153 replies to this topic

[ris242]

The difference between using a TSFLAT2 and not using it is pretty remarkable, sort of like looking though an f/4 or f/5 Newtonian with and without a Coma Corrector. 

 

You might have to try the 0.6x reducer on your scope.
 

[maniack]

Phil is right on.  The TSFLAT2 in a faster 4" (f/5, f/6.5) achromat really only flattens the field by getting rid of most of the inherent field curvature, and, in my experience, is really only useful when going for wide-field, low-power views with better-corrected 2" eyepieces like the 28mm UWA and the 30mm UFF.  Even when I used a TSFLAT2 with an 80mm APO-type refractor, I removed it for high magnification viewing.

 

The difference between using a TSFLAT2 and not using it is pretty remarkable, sort of like looking though an f/4 or f/5 Newtonian with and without a Coma Corrector.  The difference is quite apparent, whether the Newtonian has a custom mirror or a mass-produced one.  With refractors, the TSFLAT2 has an added benefit in APO-like scopes.  The difference between using a TSFLAT2 in an achromat and an ED or APO refractor shows up in the increased sharpness of the stars not just out along the edge of the field of view, but also in the middle.  At first I didn't understand why this would be so, but after looking at the Double Cluster back and forth using an APO-type refractor with the TSFLAT2 installed and removed from the 2" diagonal (same eyepiece, of course), I stopped trying to figure it out.  The stars in the middle of the field, and especially in the outer regions, were sharper with the TSFLAT2 installed.  You would definitely see improvement using a TSFLAT2 in the ST102 and ST120 (I saw it in an AT125EDL even), and also some improvement with the EON 80ED and AT102ED.

But topomountain is asking about wide field views, specifically the 28mm UWA you mentioned.

 

I would expect the view at 14-15x to be a bit better in the Televue petzvals, but still pretty close. CA isn't a problem for such low magnification.

[Oldfracguy]

But topomountain is asking about wide field views, specifically the 28mm UWA you mentioned.

 

I would expect the view at 14-15x to be a bit better in the Televue petzvals, but still pretty close. CA isn't a problem for such low magnification.

As has been discussed in another recent thread somewhere, CA is still present even at low magnifications.  With achromats at low magnifications, you don't necessarily see that bluish/purplish fringing indicative of CA, but still all the colors (wavelengths) are not focused on the same plane.  That leads to an image that is degraded compared with the images created by ED or APO refractors.  Those scopes do a much better job at concentrating all the incoming light at the same point, which is why things look sharper with more fine detail in ED and APO refractors compared with achromats.

Edited by Oldfracguy, 26 July 2024 - 08:37 AM.

[Phil Cowell]

Try looking at the moon.

 

But topomountain is asking about wide field views, specifically the 28mm UWA you mentioned.

 

I would expect the view at 14-15x to be a bit better in the Televue petzvals, but still pretty close. CA isn't a problem for such low magnification.

[maniack]

Try looking at the moon.

Why would you look at the moon at ~15x magnification using a 4" ~f/5 refractor? An ST102 with the 28mm UWA provides a 4.6 degree FOV, 4.2 degrees with the 30mm UFF I have.

 

The point being asked is would a 4" achromat with a visual flattener be an adequate choice for those 4-5 degree wide views. Jon says yes, and there are others on these forums that use that combination. I'm truly interested in how much worse the achromat would be since I intend to get an ST102 at some point (I already have a TSFLAT2). But I'm not interested in solar system objects or splitting double stars or other high magnification views as I have much better scopes for those views.

[Jon Isaacs]

As has been discussed in another recent thread somewhere, CA is still present even at low magnifications.  With achromats at low magnifications, you don't necessarily see that bluish/purplish fringing indicative of CA, but still all the colors (wavelengths) are not focused on the same plane.  That leads to an image that is degraded compared with the images created by ED or APO refractors.  Those scopes do a much better job at concentrating all the incoming light at the same point, which is why things look sharper with more fine detail in ED and APO regractors compared with achromats.

 

At F/5, the 28mm UWA provides a 5.6 mm exit pupil.  The eye's resolving power at a 5.6 mm exit pupil is not good at all. If it were, we would never need to use those 0.6mm exit pupils to resolve fine details.. 

