED F/11 with a lanthanum element that will should have better performance than my Altair 102ED F/11. 

 

Jethro 

[balcon3]

Hello CS Pete,

Here is my take on the slower scopes. My Altair Starwave Accent 102ED F/11 became my best refractor and It replaced my AT102ED F/7 as my go to scope. I enjoyed the Altair 102ED for several years. Then I made the jump to premium Takahashi and TEC optics with medium focal ratios of F/7 to F/8 and the Altair 102ED F/11 is still sitting  exactly where I put it five years ago. So I guess that the theme of my story is, if you want better optical performance, it will come with better optics. longer focal lengths will only take you so far. With that being said. TS optics sell a version of my Altair 102ED F/11 with a lanthanum element that will should have better performance than my Altair 102ED F/11. 

 

Jethro 

I already have a Takahashi FC100DF f/7.4. So I am not looking for better optics. Just something that will work better with my Masuyama 85 degree AFOV eyepieces. Maybe a 1.5x Barlow (to get to effective f/11) will be a better and cheaper way to achieve this. 

[Lookitup]

Hello CS Pete,

Here is my take on the slower scopes. My Altair Starwave Accent 102ED F/11 became my best refractor and It replaced my AT102ED F/7 as my go to scope. I enjoyed the Altair 102ED for several years. Then I made the jump to premium Takahashi and TEC optics with medium focal ratios of F/7 to F/8 and the Altair 102ED F/11 is still sitting  exactly where I put it five years ago. So I guess that the theme of my story is, if you want better optical performance, it will come with better optics. longer focal lengths will only take you so far. With that being said. TS optics sell a version of my Altair 102ED F/11 with a lanthanum element that will should have better performance than my Altair 102ED F/11. 

 

Jethro 

I don't know if the Altair Planeta 101 F/11 has a lanthanum element. I didn't notice any red hue like in my SM 125ED. The perfect FPL53 equivalent color correction, most noticeable in challenging seeing and the depth of focusing makes this scope a real bargain IMHO. 

[Jethro7]

I don't know if the Altair Planeta 101 F/11 has a lanthanum element. I didn't notice any red hue like in my SM 125ED. The perfect FPL53 equivalent color correction, most noticeable in challenging seeing and the depth of focusing makes this scope a real bargain IMHO. 

Hello Lookitup,

Thank you for the correction. I remembered that something was improved on the TS Optic F/11, It was better glass (FPL 53 intead of FK61) and not a lanthanum element that was discussed in the threads that would be a nice to have but not reality. I had to go back to the CN archives and read the posts. I will certainly agree that these scopes are a bargain for how well they perform. However, in the end, premium optics with the mid focal length scopes will still win out but there is a cost for the optical advantage.

 

HAPPY KEEP  LOOKING UP Jethro

Edited by Jethro7, 17 June 2025 - 03:36 PM.

[glittledale]

for years i rolled a D&G 5" f/12 on an AP 706 mount into the driveway.  it was a wonderful set up and extremely forgiving of older eyepiece designs.  one day i decided it was a little cumbersome.  silly of me.  i briefly had a 5" f/15. now that was cumbersome.  

Edited by glittledale, 17 June 2025 - 11:07 AM.

[Oldfracguy]

I am aware of the arguments for why medium focal ratio refractors are preferable to longer focal ratios in today's world. With ED glass you can get excellent correction of chromatic aberration at shorter focal ratios. Shorter focal length scopes are easier to mount. They have wider maximal fields. You can get to the same magnifications as with slower focal ratios by using different eyepieces and/or Barlows. I get it. I have a  couple medium focal ratio refractors (f/7 to f/7.4) and I like them very much.

