Good shortish ~4" refractors

[RogerLightyear (Roger)]

Hi all,
1st time poster here. I'm interested in a good value/performance refractor of approx 4"' aperture and wondered if there's any recommendations available in Australia other than f/9 doublets that are ~90cm long and triplets.

I'd prefer something well below 90cm in retracted length and for the $ I'm interested in what well performing doublets there might be. I'm not completely disregarding triplets but I'm a beginner and would intend to use it visually at first.

I may dabble in photography down the track (I wouldn't have the mount for astro initially). If a good shortish ~4" aperture doublet is not a realistic proposition I could start out with one of the 80mm units from various brands or a used scope but I've read some appealing feedback on these units from overseas suppliers:

TS Optics Doublet SD APO 102mm f/7
Astro Tech AT102ed 4" f/7
Altair StarWave 102 ED f/7

Do we have similar here? Thanks for any suggestions

[MortonH]

A 4" f/7 ED doublet with sliding dew shield is a capable and fairly compact scope. I've wanted one for a while myself. As far as I know the only retailer in Australia with a 4" f/7 doublet on their website is Andrews Communications with the William Optics ZS103.

I probably would have bought the TS 102mm f/7 doublet a couple of months ago but they were (and still are) out of stock. Supposedly they are getting more next month. It's a shame that no-one stocks these locally but plenty of people in Aus have ordered scopes from Teleskop Service with no problems.

[AdamJL]

Edit ignore me. I missed the doublet requirement and posted a triplet and quadruplet

[RogerLightyear (Roger)]

Hi and thanks for the replies. Doublet isn't absolutely a requirement, I just noticed recommended doublets at 100mm tended to be long f/9 tubes an wondered if there were good doublets with similar dimensions to triplets available in Aus. I had thought perhaps WO didn't have a 4" since a couple of online shops I had browsed seemed to skip from the ~80mm up to their 132mm models so that is a good option to consider.

Aside from that and keeping an eye on used items, in browsing around I have noticed some options ranging from FPL53 102mm doublets to a few fpl51 or equivalent triplet 115mm telescopes:

4" FPL53 doublet https://www.altairastro.com/starwave...ctor-459-p.asp
4.5" FPL51 triplet https://www.altairastro.com/ts-optic...or-10171-p.asp
4" FPL53 triplet https://www.altairastro.com/altair-w...2715-452-p.asp
4.5" Japanese glass triplet https://www.altairastro.com/altair-w...-apo-453-p.asp

The 115s are probably about the limit I'd consider in physical size and would be less likely to spring for the Wave series triplet but I wonder if any of them stand out as better value than the others. It looks like some Astro Tech, Altair and TS units are basically interchangable. Also it's curious that the most expensive of these, the Wave series 115 has "Japanese glass" but they don't boast what type.

Another model from Teleskop Service is this Tecnosky which seems almost identical to the TS Optics 102mm doublet except maybe more backfocus if that's useful to me https://www.teleskop-express.de/shop...mat---OTA.html

I also read through this post re: Apo, doublets and glass types:
https://www.iceinspace.com.au/forum/...d.php?t=173985
It refers to "fast" scopes being harder to make optically high quality. I'm aware the various 100mm f/9 doublets are reputed to be good in part because they're f/9 but what is considered fast in this context and possibly a negative? Would f/6 be an example of too much compromise e.g. this one? https://www.teleskop-express.de/shop...h-Fokuser.html

Thanks again for your input.

[mura_gadi (Steve)]

Hello,

There is a built in correction associated with the focal length, hence the visual doublets coming in at F9+. The faster focal length is good for astrophotography, so its about trade offs. TS-Optics normally review well, Bresser is Explore Scientific partner.

The link below is from another IIS user, and is a great link for understanding the F/L trade offs for CA on doublets.


Steve
https://imgur.com/nEjoLHd

[Peter Ward]

Why compromise? There is a CFF105 going for a song in the IIS for sale section.

[RogerLightyear (Roger)]

Good point, even as a beginner I can see that's a rarity but still more than I intended to spend. I was more trying to get an idea of the market position/value of features in scopes closer to half that price.

Maybe if it lasts the week unsold and I have the funds available it could be an option given that you could buy something like that and in the event you didn't keep it you probably wouldn't lose anything moving it on!

[Camelopardalis (Dunk)]

Can’t really go far wrong with a Sky-Watcher Esprit 100 IMO.

100mm f/5.5. Works well for visual and imaging (includes field flattener good for a full frame sensor), so it really depends on your use case.

