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Comparing high-end imaging refractors under 65 mm

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#1 Neubadger

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Posted 21 November 2020 - 11:05 PM

I've spent quite a bit of time searching and reading the forums here on CN over the past many months, but despite many overlapping threads, it seemed like this was worth a post.

 

I'm pretty new to astrophotography, having only taken constellation and Milky Way landscape shots with DSLRs (APS-C and FF) and a tripod.  I'm working on a more DSO-capable grab-and-go setup as my next step in what I'm intending to be a lifelong hobby.  Right now I'm (very patiently) shopping for an optically excellent refractor that's lightweight enough to use in an imaging setup on a Star Adventurer.  Along with home use and car-based dark sky trips, I'd like to bring this setup on backpacking trips without power, so while I'll dive into astro cameras and auto-guiding eventually, they're off my radar for the moment.  I also want to be able to use the scope for visual astronomy, knowing that aperture will be more limiting here.  Price is almost a non-factor, but I do have one price-related concern that I'll mention below.  I'm looking for direct comparisons if possible, but suggestions, experience, and clarifications are welcome.  Here are my top contenders so far, but feel free to tear them apart and/or suggest your own.  We Wisconsinites have thick skin.

 

  1. Takahashi FS-60CB / FS-60Q.  The flexibility of this platform is very appealing, at F/5.9 bare, F/4.2 with reducer, F/6.2 with flattener, and F/10 with extender (the extender wouldn't be used with the SA).  My main concern, which I alluded to above, is that it sounds like it can't use standard 2" M48 filters.  Is that accurate?  I'd really rather not have multiple sizes of the same filters, both for cost and clutter reasons.  I live in deep Bortle 9 skies, so I'll definitely be playing with filters at home.  Interchangeability between my GnG setup and other future equipment is on the wish list.
  2. Vixen FL55SS.  F/5.5 bare, F/4.3 with reducer, F/5.7 with flattener.  One thread I found mentioned this Vixen as having less chromatic aberration than the FS-60CB.
  3. SharpStar 61 EDPHII / Radian Raptor 61.  F/5.5 bare, F/4.5 with reducer.  I understand these two to be optically equivalent, so really it's just comparing prices and features in the different packages you find.  Can you pull the reducer out of the Raptor if you want to, or is that permanent?
  4. William Optics RedCat 51 / SpaceCat / NightCat.  F/4.9, inherently flattened (Petzval).
  5. Borg 55FL.  F/4.5 bare, F/3.6 with reducer.  (added per Catalin81's suggestion)

Thanks in advance!


Edited by Neubadger, 22 November 2020 - 09:52 AM.

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#2 Jim Waters

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Posted 22 November 2020 - 12:10 AM

FYI

 

https://www.youtube....h?v=KAmNjvP3F20


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#3 alan.dang

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Posted 22 November 2020 - 12:33 AM

If price is not an issue, the FL55ss is one of the best designs since it has excellent illumination across the field.

Use Google Translate
http://reflexions.jp...018/08/29/6094/

Once you set it up with all of the accessories, it gets pretty expensive though and back focus needs to be precise. You can see some spectacular results on Astrobin.

https://www.astrobin...?q=Vixen FL55SS

I have the Redcat 51 which I have listed in the classifieds. Dollar for dollar, the Redcat is a killer setup which is why it is so popular/backordered.
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#4 Catalin81

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Posted 22 November 2020 - 05:46 AM


You may also want to check out some Borgs.
The 55FL is faster, lighter and a (subjectively) sexier appearance than all the scopes you mentioned. It can be reduced to 3.6 while offering FF corrected coverage.

Also, the 72FL may fit your bill in terms of portability and speed (can be reduced to f4) while offering more aperture.

To be clear, I don’t own any of these, but Astrobin photos taken with them look very good.

I did get myself a 90FL a few months ago and I’m so hyped about it that I’m already pondering an upgrade to 107. They have very good (Canon) optics, amazing build quality and are incredibly light.

C.
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#5 alfogator

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Posted 22 November 2020 - 06:40 AM

  1. Takahashi FS-60CB / FS-60Q.  The flexibility of this platform is very appealing, at F/5.9 bare, F/4.2 with reducer, F/6.2 with flattener, and F/10 with extender (the extender wouldn't be used with the SA).  My main concern, which I alluded to above, is that it sounds like it can't use standard 2" M48 filters.  Is that accurate?  I'd really rather not have multiple sizes of the same filters, both for cost and clutter reasons.  I live in deep Bortle 9 skies, so I'll definitely be playing with filters at home.  Interchangeability between my GnG setup and other future equipment is on the wish list.

