Refractor options

[Astronovice]

I am looking to buy a refractor and have narrowed my choice down to the following 3 options (unless something better second hand turns up on here at a similar price).

1. SkyWatcher ED 120 doublet
2. SkyWatcher Esprit 80 ED Triplet
3. Saxon 102mm ED Apo Triplet

All three come with a similar price tag but obviously the similarity ends there.

I currently own an F5 200mm Saxon newt on an HEQ5 Pro mount and am looking for something a bit more portable, albeit using the same mount at this stage.

I want to use the scope for both visual and imaging (deep sky rather than planetary). I will be using an unmodded Nikon D5500 for imaging and already have a T Ring adaptor.

I live in FNQ so some degree of humidity is usually a factor, as is a moderate level of light pollution at home.

All sage advice from members regarding best choice will be gratefully appreciated.

[AstroApprentice]

I have an ED120 and it's a great visual scope. I have also taken very good lunar and planetary images with it using a Nikon D5500 and BackyardNikon. Nevertheless, it's not really an ideal deep sky setup for a number of reasons, particularly the mount demands due to its weight and length. If you're set on deep sky, then the Esprit 80 ED triplet has received many positive reviews and your HEQ5 will handle it comfortably.

[Saturnine]

Would also recommend the ED120 , it will sit on an HEQ5, which will handle 10 / 12 kg easily. For deep sky , aperture wins, it will pull in fainter objects than smaller scopes. Will give good views for planetary and will work well for astrophotography . Not as well corrected around the edge of the frame as an triplet but it depends how obsessed you want to become. Can always upgrade to an triplet if and when you become more inclined to imaging.

[xelasnave]

Hi Calvin
I went with the espirt 80mm and I think it is fantastic.
Easy to manage and I dont see any need to replace the focuser which often comes up.
The images I have obtained with it sees me using it and leaving my eight inch f5 in its box...if you are thinking of ever going narrow band you will be surprised what this little scope delivers.
Anyways in time you probably will add all the scopes on your list to your collection☺.
Alex

[Ukastronomer]

I have an Esprit and it is superb in finish and views

Can't beat a triplet

Mind you the doublet is nice I have the 72mm for Solar

As for Saxon, I'll stick with known brands for after sales service

[Ukastronomer]

:thumbsup:

[skysurfer]

Go for the ED120.

I have an ED110 for both visual and photographic use, with which I am very happy. I use it as a travel scope as well, I replaced the tube rings and the dew shield with lighter ones to reduce weight for airplane travel.
Most telescopes are not optimized for lightweight.

You can do the same with the ED120, if you think it is too heavy.

[casstony]

Hi Calvin, if you haven't done any DSO imaging it's easier to start with a shorter focal length scope - you'll get good results more quickly and avoid frustration.

You can start with 30 to 60 second exposures but eventually you'll want to learn to guide to enable exposures of a few minutes.

[glend]

X2. An 80mm is a pretty good place to start, it gives you a nice field of view (like being able to get the entire Rosette Nebula). Look wider than the Skywatcher Esprit, as it's abit expensive compared to other well know 80mm triplet APO. At the very least check the Photoline scopes offered by Teleskop Services. If the cost doesn't concern you just buy from a local retailer. Get a reducer/corrector, unless your buying a dedicated astrograph type APO (which tend to have them built in). A reducer/corrector will give you a faster scope and slightly wider flat field). A scope like the one below is perfectly fine with just a flattener attached as it is fast for a triplet.

An example:

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html

[traveller]

+1 for the ED80 Triplet.
It has better colour correction than a doublet, shorter FL so easier on the guiding, plus it has a focal ratio of f/5 which cuts down imaging time.

[Astronovice]

I think the Saxon is identical to the Explore Scientific 102mm F7 FCD100 triplet. The specifications and much of the scope description wording are identical on both Saxon and ES websites.

I would be interested in any comments regarding the Esprit 80 triplet vs the Saxon 102mm Triplet (Explore Scientific 102mm clone).

[gregbradley]

There is a bit of a scale of quality with refractors.

