smallish good refractors for imaging

[gregbradley]

I am looking to get a relatively small and light refractor to complement widefield imaging.

Weight around 5-6kgs or less.

I have found these to be interesting although not a lot about them:

1. TS Optics 86mm F5.8 petzval. 60mm imaging circle:

https://www.teleskop-express.de/shop/product_info.php/info/p10417_TS-Optics-86SDQ-86mm-F5-4-Quadruplet-4-Element-Flatfield-APO.html

60mm is big enough for even a Proline 16803.

2. William Optics GT81 with reducer:

https://williamoptics.com/products/telescope/gran-turismo/new-2018-gran-turismo-81

reducer and flattener available with up to full frame corrected circle (a fraction less).

3. Astrotech AT 92mm F5.5 triplet:

https://www.astronomics.com/astro-tech-at92-f-5-5-triplet-apo-refractor-ota.html

This last does not seem to offer a flattener or reducer nor give any spec about the imaging circle or what the ED element is (therefore probably FPL51 which is quite a bit inferior to FPL53).

Any thoughts or suggestions of other scopes?

Greg.

[Ukastronomer]

I have this I keep handy

https://www.widescreen-centre.co.uk/sky-watcher-evostar-72ed-ds-pro-ota.html

Though if I didn't have the 120 version of this I would not dismiss it, one of the best reviews around for scopes are the Esprit triplets not on your list

https://www.widescreen-centre.co.uk/sky-watcher-esprit-80ed-pro-super-apo-triplet-refractor-telescope.html

I think that unlike us conned in the UK YOU get the FREE field Flatterer, we have to pay for it

https://www.youtube.com/watch?v=5WNeHM2lqDU

[Wavytone]

Get one of the TS quad APOs - faster f ratio and no mucking around with flatteners - ready to go out of the box.

Yes it will weigh more.

I have a 70 mm Photoline quad - a nice scope but it is heavy.

A Petzval is similar but typically not as fast because the original aim of the petzval was to function as a telephoto - extending the focal length, and higher f-ratio.

[Ukastronomer]

William Optics are OK! not great

[JA]

Hi Greg,

If photographic lenses are not out of the question for you then, around the focal length range you've indicated (~500mm in your examples) there are a few quality (used) manual lenses you could consider, which:
1. fall within your weight limit,
2. include (by design) a flatterner and
3. are faster (or could be used stopped down for even better IQ).

Unless of course, telescope it must be. In which case: Back to normal programming - :D

Best
JA

[gregbradley]

I have a number of good lenses. In fact for the last several months I have been imaging with a Pentax 645 double ED F4 lens stopped down to 58mm.

Its a good performer. The 67 300mm F4 EDIF will also work on a 16803 chip. The 645 may.


Greg.

[gregbradley]

Thanks for that. These ads though are worded so vaguely I always feel they are lying.

3 ED elements but not named. OK one is FPL53 what are the others? F5 is tough for FPL53 so it would need something else.
No mention of the corrected imaging circle so I assume its too small otherwise they would tout it.

Crass sort of marketing really where they try to spin an image of perfection without saying a lot about the actual specs. I don't know about you but I won't make a decision based on sketchy data.

Greg.

[gregbradley]

That's what I am worried about with them. I have only had one WO scope and it was the highly acclaimed Lomo 80mm triplet APO back about 15 years or so ago.

It was rubbish and the focuser was massively out of collimation with the lens.

Greg.

[gregbradley]

Thanks for that. I have had 2 FSQs and they were great. I might just wait longer and get another one when I have funds. But my intention was more to capture some widefields easily without a full on setup (but it probably would end up somewhat full on).

The TS site is very informative, they seem to run a higher standard on their gear from the manufacturers. Another is selling this scope but theirs is only 1 FPL53 element whereas this one has all FDC100 glass (supposedly as good as FPL53).

Weight is 4kg which is within the ability of a small Tak mount I picked up.

Greg.

[Slawomir]

Hi Greg,

Going purely from the specs provided by manufacturers, the TS 86mm has the largest corrected circle and is likely to be the strongest performer out of the three scopes that you listed. I also think that feedback on the web gives an idea how consistently good TS scopes are in general.