It has a large dew shield up front, which almost entirely retracts over the body when not in use. It’s well under 90cm in length when this is so, but it’s about 7kg in weight.

[gregbradley]

Tak FS series are doublets. Sometimes an FS102 comes up. A bit heavy though for 102mm but its a fluorite doublet and quite well corrected.

Doublets usually have a degree of chromatic aberration typically blue. That shows up as coloured fringes on bright objects visually and blue rings around bright stars in images.

Apparently a highly corrected doublet is possible - Agema doublets.

https://www.agemaoptics.com/

The graph comparing various high end refractors is interesting.

An ED80 is a very common starting scope that performs well, is inexpensive and will even do some decent imaging.



Greg.

[ausastronomer (John Bambury)]

Originally Posted by RogerLightyear

Hi all,

I'm interested in a good value/performance refractor of approx 4"' aperture and wondered if there's any recommendations available in Australia other than f/9 doublets that are ~90cm long and triplets.

Do we have similar here? Thanks for any suggestions

Apochromatic essentially means to be free of any false colour and spherical aberration. Some sources expand and define this to be free of spherical aberration at 2 wavelengths and free of false colour at 3 wavelengths.

The laws of physics (optics) determine that a 2 element objective needs a slower f-ratio (to be longer for a given aperture) to be apochromatic, than a 3 element objective. This is why the 3 element APO's (triplets) can be shorter than the 2 element APO's (doublets).

Despite the fact that newer technologies and glass types have narrowed the gap a lot and allowed for manufacture of some "decent" 2 element refractors at around F7 to F8, IMO if you want high quality optical performance in a shorter tube refractor, it needs to be a triplet.

As you mention you are just starting out and don't want to spend a fortune, which is understandable, which means for "decent" optical performance it needs to be F7 or F8, which is where I would be heading. I think you're better with something decent optically in priority to something short. If you want both it will be a triplet and a lot more money.

As Greg Bradley mentioned there are some pretty decent scopes available 2nd hand. I would prefer a high quality 2nd hand scope (unless its a gift to someone) as opposed to a new scope made in China or Taiwan. While optically some of the new stuff is reasonable, they don't have the same build quality and quality control as the premium Japanese manufacturers. I would be backing a 10 year old Takahashi FS102 to outlast and outperform a new Skywatcher doublet scope, every day of the week. It will also hold resale value better. The Takahashi FS102 is F8.1 but very good optically. Another scope to keep your eyes out for is the Vixen FL102S. Again this is a Japanese made Flourite doublet of the very highest quality and the equal of the Tak FS102.

Cheers
John B

[ausastronomer (John Bambury)]

Originally Posted by gregbradley

Apparently a highly corrected doublet is possible - Agema doublets.

https://www.agemaoptics.com/

The graph comparing various high end refractors is interesting.

Greg.

Hi Greg,

I can appreciate that modern technology and glass types have allowed for the design and construction of better and shorter doublets in more recent years, but you still can't defy the laws of physics.

These aren't short doublets BTW. The 120mm is F8.7.

The comparison graph is interesting and puts it on a pedestal above some pretty high end gear, namely the 130mmm Zeiss APQ and the Takahashi TSA 120. Both are a fraction faster at F7.7 and F7.5 respectively, but they are both the top of the crop in terms of Triplet APO's. I would really like to see some independent test data. I could be wrong, but I am suspecting the marketing department had some influence on these tests

Cheers,
John B

[RogerLightyear (Roger)]

Thanks for the replies. At the moment I'm looking closely at the 115mm FPL51 triplets from various outlets which all appear to be the same thing and review pretty well along with checking out astrobin samples.

We have:
https://www.teleskop-express.de/shop...P-focuser.html
https://www.astroshop.eu/telescopes/...v3-ota/p,57325
https://www.astronomics.com/astro-te...actor-ota.html
https://www.altairastro.com/ts-optic...or-10171-p.asp

Obviously the Altair link is the TS Optics model too while they also have their Ascent branded one with FK61 glass instead. All have 2.5" focusers and it looks like the Tecnosky and Astro Tech come with a case. Other than that it might be price and whether it's in stock.

If they're made in Asia it seems unnecessary to ship them half way around the world and back but I'm not sure there is another way. I'd probably happily take one 2nd hand but perhaps they're not common here and/or owners keep them. Does anyone order direct from Asia or Asian retailers? I'm guessing they're not stocked here although I did see a TS Optic item on one site but nothing else i.e. a range.

The WO 102 mentioned previously appealed but it seemed that would need to be ordered in too and the price might be comparable.