Battle of the budget 4-inch semi-APOs – Explorer Scientific ED 102mm F/7 FCD1 triplet vs Kunming ED 102mm F7 FPL53 doublets

[Prism]

Hello friends,

Back with a topic that never seems to never really get resolved!

I am trying to gauge the better choice between the following FCD1 triplet and these similarly priced FPL53 doublets:

a) Explorer Scientific ED 102mm F/7 FCD1 Triplet (currently retailing for around £900), vs

b) Any of the Kunming ED 102 F/7 FPL53 Doublet variants (such as the Altair Starwave 102ED-R FPL53 or the TS-Optics Doublet SD Apo 102) (all also currently retailing for around £900)

All other factors aside (such as price, cool down time, quality of the focuser, included accessories etc.), which has the superior optics for viewing and photo use – the ES FCD1 Triplet or the Kunming FPL53 Doublet? Has anyone here on SGL had the opportunity to directly A/B test them side-by-side? And if so, do you have any photographic evidence you could share? I’ve searched the forums as well as Astrobin, but have never found a satisfactory answer to this, and due to COVID (as well as the unfortunate lack of an adequate returns policy from many retailers), I am unable to test these out in the flesh. I’m a numbers guy, so looking for solid evidence or any data that can prove the superiority of one over the other. Any specialists in optics out there that care to chime in? The closest I’ve come to an answer is in this link that someone on the forums posted:

https://www.telescope-optics.net/polychromatic_psf.htm

The charts in this document seem to suggest the triplet is the better option, but I know there are a lot of variables including build quality / QC etc that come into play, hence why I am asking about these specific scopes as opposed to triplets and doublets in general.

Some background: Like many, I’m in the market for an affordable (<£1,000) semi-APO refractor for some casual observing. I plan to use this primarily for visual use (save for the odd shot of the moon taken from my smartphone with a suitable adapter) – mostly lunar and planetary viewing to be specific, from my balcony in (light polluted) central London. I already have a larger, rather heavy SCT on an EQ mount gathering dust in the basement, and I’m looking to simply my rig by switching over to a relatively light 4-inch refractor on a solid AZ mount. This size was chosen for being small / easily manoeuvrable, allowing adequate magnification for my needs (I have achieved a crisp 250x on similar sized ED scopes with good seeing), as well as for having the maximum aperture that I can afford at this time (i.e. that the Mrs will allow).

 As far as the mount is concerned, I’ll be using it with the TS Optics AZ5 (Skytee 2 clone) on vibration pads, with upgraded ADM saddles and a heavy-duty Skywatcher 1.75” steel tripod (borrowed from my EQ mount), on which it will be mounted weight-balanced with the top slot for the Telrad. The scope will be used with my Nagler 3-6mm and Pentax 8-24mm zoom eyepieces.

I should mention that I briefly owned a cheap Celestron refractor that I bought on impulse, wrongly believing that I could improve it by using a quality diagonal and eyepieces. Unfortunately, the CA was unbearable (the purple fringe was wide enough to severely compromise planetary viewing) and expensive eyepieces made no discernible improvement, forcing me to return it. Lesson learned: your rig is only as good as the weakest link in the optical chain. Apologies for the long post, but I’m trying to avoid the usual, “I haven’t tried them both, but they both seem like great scopes…” type responses.

All the best,

Al

[Adam J]

18 hours ago, Prism said:

Hello friends,

Back with a topic that never seems to never really get resolved!

I am trying to gauge the better choice between the following FCD1 triplet and these similarly priced FPL53 doublets:

a) Explorer Scientific ED 102mm F/7 FCD1 Triplet (currently retailing for around £900), vs

b) Any of the Kunming ED 102 F/7 FPL53 Doublet variants (such as the Altair Starwave 102ED-R FPL53 or the TS-Optics Doublet SD Apo 102) (all also currently retailing for around £900)

All other factors aside (such as price, cool down time, quality of the focuser, included accessories etc.), which has the superior optics for viewing and photo use – the ES FCD1 Triplet or the Kunming FPL53 Doublet? Has anyone here on SGL had the opportunity to directly A/B test them side-by-side? And if so, do you have any photographic evidence you could share? I’ve searched the forums as well as Astrobin, but have never found a satisfactory answer to this, and due to COVID (as well as the unfortunate lack of an adequate returns policy from many retailers), I am unable to test these out in the flesh. I’m a numbers guy, so looking for solid evidence or any data that can prove the superiority of one over the other. Any specialists in optics out there that care to chime in? The closest I’ve come to an answer is in this link that someone on the forums posted:

https://www.telescope-optics.net/polychromatic_psf.htm

The charts in this document seem to suggest the triplet is the better option, but I know there are a lot of variables including build quality / QC etc that come into play, hence why I am asking about these specific scopes as opposed to triplets and doublets in general.