 

I understand all about chromatic aberration.. But I also know that a large exit pupils, it's effects are minimal.. This not about the 102mm F/5 achromat providing the sharpness across the field that the NP-101 does, an APO with a TSFLAT2 doesn't do that.. It's about a nice view under dark skies without too much field curvature.  

 

I calculate the difference between center focus and edge focus using the 28mm UWA in a scope with a 500mm focal length is about 1.2 mm.  That is significant, noticeable, bothersome.. the defocused blur is about 240 microns..  The chromatic blur of a 102 mm F/5 is about 8x the Airy disk diameter 6.7 microns, the chromatic blur is 54 microns in diameter..  In the outer half of the field, the defocused blur is larger in diameter than the chromatic blur.

 

The TSFLAT2 does a decent job of correcting the field curvature.. I think it's safe to say that a 102mm F/5 achro can qualify as a poorman's NP-101.  When I have compared them without the TSFLAT2, the views are very similar, I pretty much see the same exact stuff, one is just more perfect than the other.. 

 

The virtue of the NP-101 is that it does it all.. the 4.9 degree flat field on up to 300x on a close double..  But what sets it apart from other 4 inch refractors is that flat field at low magnifications and a 102mm F/5 with a TSFLAT2 does a pretty decent imitation.. 

 

Jon

[bobhen]

Why would you look at the moon at ~15x magnification using a 4" ~f/5 refractor? An ST102 with the 28mm UWA provides a 4.6 degree FOV, 4.2 degrees with the 30mm UFF I have.

 

The point being asked is would a 4" achromat with a visual flattener be an adequate choice for those 4-5 degree wide views. Jon says yes, and there are others on these forums that use that combination. I'm truly interested in how much worse the achromat would be since I intend to get an ST102 at some point (I already have a TSFLAT2). But I'm not interested in solar system objects or splitting double stars or other high magnification views as I have much better scopes for those views.

The answer is yes. You will not see much if any CA at the low powers used. There are plenty of fast, achromatic binoculars out there that people enjoy without complaint. As a matter of fact, the 4" F5 will be MORE than adequate for what you are intending to use it for.

 

I've owned mine for 20-years and it has provided wonderful, dare I say, thrilling views under a dark sky. Now, I use mine with an image intensifier and the low power, deep sky views are absolutely breathtaking. Get one, use it for what it's best at, and you will not be disappointed.

 

Below is my 20-year-old, 4" F5 with the image intensifier in the diagonal. A deep sky, wide field machine and still going stronger than ever. 

 

Bob

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[Phil Cowell]

No matter how you spin it there is bloating of stars even at low power. Pin point stars across the field are not going to be there even with the flattener. 
So to answer the question, no it’s not going to give 101 performance. Bright stars even at low power would display CA’s presence. If CA bothers you once you see it the eye looks for it. If you’re tolerant to it as said before go for it. The CA might be minimal to some and bloody annoying to others. 

A fast achro is a fast achro.

If someone is in central NY and has the fast achro(s) in question, I have mounts to do a side by side test. Heck can put up an FSQ106 as well so you can check between the three.

I’m willing to wager the difference would be easily noticeable.

Edited by Phil Cowell, 26 July 2024 - 07:11 AM.

[tony_spina]

No matter how you spin it there is bloating of stars even at low power. Pin point stars across the field are not going to be there even with the flattener. 
So to answer the question, no it’s not going to give 101 performance. Bright stars even at low power would display CA’s presence. If CA bothers you once you see it the eye looks for it. If you’re tolerant to it as said before go for it. The CA might be minimal to some and bloody annoying to others. 

A fast achro is a fast achro.

If someone is in central NY and has the fast achro(s) in question, I have mounts to do a side by side test. Heck can put up an FSQ106 as well so you can check between the three.

I’m willing to wager the difference would be easily noticeable.

You are missing the whole point of the request. 

 

Is an ST102 with a TSFLAT2 a good poor man's TV101.   At low to mid powers The OP will enjoy the views. No one questioned that an APO is better 

 

And before you ask.  I have all the equipment that the OP is asking for the comparison. 