You can user 2" filters on the FS-60CB with a DSLR by using Philipp Salzgeber's trick:

 

https://www.salzgebe...n-the-baby-tak/

 

I used it and works fairly well.

 

Alternatively you can reduce the M52 thread from the adapter to a M48 (I'm using the Lacerta adapter) and use more common parts.


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#6 Neubadger

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Posted 22 November 2020 - 11:24 AM

Thanks for the responses so far.  I'll compile some notes and post those later, both for anyone else looking at these scopes and for anyone who can further comment or correct my notes.



#7 andysea

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Posted 25 November 2020 - 02:49 AM

I used the fs60 quite a bit with a Canon 6d and the reducer, its a very good imaging scope. I put it on the pm-1 which would carry it with ease. However it’s a pretty heavy scope for its size, I would make sure that the SA can carry it, with all the accessories etc...

The Borg should be equivalent as far as optical quality but it’s probably lighter.


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#8 DroneSaif

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Posted 26 November 2020 - 01:10 AM

Hi.

Following.

I am in at the same stage right now, with a soft spot for the Vixen.

#9 Jaimo!

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Posted 26 November 2020 - 01:27 AM

While only an FPL-53 Doublet the WO Z61 / AT60ED / Apetura 61EDR should also be considered.  I have had the WO (in red) for about 6 months and I enjoyed it very much, awaiting the next clear evening to shoot the Pleiades, Andromeda, and Orion Neb.  As mentioned above search Astrobin.



#10 Neubadger

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Posted 26 November 2020 - 12:20 PM

Maybe I'm overvaluing this as a newcomer, but the flexibility of the Takahashi and Vixen scopes is really drawing me in.  Unfortunately, I'm having a pretty hard time finding direct comparisons between the two.  Has nobody done a shootout between these two or the other scopes on this list?  Maybe the Vixen isn't popular enough for this sort of direct comparison?  I can find plenty of FS-60CB vs FOA-60, for example, or FS-60Q vs FOA-60, or FS-60Q vs FOA-60Q.  And at least in passing forum comments, a fair amount of people have compared performance between the RedCat51 and the EDPHII.  Here's a bit of what I've been able to find so far:

 

http://k-astec.cocol.../11/post-3.html

  • This one looks at the RedCat 51, FL55SS at F/4.3, and FS-60CB at F/6.2.  To my novice eyes, the stars on the RedCat and Takahashi look great, but the FL55SS stars are showing some coma.

http://k-astec.cocol...18/08/fs60.html

  • This indicates the FL55SS has better CA performance than the FS-60CB, but that's for the bare scopes rather than in any sort of flattened/corrected setup.

http://k-astec.cocol...8/6cm-ffd8.html

  • This seems to contradict the post above, saying the CA performance of the FS-60CB and FL55SS are equivalent (but both inferior to the FOA-60, which I expected).

https://www.cloudyni...-ongoing-review

  • Hesiod's ongoing review of the Vixen in this thread has been great to read, but he's a fairly lone voice here.


#11 N-1

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Posted 26 November 2020 - 03:05 PM

You can user 2" filters on the FS-60CB with a DSLR by using Philipp Salzgeber's trick:

 

https://www.salzgebe...n-the-baby-tak/

 

I used it and works fairly well.

 

Alternatively you can reduce the M52 thread from the adapter to a M48 (I'm using the Lacerta adapter) and use more common parts.

Excellent info, thanks!



#12 alan.dang

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Posted 28 November 2020 - 11:28 PM

vixen-fl55ss-spotdiagram.jpg

fl55sl-img02.jpg

I think your comments are accurate about coma vs sharpness but you are gaining quite a bit of imaging speed.


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#13 Hesiod

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Posted 30 November 2020 - 12:08 PM

The FL55 is remarkably well corrected as far as CA goes, both visually and on the digital sensor (I think posted a shot of an antenna, which is a pretty common "test" for CA).