Some background: Like many, I’m in the market for an affordable (<£1,000) semi-APO refractor for some casual observing. I plan to use this primarily for visual use (save for the odd shot of the moon taken from my smartphone with a suitable adapter) – mostly lunar and planetary viewing to be specific, from my balcony in (light polluted) central London. I already have a larger, rather heavy SCT on an EQ mount gathering dust in the basement, and I’m looking to simply my rig by switching over to a relatively light 4-inch refractor on a solid AZ mount. This size was chosen for being small / easily manoeuvrable, allowing adequate magnification for my needs (I have achieved a crisp 250x on similar sized ED scopes with good seeing), as well as for having the maximum aperture that I can afford at this time (i.e. that the Mrs will allow).

 As far as the mount is concerned, I’ll be using it with the TS Optics AZ5 (Skytee 2 clone) on vibration pads, with upgraded ADM saddles and a heavy-duty Skywatcher 1.75” steel tripod (borrowed from my EQ mount), on which it will be mounted weight-balanced with the top slot for the Telrad. The scope will be used with my Nagler 3-6mm and Pentax 8-24mm zoom eyepieces.

I should mention that I briefly owned a cheap Celestron refractor that I bought on impulse, wrongly believing that I could improve it by using a quality diagonal and eyepieces. Unfortunately, the CA was unbearable (the purple fringe was wide enough to severely compromise planetary viewing) and expensive eyepieces made no discernible improvement, forcing me to return it. Lesson learned: your rig is only as good as the weakest link in the optical chain. Apologies for the long post, but I’m trying to avoid the usual, “I haven’t tried them both, but they both seem like great scopes…” type responses.

All the best,

Al

Both will have some CA in imaging and very little visually, but if it was me I would be going with the FPL-53 / lanthanum Doublet for imaging and even more so for pure visual. Cools down faster, optically simpler and so you are likely to get a better sample and the ES focuser is not the best anyway.   

Bottom line for me is that cheaper triplets can be trouble and in that price bracket would I like my chances better with a quality doublet than a cheap triplet.

This chap has lots of example shots from the TS 102mm FLP-53 doublet in his profile on telescopious:

https://telescopius.com/profile/cvbadengoth

https://telescopius.com/pictures/view/33985/deep_sky/by-cvbadengoth

He also has a couple of youtube vids on the scope with a good review but in german, subs work ok though. 

Adam 

Edited October 8, 2020 by Adam J

[Prism]

9 minutes ago, Adam J said:

Both will have some CA in imaging and very little visually, but if it was me I would be going with the FPL-53 / lanthanum Doublet for imaging and even more so for pure visual. Cools down faster, optically simpler and so you are likely to get a better sample and the ES focuser is not the best anyway.   

Bottom line for me is that cheaper triplets can be trouble and in that price bracket would I like my chances better with a quality doublet than a cheap triplet.

Adam 

Adam have you have a chance to compare them side-by-side? 

I'm aware of the additional cool down time of a triplet as well as the superiority of the Kunming focusers vs the ES, but I'm strictly interested in which has the better visuals. The ES having ED glass and being a triplet should be (at least slightly) better than a doublet, albeit one with better glass, no? 

[Adam J]

9 minutes ago, Prism said:

Adam have you have a chance to compare them side-by-side? 

I'm aware of the additional cool down time of a triplet as well as the superiority of the Kunming focusers vs the ES, but I'm strictly interested in which has the better visuals. The ES having ED glass and being a triplet should be (at least slightly) better than a doublet, albeit one with better glass, no? 

See my edit / link above, 

I have not had chance to compare them no, i doubt many will had done.

I have looked through the doublet but not the triplet, the rest of my comments are a technical assessment based on my general experience with optics. 

Looking at the moon I only saw the slightest CA with the doublet, it was not intrusive. It was not my scope mind you it was someone at the local club when i first started out. I remember it though as i was considering the same choice before i decided to save for my Esprit 100, and even for imaging I concluded I would go with the doublet. 

Looking at images online I would say that both have CA (always more apparent in imaging) but I don't really see much difference between images from the two.  