 

But here's the thing. Adding a Barlow to a faster scope will not correct the aberrations that come with faster focal ratios, such as astigmatism and coma. Even chromatic aberration, which can be nicely corrected at medium focal ratio ED scopes, will be even better corrected at slower focal ratios. This point is driven home to me when I use my Masuyama 5-element 85 degree AFOV eyepieces. I love these eyepieces. Even though they have significant outer field aberrations at f/7.4, I still prefer them at that focal ratio over my Morpheus's. The Morpheus's have a much better-corrected field over the whole field of view. But the view is much more alive in the Masuyamas. 

 

So this got me wondering, would it be worth trying to find a slower refractor where the Masuyamas would be better corrected across the field ? This is the first time I am looking for a telescope to match an eyepiece, but I like the Masuyamas so much I am considering it. What do people think of this idea?  I know that many here will not understand my quest. But I find that I prefer low-element eyepieces, and Masuyamas allow for wide AFOV provided you are at slower focal rations. What are good slower refractors (say f/11 or above) ? Probably on the used market since the better ones are probably not made anymore. But not expensive collectors items. I would be interested in achromats as well as ED scopes. 

 

You are very correct in your assessment here.  There is something special about the quality of the views provided in these long, slow refractors.  I've had a few of the longer f/11 models, and the best one I had, which I regret selling because I now have a pier extension column tall enough to manage that long scope, is a TS-Optics 102mm f/11 FPL-53 APO:

 

 

 

The field curvature seen in f/6 and f/7 scopes is virtually undetectable.  Stars are pinpoints across the entire field of view.  I had a Nagler 3.5mm Type 6 in this scope for 321x and details on Jupiter were still clear.  I also had a 115mm f/7 ED Triplet at the time, and this 102mm f/11 ED Doublet beat it over and over again on double stars.  For example, the three stars of Iota Cassiopeia appeared smaller and "tighter", with more black sky in between them, at very nearly the same magnifications using the same type of eyepieces, KK Fujiyama HD-OR Orthoscopics.  Even the "regular ED" version of this scope that I had before this FPL-53 APO version bacame available did wonderfully:

 

https://www.teleskop...p-focuser-11262

 

I distinctly remember one Winter night using an APM 30mm UFF in that scope to look at M42.  Even at 37x with a 1.85-deg TFOV I could see the four stars of the Trapezium as tiny pinpoints of light.  By the way, unlike the f/7 versions of these scopes where the better SD glass (FPL-53 or FCD-100) rather than the regular ED glass (FPL-51, FCD-1 or FK61) makes a noticeable difference in the reduction of residual color fringing (CA) seen around bright targets, I really don't think it is that much of an advantage in the longer f/11 version.  The regular ED model will be more than adequate.

 

If you have a mount that will handle a long scope weighing about 6.5 kg with a 2" diagonal, finder scope and eyepiece installed, and a tripod with a tall enought column, one of these 4" f/11 ED refractors would be an amazing scope for visual observation.

Edited by Oldfracguy, 17 June 2025 - 12:37 PM.

[balcon3]

You are very correct in your assessment here.  There is something special about the quality of the views provided in these long, slow refractors.  I've had a few of the longer f/11 models, and the best one I had, which I regret selling because I now have a pier extension column tall enough to manage that long scope, is a TS-Optics 102mm f/11 FPL-53 APO:

 

101_3418.JPG

 

 

The field curvature seen in f/6 and f/7 scopes is virtually undetectable.  Stars are pinpoints across the entire field of view.  I had a Nagler 3.5mm Type 6 in this scope for 321x and details on Jupiter were still clear.  I also had a 115mm f/7 ED Triplet at the time, and this 102mm f/11 ED Doublet beat it over and over again on double stars.  For example, the three stars of Iota Cassiopeia appeared smaller and "tighter", with more black sky in between them, at very nearly the same magnifications using the same type of eyepieces, KK Fujiyama HD-OR Orthoscopics.  Even the "regular ED" version of this scope that I had before this FPL-53 APO version bacame available did wonderfully:

 

https://www.teleskop...p-focuser-11262

 

I distinctly remember one Winter night using an APM 30mm UFF in that scope to look at M42.  Even at 37x with a 1.85-deg TFOV I could see the four stars of the Trapezium as tiny pinpoints of light.  By the way, unlike the f/7 versions of these scopes where the better SD glass (FPL-53 or FCD-100) rather than the regular ED glass (FPL-51, FCD-1 or FK61) makes a noticeable difference in the reduction of residual color fringing (CA) seen around bright targets, I really don't think it is that much of an advantage in the longer f/11 version.  The regular ED model will be more than adequate.