 

To the OP. The views with an ST120 and the TSFLAT2 are also very impressive if you are considering a bigger scope

[Jon Isaacs]

No matter how you spin it there is bloating of stars even at low power. Pin point stars across the field are not going to be there even with the flattener. 
So to answer the question, no it’s not going to give 101 performance. Bright stars even at low power would display CA’s presence. If CA bothers you once you see it the eye looks for it. If you’re tolerant to it as said before go for it. The CA might be minimal to some and bloody annoying to others. 

A fast achro is a fast achro.

If someone is in central NY and has the fast achro(s) in question, I have mounts to do a side by side test. Heck can put up an FSQ106 as well so you can check between the three.

I’m willing to wager the difference would be easily noticeable.

 

Phil:

 

I have the fast Achro in question.  I also have an ST-80 with a 2 inch focuser.  I also have a TSFLAT2.. I also have an NP-101 as well as a 31mm Nagler..  I do know something about this, I have experience under dark skies with these combinations.  I am not speculating.. Been there, done that.   

 

I am plenty sensitive to chromatic aberration but the reality is that at low magnifications and large exits pupils, it's not a big deal. The math shows that the defocused blur from field curvature is greater by far in the outer field than the chromatic blur.. Sure, maybe bright stars will show some CA but the Pleiades in an ST-80 with a TSFLAT2 and the 31 mm Nagler are pretty awesome... 

 

Keep in mind that a Poorman's NP-101 only means that it does a pretty good imitation of an NP-101 at low powers, it does not mean it duplicates it.  Certainly a camera would show bloated stars but the eye is not a camera and has limited response in the violet and red.. 

 

Bob's mention of large aperture binoculars is something to consider..  The large aperture Fujinons like these 25s x 150 are achromats with a flat field.

 

https://www.cloudyni...binoculars-r195

 

Of course they hardly qualify as a budget solution, as they are around $7000 without a mount.. 

 

Jon

[Oldfracguy]

Does anyone have any experience with this type of scope, particularly how it performs vis a vis the NP-101?

 

https://www.cloudyni...ion-vixen-120s/

[Jon Isaacs]

Does anyone have any experience with this type of scope, particularly how it performs vis a vis the NP-101?

 

https://www.cloudyni...ion-vixen-120s/

 

I saw that too.  I have never looked through any of the Vixen Modified Petzval achromats but it is a 120mm F/6.7 so field curvature in doublet or triplet is not so much of an issue.. 

 

jon

[RichA]

As has been discussed in another recent thread somewhere, CA is still present even at low magnifications.  With achromats at low magnifications, you don't necessarily see that bluish/purplish fringing indicative of CA, but still all the colors (wavelengths) are not focused on the same plane.  That leads to an image that is degraded compared with the images created by ED or APO refractors.  Those scopes do a much better job at concentrating all the incoming light at the same point, which is why things look sharper with more fine detail in ED and APO refractors compared with achromats.

The visible halo has never been the main problem, just the most obvious.  It's the overall contrast-sapping blur of blue and red light over the entire image that is.  In fact, viewing the moon at higher powers in a fast achromat is akin to viewing in non-dark conditions because scattered blue light from a daylight or twilight sky is like blue-blur chromatic aberration.

[Terra Nova]

I would at least try for an SDF.

The SDF is indeed a wonderful scope and easily a 90% visual apo. Be advised however that it is longer and heavier than the Genesis F5 before it or the later NP101. I enjoyed mine for a number of years.

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[kmparsons]

You don't need a premium instrument to get thrilling views. I have been tempted by this one:

 

https://www.teleskop...riebauszug-2229

 

Yeah, i know, lots of CA on bright targets, but with the large aperture, under dark skies I imagine you could get some terrific views of the Sagittarius star clouds and other deep sky targets. 

[Phil Cowell]

I’m very sensitive to CA and the ST80 we called magic mushroom. Bloody awful thing.

As for Binoculars I’ve been using ED for years. Currently Zeiss Terra’s.

With regard to star bloat look at both side by side it would be noticeable and correction will not go to the edge.

As stated it’s dependent on the OP’s tolerance for CA.

I have two NP101’s here but no achro’s due to CA. 
To each their own but I think it would be a poor excuse compared to an np101. The NP isn’t just a one trick pony the achro combo would be.

The OP asked I gave an honest answer.