Here are some known good companies:

1. Teleskop Services. They have a broad range of scopes. A number of these are clones that are simply wearing their brand and you'll see the same scope elsewhere rebadged. But it seems like occasionally they demand a bit more from the same scope which is good.

2. Stellarvue. They have a good name. Their prices are probably a bit higher but you are less likely to have issues.

3. William Optics. They can sometimes be good. Check reviews though as I do read of the occasional dud.

4. Esprit. It seems these are quite high end triplets.

5. Astrotech - they often rebrand these same scopes as Teleskop Services, Skywatcher and others.


As a general rule triplets are better corrected for false colour than doublets although I see a lot of FPL53 (premium ED glass) and Lanthanum doublet scopes these days. How they perform is a bit hard to tell but from sample images some brands post they perform quite well.

Also keep in mind that the Q style refractors are more suitable for imaging than the straight triplets etc. Teleskop services lists their Q type astrograph refractors separately. They are often around F5 which is nice and fast and they should have more perfect stars in the corners.

As I understand it, a lot of these FPL53 triplets are lenses made by Canon iOptron and then they are fitted into various telescope bodies so the standard seems to be quite high.

APO refractors used to be extremely expensive and in the last several years they have come down a lot in price and the quality has risen.

To complete the list the high end scopes are:

1. Takahashi.
2. AstroPhysics.
3. TEC (Telescope Engineer Company).
4. APM.
5. CFF.

Astrophysics have a wait list so not that practical to list them but their scopes often come up in 2nd hand marts.

CFF would be my current top notch choice or Takahashi. APM used to be super high end but their strehl ratios are lower than Takahashi, CFF.
TEC are very good but optimised for visual so a bit weak in the red channel.

When considering a refractor I would;

1. Consider the brand as above.
2. Triplet is usually better than a doublet.
3. FPL53 is the premium glass and usually only 1 element is FPL53 and then the other 2 elements are some lesser glass. That is normal. If it were FPL51 I would be less interested.
4. Focuser. A good focuser is a must. It needs a micro adjuster as well.
Larger focusers are better than shorter ones.
5. How wide is the corrected circle of the scope? That means the circle where all the stars are round and not distorted.
6. Accessories - guide scopes, mounting rings, flatteners and reducers (a flattener makes stars round in the corners, a reducer makes the F ratio lower and the field of view wider making for faster imaging and wider views).

I hope that helps.

Greg.

[casstony]

The Esprit is a safe buy and includes a flattener. It's a high quality scope.

The Saxon 102 FCD100 triplet would be better for double duty as a visual/imaging scope. The quality may not be quite as good. Astro anarchy (astro pete's) is a reputable dealer which sells both brands - ask them if the focuser is good enough for imaging and also ask if they can supply a suitable reducer/flattener.

[Ukastronomer]

This is the problem when you don't have scopes lile this in the UK, even Explore Scientific are not "over" known, then you have no benchmark, as yet I have never seen an ES scope at a star party in the UK, just Skywatcher and better known ones.

You are probably 100% right and it is a good scope I would like to hear of peoples views on Saxon also

Thanks:thumbsup:

[Ukastronomer]

:thumbsup::thumbsup::thumbsup:

[brian nordstrom]

:D Jeremy , Saxon is a big brand down here , your run of the mill Synta scopes but the 102mm triplet is not one , its a Kuming Optical I think and would be the sweet spot between these 3 scopes .

Brian.

[Astronovice]

Thank you Greg,

Your comments are most helpful.

Am I right in assuming that Hoya FCD100 and Ohara FPL53 are broadly equivalent?

Calvin

[Ukastronomer]

I thought that may be the case, being well known there but not here

:thumbsup:

[glend]

It depends on how broadly you want to define equivalent.