There is one more potential candidate - FSQ 85. Going from spot diagrams it has significantly sharper lens on axis and up to about 15mm off-axis (30mm imaging circle) than TS 86mm, but then the 86 has smaller spot size all the way up to 36mm off axis (72mm circle!). So if you are planning to use this scope for wide fields with KAF-16803, then perhaps the TS 86mm is a more suitable option.

I have not used either of the scopes so all of the above is just some theoretical waffle...:)

[gregbradley]

FSQ85 has a variable reputation on the net. Even a casual research had it go from junk to amazing.

Tak needs to up its quality control on that one. Perhaps they already have.

One guy on Cloudy Nights had 2 and both were defective.

For the money it needs to be spot on.

FSQ106ED had its share of focuser issues which as far as I know are now sorted but there were endless "my focuser is flexing" type posts at one stage several years ago. Hence the 4 iterations of that model.

I still want to get a CFF later on but they are quite expensive and there is a wait list.

Greg.

[gregbradley]

TS 86 F5.4 quad.

I have decided not to get one of these. I looked at a couple of images (not many around which is not a positive thing) from these scopes.

They were OK but brighter stars all had a halo from fairly minor to noticeable on the larger ones.

Not good enough.

Greg.

[casstony]

The WO GT81 is most likely the same objective as the 80mm f/6 triplets sold by TS and Astrotech/Astronomics.
TS says the 2.5 flattener and 3" reducer are good for FF.

The focusers vary on these models with a shift towards a newer, cheaper style where the pinion body is cast into the focuser (eg. WO GT81). Note that the TS picture still shows a separate pinion body which is easier to adjust.
https://www.teleskop-express.de/shop/product_info.php?products_id=3881#c s

[glend]

I just bought the TS71 SDQ:

https://www.teleskop-express.de/shop/product_info.php/info/p4639_TS-Optics-71SDQ---71-mm-f-6-3-Quadruplet-Apo---Field-correcture-for-big-sensors.html

It just cleared customs last night.

It is new, and no apparent images yet on Astrobin. They (TS) claim it is halo free, we shall see. FPL-53 Triplet with ED Petzval, f6.

[gregbradley]

I'll be interested in in reading your review of it.

The small refractor market has lots of choices. I just don't know how good many of them are.

I just read an horrendous thread by a guy who bought a CFF 92mm F6. Problems went on for 16 months! That's a premium scope.

I was considering a CFF refractor now I am not so sure.

The best 2 smaller refractors I have had were an FSQ106EDXiii and a TEC110 F5.6.

I preferred the TEC slightly due to the nicer colour transmission from the fluorite but the focuser really needed to be replaced with a Feathertouch. The FSQ just worked and was perfect even with a Proline 16803 sensor and also with the reducer.

The main "issue" with an FSQ is they are heavy.

Greg.

[Camelopardalis]

Depends on how “serious” you want it to be...

Some folk in here like to bash WO but I have a couple of their scopes and know others with some across the aperture range and everyone is happy. They’re not as cheap as some, but generally well made and live up to the price tag. I’ve recently been imaging with the Z61 and the aberrations in the corner of a full frame are minor.

Of course, for any company there’s always one that slips through the QA net, or so it seems. You wouldn’t think it should happen on the premium brands, but I’ve heard too many horror stories of scopes out of my price range.

If you wanted to limit yourself to APS-C, the SW Esprit 80mm should perform well. I have the 100 but it weighs 7kg plus rings, guide scope, etc.

A WO GT81 plus reducer should perform well. Disclaimer: your determination of well may differ from mine!

[gregbradley]

I've had a look at some WO scopes. Some look interesting. They all look good with the bright coloured anodising.

I also like the Stellarvue SVQ100. Seems like a competitor to an FSQ106.

The thing about an FSQ that no one else seems to be able to match is the large backfocus plus an 88mm corrected circle. So a 16803 chip can produce round stars to the corners.

Greg.

[glend]

Sounds like you have made up your mind. You won't be happy if it's not an FSQ.

[Camelopardalis]

Hence the reason I asked how "serious" it needed to be...