 

Phil:

 

I have the fast Achro in question.  I also have an ST-80 with a 2 inch focuser.  I also have a TSFLAT2.. I also have an NP-101 as well as a 31mm Nagler..  I do know something about this, I have experience under dark skies with these combinations.  I am not speculating.. Been there, done that.   

 

I am plenty sensitive to chromatic aberration but the reality is that at low magnifications and large exits pupils, it's not a big deal. The math shows that the defocused blur from field curvature is greater by far in the outer field than the chromatic blur.. Sure, maybe bright stars will show some CA but the Pleiades in an ST-80 with a TSFLAT2 and the 31 mm Nagler are pretty awesome... 

 

Keep in mind that a Poorman's NP-101 only means that it does a pretty good imitation of an NP-101 at low powers, it does not mean it duplicates it.  Certainly a camera would show bloated stars but the eye is not a camera and has limited response in the violet and red.. 

 

Bob's mention of large aperture binoculars is something to consider..  The large aperture Fujinons like these 25s x 150 are achromats with a flat field.

 

https://www.cloudyni...binoculars-r195

 

Of course they hardly qualify as a budget solution, as they are around $7000 without a mount.. 

 

Jon

Edited by Phil Cowell, 26 July 2024 - 10:48 AM.

[Oldfracguy]

You don't need a premium instrument to get thrilling views. I have been tempted by this one:

 

https://www.teleskop...riebauszug-2229

 

Yeah, i know, lots of CA on bright targets, but with the large aperture, under dark skies I imagine you could get some terrific views of the Sagittarius star clouds and other deep sky targets. 

I was thinking the same thing.  I just picked up a 4" f/6.5 achromat and have been quite satisfied with its performance at lower powers, especially with a TSFLAT2 ahead of the 2" diagonal.  For kicks I replaced that diagonal with a Baader T2 Prism and a short 1.25" Orthoscopic eyepiece and took a look at Izar.  This scope is not far behind what I remember seeing using an AT102EDL on those double stars.  However, a 4" aperture does leave you wanting for more when viewing globular clusters.  I picked up a 150mm Mak, and the difference in the views of M13 using that scope and the 102mm achromat (or any 4" scope for that matter) are quite noticeable, which is why It crossed my mind (again) about the idea of one of these 6" f/5.9 achromats.  The same basic scope is also available from Altair Astro:

 

https://www.altairas...scope-461-p.asp

[maniack]

I’m very sensitive to CA and the ST80 we called magic mushroom. Bloody awful thing.

As for Binoculars I’ve been using ED for years. Currently Zeiss Terra’s.

With regard to star bloat look at both side by side it would be noticeable and correction will not go to the edge.

As stated it’s dependent on the OP’s tolerance for CA.

I have two NP101’s here but no achro’s due to CA. 
To each their own but I think it would be a poor excuse compared to an np101. The NP isn’t just a one trick pony the achro combo would be.

The OP asked I gave an honest answer.

I'm also quite sensitive to CA (one of the reasons I moved on from an AT80ED), but the ST80 I have is fine in that regard for low magnification wide-field. What bothers me with the ST80 in those wide views is field curvature. The TSFLAT2 seems to do a pretty good job cleaning that up in my AT72EDII (similar focal length to the ST80) on even wider views with a 2" 30mm UFF.

 

I agree that the NP isn't a one-trick pony, it's a scope that does everything well. I'd love to have one, or even better an FSQ106. But the NP costs more than all of my current equipment combined, and the FSQ is almost double that. Not to mention the more robust mount I'd have to acquire for those scopes. If I were to travel to dark skies a lot I could possibly justify the expense. But for now a $200-300 one-trick pony for the views that I can't get out of my SCTs or smaller refractors sounds pretty good.

[bobhen]

I’m very sensitive to CA and the ST80 we called magic mushroom. Bloody awful thing.

As for Binoculars I’ve been using ED for years. Currently Zeiss Terra’s.

With regard to star bloat look at both side by side it would be noticeable and correction will not go to the edge.

As stated it’s dependent on the OP’s tolerance for CA.

I have two NP101’s here but no achro’s due to CA. 
To each their own but I think it would be a poor excuse compared to an np101. The NP isn’t just a one trick pony the achro combo would be.

The OP asked I gave an honest answer.

Below are 2 different reviews. Regarding deep sky, both praise the scope as a low power, deep sky telescope that when used as such is not too far from an apo. My experience agrees. And I've had enough apos to compare. And I once owned a TV F5 Genesis. 