FPL-53 offers almost indistinguishable performance compared to Fluorite/CaF2, in an astronomical refractor, it’s more stable, has better physical characteristics, cost far less, and it's environmentally better to work with. FPL-53 has an Abbe number of 94.99 .
FCD-100. The relatively new Extra Low Dispersion Hoya FCD-100 glass has an Abbe number of 94.66 compared to 94.99 of S-FPL53.
To most, if not all eyes using it visually, Flourite, FPL-53, and FCD-100, will appear to be the same. FCD-100 is more cost effective currently, but many astronomers buy on reputation, even if they cannot visually tell the difference, and Flourite and FPL-53 continue to sit at the top in people minds.

[Ukastronomer]

You see you get the FF free over there in the UK we have to buy it :(

[Ukastronomer]

"Am I right in assuming that Hoya FCD100 and Ohara FPL53 are broadly equivalent?

Calvin"

I bet no one could tell the difference visually

I have an argument with other photographers about prime v zoom lenses ED glass, I prefer zoom and use them as a paid sports photographer.

Put the same back on an 80-400 ED zoom such as the Nikkor and get three other bodies place a Nikkor 80-200-400 prime on each, shoot the same image NOT in a lab but real life, same settings and I challenge anyone to see any difference.

[Ukastronomer]

CaF2 “Fluorite” (Various manufacturers): 94.99 (incredibly expensive and hard to work with, often has inhomogeneity, internal stresses, can fracture during polishing, not very stable in a chemical or thermal sense).

S-FPL-53 (Ohara Japan): 94.94 (more even internal structure, freedom from internal stresses can be attained by baking and annealing, more chemically stable, less expensive and easier to work with than Fluorite, but harder to work with than S-FPL51).

S-FPL-51 (Ohara Japan): 81.54 (easier to work with, and much less expensive than S-FPL53, very stable by comparison).

FCD-100 (Hoya): 94.66 (less expensive than S-PL53, thanks Hoya!)

By combining multiple lens elements and using ED glasses in the same refractor lens cell, we can get very good or essentially perfect “colour correction” in a refractor, such that there are no false colours visible in the in-focus image. This is particularly evident in our FPL-53 based triplet refractors which have perfect colour correction to the eye both inside and outside of focus, and amongst the best photographic colour correction on the market today, especially in the red and blue ends of the spectrum which CCD cameras are very sensitive to. This is borne out in the profusion of images taken with our telescopes on the internet. It’s important to remember, however, that glass type and the Abbe number of that glass type isn’t the only factor in controlling colour correction or optical quality overall. The geometry of the surface being polished, the relative spacing between the lens elements, coatings, surface smoothness, and deviation from the ideal design, plus the final figuring process are all very important. In fact, we are able to get near perfect colour correction in focus in our FPL-51 based triplets, and better blue colour correction than many FCD1 and H-FK61 based lenses we’ve tested. This is achieved by using a better match of mating elements and geometry, and taking time to adjust the lens arrangement and surfaces during production. Once again, this is borne out by the images taken with our larger FPL-51 based refractors which show better blue channel colour correction than some competing refractors with FCD1 and H-FK61 lenses. The point is that, while a lower dispersion element helps, it's not the only factor. But for now, let’s concentrate on the main ED glass types we use:

[casstony]

The difference between fpl53, fpl55, fcd100 and fluorite is negligible compared to how well the overall lens and cell are made.

Some fpl53 doublets and fpl51 triplets are fine for imaging too, such as the two scopes in my signature.

[glend]

Yes indeed, FPL-51 Triplets which include a Lanthanum partner element, are highly regarded. The TS115 being a good example.

[gregbradley]

A lot of theory. I would want to simply evaluate images taken with the scopes/ The images tell you if the lens is OK or not.

I have used several fluorite scopes and they are a slight cut above FPL53 scopes. Usually better colour in the images and a little bit brighter image.

Fluorite allows the optical designer the freedom to achieve a faster F ratio without sacrificing optical performance. It also has less light scatter.

But a fluorite doublet - is it really an APO? Tak calls them APOs but I my old FS152 was superb as a visual instrument but you did get the odd blue ringed bright star with it imaging.

These ED glass/Lanthanum element lenses look interesting. I have not heard of one in a triplet yet only doublets. Which scopes have them in a triplet?



Greg.