Adam

 

Edited October 8, 2020 by Adam J

[mikeDnight]

I too would not hesitate to go for the doublet FPL53/Lanthanum ED. A couple of years ago i had chance to use an older AstroTech 102 ED alongside a beautiful Takahashi FC100DC.  At no time did the AstroTech look like the poor cousin. There was some minimal CA, very well controlled, along the limb of the Moon and when looking at brilliant stars like Vega. This was in no way obtrusive! When in focus the AstroTech was a quality ED apo that happily ran alongside a Tak fluorite with little difference between them. As a lunar and planetary refractor the AstroTech doublet ED performed very well indeed. 

If you are an observer who obsessively looks for CA as some do, you'll find it, but there will be more colour fringing from atmospheric dispersion and from many eyepiece types than from the ED objective itself. The Triplet may boast better colour control, but you may find the more rapid cool down of the doublet will give you finer planetary definition more quickly and more regularly than the triplet lens will, especially if you're observing straight from a warm room out into the cooler night air. Both will do the job, its just that the doublet may be a more reliable option.

[Prism]

On 08/10/2020 at 13:42, mikeDnight said:

I too would not hesitate to go for the doublet FPL53/Lanthanum ED. A couple of years ago i had chance to use an older AstroTech 102 ED alongside a beautiful Takahashi FC100DC.  At no time did the AstroTech look like the poor cousin. There was some minimal CA, very well controlled, along the limb of the Moon and when looking at brilliant stars like Vega. This was in no way obtrusive! When in focus the AstroTech was a quality ED apo that happily ran alongside a Tak fluorite with little difference between them. As a lunar and planetary refractor the AstroTech doublet ED performed very well indeed. 

If you are an observer who obsessively looks for CA as some do, you'll find it, but there will be more colour fringing from atmospheric dispersion and from many eyepiece types than from the ED objective itself. The Triplet may boast better colour control, but you may find the more rapid cool down of the doublet will give you finer planetary definition more quickly and more regularly than the triplet lens will, especially if you're observing straight from a warm room out into the cooler night air. Both will do the job, its just that the doublet may be a more reliable option.

mikeDnight I managed to get both scopes on hand and will post a brief comparative review of them shortly (with a few photos). First impressions are that the triplet has slightly better control of CA but the doublet is brighter and has more contrast, likely owing to the superior glass! CA on the doublet is definitely visible even without imaging, but is relatively mild and drastically less than my achromat. Most surprisingly, while I was certainly expecting a difference in cool down time, the effect is even more pronounced than I expected with the doublet literally ready for use right out a warm room vs 30-40 minutes with the triplet. 

[Adam J]

On 12/10/2020 at 16:01, Prism said:

mikeDnight I managed to get both scopes on hand and will post a brief comparative review of them shortly (with a few photos). First impressions are that the triplet has slightly better control of CA but the doublet is brighter and has more contrast, likely owing to the superior glass! CA on the doublet is definitely visible even without imaging, but is relatively mild and drastically less than my achromat. Most surprisingly, while I was certainly expecting a difference in cool down time, the effect is even more pronounced than I expected with the doublet literally ready for use right out a warm room vs 30-40 minutes with the triplet. 

So are you willing to trade the other advantages for slightly better CA? 

[mikeDnight]

6 hours ago, Adam J said:

So are you willing to trade the other advantages for slightly better CA? 

Hi Adam,

It was Prism's experience that was quoted and not mine. It confused me to initially, as it begins with "mikeDnight".

Anyhow, from a personal point of view I've never seen a triplet of any make produce a visual planetary view that was meaningfully better than that produced by a good doublet. For me there are more drawbacks than gains when it comes to triplet apo's. With an FPL53 doublet, the colour correction is so well controlled that the added cost, weight & cool-down time of a triplet just isn't worth the difference. Visually a FPL53 doublet could be considered apochromatic, because the scope itself produces very little residual CA; and many older triplets, including the likes of AstroPhysics, can show more colour than a modern FPL53 doublet. Often it can be local conditions that produce colour fringing. Thin cloud or haze can cause a colour fringe around bright objects, and the UK is often cloaked in some degree of haze or cloud even on apparently clear nights. Eyepieces, particularly wide angle designs, can create a colour fringe if the eye is slightly off axis. And of course atmospheric dispersion and turbulence play their part too. So I'd happily grab a good doublet over a triplet without ever believing I've ended up with a raw deal. Conversely, if I had a good triplet, I'd use it to its full potential.