 

First Review...

 

Deep Sky

The StarTravel 102 (F5) excels as a wide-field scope as you would expect and does the job very well indeed. All the refractor hallmarks of wide sharp field and diamond pinpoint stars are there in abundance. In fact, apart from a little extra field curvature and off-axis coma, this could be mistaken for a premium refractor when looking at the Orion Nebula or a cluster like M36 in Auriga. The extra aperture really makes a difference for deep sky when compared to most 3” grab-and-go scopes (around 45% more light gathering than a 76mm, for example). Note that for visual use on Deep Sky, a premium 4” APO from one of the top makers really won’t give a much better view than this cheap achromat!

 

Overall the contrast with another scope I’m testing – the Vixen VMC95 Maksutov – was interesting. The Vixen of course shows little CA, but in almost every other respect its view was inferior, even on Jupiter where you’d think it would excel.

 

Summary
What’s not to like about Sky-Watcher’s simply excellent StarTravel 102? It is cheap, light, fast and grabs a lot of light. Its compact size makes it easy to mount and the optical quality, on my sample at least, was very good indeed – better in fact than you have any right to expect from a scope of this type and price.

 

HERE is a link to the complete review

 

Another Review...
 

But the real beauty of the lightweight f/5 is best found at low power where its 2-inch focuser and wide field-of-view come into play. At 20x, using the standard 25mm Plossl supplied with the scope, the Andromeda Galaxy filled most of the field with delicate nebulosity, its two companion galaxies, a hint of dark lanes, and myriad foreground stars. In similar fashion, the Pleiades were neatly framed and quite striking with the faint stain the nebulosity around Merope being easily visible.

 

With the magnification increased to 56x by a 9mm Type 6 Nagler, the numerous open star clusters of autumn became attractive targets. The Double Cluster in Perseus was beautifully framed at this power, the “diamonds on black velvet” effect not too far from what one would expect from a good APO. The trio of Auriga clusters, M 37, 38, and 36 also revealed pinpoint stars, the scope’s inherent chromatic aberration being a non-factor on stellar objects fainter than magnitude 5 or so.

 

HERE is a link to the complete review

 

Bob

Edited by bobhen, 26 July 2024 - 12:38 PM.

[Echolight]

No matter how you spin it there is bloating of stars even at low power. Pin point stars across the field are not going to be there even with the flattener. 
So to answer the question, no it’s not going to give 101 performance. Bright stars even at low power would display CA’s presence. If CA bothers you once you see it the eye looks for it. If you’re tolerant to it as said before go for it. The CA might be minimal to some and bloody annoying to others. 

A fast achro is a fast achro.

If someone is in central NY and has the fast achro(s) in question, I have mounts to do a side by side test. Heck can put up an FSQ106 as well so you can check between the three.

I’m willing to wager the difference would be easily noticeable.

Certainly the price is easily noticeable.

[Terra Nova]

Does anyone have any experience with this type of scope, particularly how it performs vis a vis the NP-101?

 

https://www.cloudyni...ion-vixen-120s/

I owned a pure Vixen 120S for a number of years. It’s not an apo but color is very well corrected, about the same as an F12 or F13 120mm achromat. It also delivers pretty wide fields (not as wide as an ST120) and the field is fairly flat. It’s really a multipurpose telescope, in a whole other league compared to an ST120. I’ve had three ST120s. In my book, ST120s (and ST102s) are cheap thrills but they’re strictly fast RFTs. The 120s can deliver good views of the moon and planets at 200X, (w/ 4mm Delite). In fact, on one night of excellent seeing, I pushed mine up to 266X with a 3mm Delite and the views of Jupiter and Saturn were quite serviceable. Note the 120S is the five element model, the petzval unit has three elements. It’s a level higher than the more common NA120 (Vixen called that one a neoachromat) which has four elements and not as good of color control. I liked mine a lot and only sold it because it’s a fairly large telescope and I was moving from my house into a condo where it would have no longer been practical.