[skysurfer]

This does not cut down imaging time. Only a larger aperture in mm cuts down image time to get the same faint objects.

But, indeed a triplet wins over a doublet if you are a real pixel peeper. Otherwise, rather get a 100...120mm ED doublet plus reducer.

[Camelopardalis]

I bought and Esprit 100 a few years ago and I image with it (almost) all the time. Well corrected and super sharp. If you can spring for it, it’s well worth it. Note that these are not rebadged and sold by someone else, the Esprit are made by Skywatcher for Skywatcher.

On a practical note, the few shield is huge and I rarely use a few heater in SE QLD...

[gregbradley]

F5 is indeed quicker but yes aperture is the key. F5 means a wider swath of the field ends up on the same real estate of the sensor. So its collecting more total light.

I think also you'll find that more of the light that enters the lens actually lands on the sensor rather than only a large slice of it with a substantial amount missing the sides of the sensor.

Greg.

[JA]

Hi J,

At the risk of opening a second front :D, I can't agree: I think one would see differences in image quality between the Nikkor 80-400 ED and certain Nikkor primes in the same focal length range. Demonstrating that over the internet is not an easy task, since it would require identical subjects photographed with the same settings and then analysed in some way: quantitatively or qualitatively.

Identical images to compare are difficult to come by (unless one owns all the lenses) or is prepared to accept images of optical test charts (±some analysis). You also expressed a certain reluctance/disinterest in LAB tests. If that extends to images of optical test charts, it may be difficult to provide objective evidence of any differences.

As the discussion may grow and it's a little off-topic already, It may be better to discuss this in a new thread, if you want to.

Best
JA

[glend]

Greg, my TS115 Triplet uses FPL-51 and Lanthanum elements. I am very happy with it.

To quote TS:

Triplet Apo Objective with two special glasses (Lanthanum glass and FPL51) - Ohara Japan - colour correction comparable with FPL53 triplet objective

[Ukastronomer]

It has been done by us as a club, and at A3+ there is NO visible difference

[JA]

Hi J,
May I ask: which Nikkor lenses and camera did you use and what was the subject image?

Best
JA

[Ukastronomer]

No problem

D4s, D810, My Nikkor 80-400, 70-210 f2.8 and Nikkor 14-24 with others Nikon 200mm f4 AF Micro, Nikon 300mm f4, Nikon 24mm f1.8

Outdoors, same conditions/lighting etc, real life situation, sports and landscape

[Ukastronomer]

Over here that would be considered racist, sorry for being WELSH and NOT a BRIT, please get it right, thank you

In fact my Mother was born in Swansea, Wales and my father Scotland I am actually a Cornish living in Wales my son is Devonian born Devon/Cornwall

I was simply making the comparison that most people would not see any difference between different types of GLASS used in scopes, fluorite etc, may be a bit complicated to understand, whicgh I have now highlighted in my earlier response to make it easier to understand the relevance, sorry

http://projectbritain.com/nationality.htm[/QUOTE]

[Astronovice]

[/QUOTE]

Hi Jeremy,

As the originator of this topic I fail to see what you find offensive or incorrect about being called a Brit. I was born in England and to my knowledge all of us born in Great Britain (which includes Wales, as the website you linked quite clearly points out) are labelled as British, colloquially known as Brits.

Back to the original topic, I have found your input informative in regard to the differences between FCD100 and FPL53 as it addresses the question whether there would be a significant glass driven difference between the Saxon scope and the SkyWatcher alternatives. Having acknowledged that, the ongoing discussion relating to zoom or fixed focus camera lenses is spurious to the original post as it contributes nothing to making the choice of which scope to buy.
Thanks again for your input. Calvin, naturalised Australian.

[JA]

Life is a conversation. Sometimes it wanders from person to person as people dropout of the discussion and/or make a point that someone picks up on and shows interest in discussing further.

Best
JA

[glend]

Greg, you wanted to see some TS115 images, here are the ones Astrobin finds:

https://www.astrobin.com/gear/13409/teleskop-service-ts-115-800-triplet-apo/