 

If you have a mount that will handle a long scope weighing about 6.5 kg with a 2" diagonal, finder scope and eyepiece installed, and a tripod with a tall enought column, one of these 4" f/11 ED refractors would be an amazing scope for visual observation.

Very intriguing. I was wondering if there are any slow 80-90mm ED refractors. The 100mm f/11 sounds great, but I am hesitant because I already have the wonderful Tak FC100DF f/7.4 which I use al the time. I was thinking to try to find a somewhat smaller aperture. But the 102 f/11 does sound interesting.

[Lookitup]

Tak extenders are another option:

https://www.cloudyni...tender-madness/

[glittledale]

Very intriguing. I was wondering if there are any slow 80-90mm ED refractors. The 100mm f/11 sounds great, but I am hesitant because I already have the wonderful Tak FC100DF f/7.4 which I use al the time. I was thinking to try to find a somewhat smaller aperture. But the 102 f/11 does sound interesting.

the vixen Japanese made 80ish mm f/11's are very nice and date back some time.  the later iterations had nice 2" focusers.  i've never used the Chinese made vixen 80ish mm f/11's but i bet they are nice too.  i don't know if the glass is different or lens cells changed but the advertised diameter on these changed by a couple of mm at various times.  the celestron/vixen version is the same thing.  i've probably used a dozen of them over the years and never saw a bad one.  

[Jeff B]

Balcon3, if I may suggest, a really easy and immediate way to see what a slower native focal ratio can bring to the visual table for you is to make a 70mm paper aperture stop for your Tak.  You can compare edge performance between the focal ratios almost immediately.

 

Just a thought.

 

Jeff

[daquad]

Very intriguing. I was wondering if there are any slow 80-90mm ED refractors. The 100mm f/11 sounds great, but I am hesitant because I already have the wonderful Tak FC100DF f/7.4 which I use al the time. I was thinking to try to find a somewhat smaller aperture. But the 102 f/11 does sound interesting.

Agena Astro has an 80 mm f/15 from Kasai Trading.  

[Oldfracguy]

Very intriguing. I was wondering if there are any slow 80-90mm ED refractors. The 100mm f/11 sounds great, but I am hesitant because I already have the wonderful Tak FC100DF f/7.4 which I use al the time. I was thinking to try to find a somewhat smaller aperture. But the 102 f/11 does sound interesting.

Yes.  There is the StellaMira 80mm f/10 ED Doublet:

 

https://www.firstlig...rp-focuser.html

 

Rochelimit has one that I think he could part with.

[balcon3]

Tak extenders are another option:

https://www.cloudyni...tender-madness/

I looked into the Tak extenders. I am not sure. First, I don't know if I want to add a 5-glass element to the optical train. The whole point is that I like my low-glass count eyepieces. If I add an extender, would the image lose some of its sparkle, like when I swap the Masuyama for the Morpheus eyepieces.

 

Another question is that the extenders, even the 2 inch versions, seem to have a limited optical opening, so one couldn't get a full field of view from a wide 2 inch eyepiece. One of my goals is to use 2 inch eyepieces and get as wide a field of view as possible. 

 

I also thought about a 2-inch Barlow. But they are huge and heavy and I don't like huge and heavy things hanging off my scopes (except binoviewers). 

 

So I still think ia slow objective will be the best for me.

[balcon3]

Balcon3, if I may suggest, a really easy and immediate way to see what a slower native focal ratio can bring to the visual table for you is to make a 70mm paper aperture stop for your Tak.  You can compare edge performance between the focal ratios almost immediately.