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[Phil Cowell]

To each their own. I’ve owned several achro’s and been unimpressed fa@tless. None remain in the stable. As stated I’m very sensitive to CA and it’s going to be there in a fast achro. As an imager, a less than stellar field sticks out like a sore thumb. You might have a different view or different expectations, everyone is different. I provided an opinion that I stand by. You made your opinion and I dare say you stand by that. The OP now has a few opinions to add to the mix. The OP might be happy with the view that’s provided, I wouldn’t, knowing the imperfections of the system it would be something looked for and found. Like I say, I stand by my comment. 

I have currently 35+ scopes so have enough to compare against as well, including two NP-101’s being converted to an NV binoscope.

The APO comment you added bold to makes me think the reviewer is suspect at best and to top it off this completely makes it not reliable. 
“Overall the contrast with another scope I’m testing – the Vixen VMC95 Maksutov – was interesting. The Vixen of course shows little CA, but in almost every other respect its view was inferior, even on Jupiter where you’d think it would excel.”

 

The second not far comment you added bold text to what’s the reviewers definition of not far?
Like a meat substitute that many folks say tastes just like meat? I disagree there too.

 

It’s all subjective and individual observer dependent. 

 

Below are 2 different reviews. Regarding deep sky, both praise the scope as a low power, deep sky telescope that when used as such is not too far from an apo. My experience agrees. And I've had enough apos to compare. And I once owned a TV F5 Genesis. 

 

First Review...

 

Deep Sky

The StarTravel 102 (F5) excels as a wide-field scope as you would expect and does the job very well indeed. All the refractor hallmarks of wide sharp field and diamond pinpoint stars are there in abundance. In fact, apart from a little extra field curvature and off-axis coma, this could be mistaken for a premium refractor when looking at the Orion Nebula or a cluster like M36 in Auriga. The extra aperture really makes a difference for deep sky when compared to most 3” grab-and-go scopes (around 45% more light gathering than a 76mm, for example). Note that for visual use on Deep Sky, a premium 4” APO from one of the top makers really won’t give a much better view than this cheap achromat!

 

Overall the contrast with another scope I’m testing – the Vixen VMC95 Maksutov – was interesting. The Vixen of course shows little CA, but in almost every other respect its view was inferior, even on Jupiter where you’d think it would excel.

 

Summary
What’s not to like about Sky-Watcher’s simply excellent StarTravel 102? It is cheap, light, fast and grabs a lot of light. Its compact size makes it easy to mount and the optical quality, on my sample at least, was very good indeed – better in fact than you have any right to expect from a scope of this type and price.

 

HERE is a link to the complete review

 

Another Review...
 

But the real beauty of the lightweight f/5 is best found at low power where its 2-inch focuser and wide field-of-view come into play. At 20x, using the standard 25mm Plossl supplied with the scope, the Andromeda Galaxy filled most of the field with delicate nebulosity, its two companion galaxies, a hint of dark lanes, and myriad foreground stars. In similar fashion, the Pleiades were neatly framed and quite striking with the faint stain the nebulosity around Merope being easily visible.

 

With the magnification increased to 56x by a 9mm Type 6 Nagler, the numerous open star clusters of autumn became attractive targets. The Double Cluster in Perseus was beautifully framed at this power, the “diamonds on black velvet” effect not too far from what one would expect from a good APO. The trio of Auriga clusters, M 37, 38, and 36 also revealed pinpoint stars, the scope’s inherent chromatic aberration being a non-factor on stellar objects fainter than magnitude 5 or so.

 

HERE is a link to the complete review

 

Bob

[ris242]

Maybe I should have named the topic - a decent flatfield short 4" refractor for a tighter budget.

 

I was never intending in discussing longer FL refractors.  I have 3 already and a forth on order.

 

The original 103 mentioned with a reducer would be half the FL of my other scopes.

I would have thought if the distance was correct it would give views like the NP101?

[Phil Cowell]

You should drop the NP101 comparison and just ask if it would give satisfactory wide field low power views.

An NP101 it is not.

 

Maybe I should have named the topic - a decent flatfield short 4" refractor for a tighter budget.

 

I was never intending in discussing longer FL refractors.  I have 3 already and a forth on order.

 

The original 103 mentioned with a reducer would be half the FL of my other scopes.

I would have thought if the distance was correct it would give views like the NP101?

[Echolight]

Welp, I guess there goes my idea for a poor man's Genesis thread when I get a TSFLAT2 for my StarTravel 102.