A smaller scope and a smaller camera (with smaller pixels) can give you the same FOV and resolution, or better. All I'm saying is that it doesn't have to be big and heavy to be productive :D

[gregbradley]

Good point although I already have cameras so I don't really want to buy another one.

I have had 2 FSQs Glen and they are great for sure but another one would be a tad "boring" as been there done that.

Keep coming back to the Teleskop Optics 86mm quadruplet. 60mm imaging circle is the largest of any of these scopes. Also light enough for a 2nd imaging rig I am putting together. The mount is a lightweight Takahashi EM10 which has a load limit of around 6kg. It probably can cope overloaded if the polar alignment is very good as Tak is conservative in its numbers (my NJP mount used to hold a TEC180 reasonably well).

FSQ and cameras may be pushing it though at around 11kg. I haven't used the mount yet so I don't know but the previous owner said he had overloaded it to 12kg and it worked. Time will tell. I don't like losing subs to tracking errors either.

These new Stellarvue SVX102mm sound amazing with strehl around .98, wow.

Focal length is longer. Perhaps that is not an issue though. They won't handle the 16803 chip though but I can use the 16200 which is APSh in size around 29 x 24mm or so.

Anyone using one of these Stellarvue's? Stellarvue seems to be a fairly high end scope maker and also has been around for a long time.

Tak seem to mainly have visual scopes with several 100mm fluorite doublet variants. Not sure why they have so many.
For imaging that just leaves the FSQ's and TOA and TSA's but really mainly the FSQ.

Greg.

[glend]

But is that Strehl noticeable in the use you want the scope for? Even visually, it just becomes a bragging number because neither the average eye nor sky conditions are going to be able to exploit that.

The colour correction (don't forget most Strehl is measured in green from the centre), and collimation are two big factors that can support or degrade actual field Strehl. Anything less than perfect collimation, thanks for the bump UPS, can negate the difference between 0.93 and 0.98. And then you have focus precision, etc etc.

[Slawomir]

That Stellarvue looks good, but it is not cheap for sure.

With dedicated flattener (43mm corrected circle) it would cost about 4700 AUD + shipping cost and import duty + motorising the focuser (Stellarvue recommends Optec = 600 AUD just the motor or about 1000 AUD with controller). Al in all, easily over 6000 AUD landed for a 4" f/7 with 43mm corrected circle and motorised Stelarvue focuser. Makes Esprit 100mm look rather affordable...

[gregbradley]

I am not an expert on the intracies of Strehls but it is a normally quoted measure for the sharpness of optics. I presume it also takes into account other aberrations.

For imaging >.95 strehl seems to be a common standard for todays APOs and >.97 only by the very best. >.90 was considered good about 10years ago.

So Strehl number for me rightly or wrongly would be very important as an objective measure of the perfection of the lenses. But yes TEC for example tune their lenses for the best result in green (more green in the eye than other colours and the Bayer matrix has 2 green filters for every 1 of red and blue). But red correction can be weak.

Astrophysics scopes often are optimised for imaging and it shows in their slightly better performance than TEC which are very good but not top of the class.

I have had some very high end refractors and yes you definitely notice it.
An Orion ED80 is a great little entry scope. Put it up against a TEC110 and the differences stand out.

Where the better scopes seem to rise above is in size of the corrected field (some are just too small limiting what camera you can use). The lack of false colour - a big one. I had a Tak FS152 which was the best visual scope I have had but for imaging it tended to give blue rings around bright stars.

Look through an AP140 and the stars suddenly are super tiny pinpoints. Never seen anything like that with any other scope.

Greg.

[gregbradley]

Yes it becomes too much. Good point. Also the focal length for my use is too long. I want a more FSQ type field of view so F5.5 at 100mm is about right.

Now if you sold me your CFF105 F6 everything would be well in the world!

Greg.

[Slawomir]

I consider you a good friend Greg, but not this good :lol:

But it would certainly be very interesting to see how well does the dedicated in-house made flattener with 82mm of clear aperture performs with KAF-16803.

[gregbradley]

I doubt it would have any trouble with the 16803 sensor and the flattener.

Greg.