 

Just a thought.

 

Jeff

Good suggestion. I will try this. 

[balcon3]

Agena Astro has an 80 mm f/15 from Kasai Trading.  

Wow, that might be too long. 

[balcon3]

Yes.  There is the StellaMira 80mm f/10 ED Doublet:

 

https://www.firstlig...rp-focuser.html

 

Rochelimit has one that I think he could part with.

Thanks. I'm not a huge fan of lanthanum-containing lenses. Of course each case is different. But on my Lanthanum TS80EDL and several of my eyepieces I can detect a warm tone that I am not crazy about.

[25585]

Masuyamas are one of those designs where you choose a scope to suit the eyepiece 

[balcon3]

Tak extenders are another option:

https://www.cloudyni...tender-madness/

Regarding the Tak extenders:  Can they cover the full 47mm field stop of a 2 inch eyepiece, or will there be some vignetting?

[Oldfracguy]

Thanks. I'm not a huge fan of lanthanum-containing lenses. Of course each case is different. But on my Lanthanum TS80EDL and several of my eyepieces I can detect a warm tone that I am not crazy about.

It's refreshing to read that I'm not the only one.  I've been saying for some time now that, to my eyes, those Lanthanum mating elements tend to alter the overall cast and color of things.  I suspect they attenuate some of the wavelengths down in the blue region, which is why I never see any traces of bluish fringing around bright targets with these scopes.  The difference is that the colors of stars, especially the blue ones in colorful double lake Almach, Iota Cancri and Albireo to name a few, seem muted, and less vibrant.  For example, my little AT70ED with its basic FK61 ED element and no Lanthanum mate will show some residual color fringing (CA) around bright targets, but the colors of stars look the the same to my eye as they do in a pure reflector telescope.  That's why I have moved to ED triplets without any Lanthanum.  They also render true star colors, are virtually color-free using good mirror diagonals, and are usually better corrected for Spherical Aberration than the ED Doublets.

 

I have to keep my fingers crossed because I just bought a used WO Zenithstar 103.  I'm thinking that if it does have a Lanthanum mate along with its FPL-53 ED element then William Yang would have made a point to mention that in the product description.  I'll be able to tell right away.  If I need to use my Baader Zeiss-spec Prism diagonal to eliminate any color fringing on bright targets, and I see some bluish fringing on one side of focus and reddish/pinkish fringing on the other side using a Baader BBHS Silver Mirror diagonal, the verdict will be in: the scope contains a Lanthanum mating element.  That has been a consistent pattern I have seen with these FPL-53/FCD-100 and Lanthanum scopes.

Edited by Oldfracguy, 17 June 2025 - 06:08 PM.

[betacygni]

If you can find one a Vixen FL-90S might be a fine contender (though only f9, but should still do better then f7.5). They sell significantly under their performance to price ratio, but are quite uncommon to find. Another good contender would be the Tak 100DL on the used market. If you really want to go crazy there is the new TEC 140 f9.6 fluorite doublet…

For the record I don’t think you’re crazy, eyepieces are an important interface between us and our views, I certainly have considered eyepiece preferences in scope purchasing decisions.

[Sacred Heart]

balcon3,

 

I think I sort of understand the question, you are wanting a well corrected refractor so you see no imperfections. The Takahasi gives you this.   The Tak is 740mm in focal length,   what focal length are you wanting??  An 80mm F11 is up there in focal length also, a little more than your Tak.   Are you not wanting something in the 400mm to 550mm range??  An 80mm F10 / F11 is basically replicating the Tak in focal length.  To me anyway.

 

If I'm thinking well corrected small refractors, I'm thinking Televue 76 and 101, AP 92 and the CFF92.  I do not have that eyepiece, I have a TV 22mm Nagler and like the views through my TV76, CFF92 F6, as well as my Agema SD130.

 

Joe