Anyway, when made my choice I ruled out the Borg because of practical concerns (the brand is not dealt by my usual vendors and building up the right setup is quite nightmarish due to the modular nature), and pondered a while between the Takahashi and the Vixen.

The former was cheaper but my previous experience with setting up a "visual" Takahashi for AP was not exactly thrilling; also I had the impression that it was a bit heavier; on the other hand I was aware that Vixen can made really good stuff but they have also the unpleasant habit of throwing in junk-grade accessories.

In the case of the FL55 however they did all right, and both the tiny telescope and its accessories are well crafted and, even before that, very well designed (I have liked above anything else the fact that a minimal number of parts is required to set up the telescope for the different applications); and it is already equipped with a much better focuser than that of the FS60. Indeed in my opinion the FL55 focuser is better than the focuser installed on my FC100DF/DL, which itself is regarded by Takahashi as an upgrade from the unit equipping the FS60 and the FC100DC.

While I use mostly a RDF placed atop the camera's hotshoe for quick aiming, the FL55 can be equipped with a Vixen-type quick release clamp for finderscopes (their own RDF is however a piece of trash); the FS60 has an excellent, but ultimately useless, 6x30 optical finderscope and should resort to third parts accessories to install a RDF. Also do not remember if the optional quick release device is available also for the FS60, or just for the larger Taks.

A major difference is also about how the tube is linked to the mount: the Tak through a clamshell, the Vixen instead has a dovetail bar linked to its "bottom". Tak's solution allows to rotate the tube to place the finderscope in the most convenient place, and IME gives a feeling of better safety while balancing; Vixen's on the other hand brings to a substantial weight saving. In my agenda the latter outweighted the clamshell's PROs (as said earlier, the RDF is on the hotshoe).

Last, Vixen did not developed a separated device to rotate the camera like the Takahashi's Camera Angle Adjuster but it is the very same adapter ring to allow the rotation: when bought mine it was available just for Nikon and Canon mounts so, if nothing has changed, to link other cameras need two adapters (the Canon or Nikon ring and a Canon/Nikon to whatsoever adapter). I use Canon cameras so for me it is just brilliant (a single piece, where I need two adapters plus the CAA and Canon ring to assemble the imaging train with the FC100DF), but those using different cameras may easily and rightly disagree...

 

As for the flexibility, mind that is rather relative: granted that the last thing you want to do is to use the Q module for deep sky imaging with the SA, switching between 230 and 320mm does not really change the kind of target/framing you are for. Unfortunately, to attain more flexibility a larger and stronger mount than the SA would be needed.

Personally have used the old aps-c camera mostly with the flattener + reducer, and the FF with just the flattener; with the latter use the reducer basically to gain speed, as mostly have enough fov already at 300mm (for the kind of pictures I am interested in).

If are shooting from LP sites however the extra speed provided by reducers is a strong asset to cut the integration time (while, for the single sub, is is a bitter-sweet advantage especially if do not use a LP filter: dealing with a gazillion 5" .raw is quite nasty)


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#14 Neubadger

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Posted 02 December 2020 - 10:40 AM

 

 

I think your comments are accurate about coma vs sharpness but you are gaining quite a bit of imaging speed.

Alan, thanks for sharing those images and diagrams.  Where did you find them?

 

I guess I was surprised to see that much more coma on the FL55SS in its corrected state.  But maybe that's something you just have to accept at F/4.3 versus the Petzval RedCat at F/4.9.  I'm too new to this to know where physics lays a heavy hand versus where humans can effectively fight back.

 

The FL55 is remarkably well corrected as far as CA goes...and it is already equipped with a much better focuser than that of the FS60...

 

As for the flexibility, mind that is rather relative... switching between 230 and 320mm does not really change the kind of target/framing you are for. Unfortunately, to attain more flexibility a larger and stronger mount than the SA would be needed.

 

If are shooting from LP sites however the extra speed provided by reducers is a strong asset to cut the integration time (while, for the single sub, is is a bitter-sweet advantage especially if do not use a LP filter: dealing with a gazillion 5" .raw is quite nasty)

Alberto, thanks for the detailed response.  That was extremely helpful.  You and Alan currently have me leaning toward the FL55SS.  To respond to some particular comments you made:

  1. I'm glad to hear that you like the FL55SS stock focuser.  This reviewer felt that it needed to be upgraded right away, so it's good to hear a contrasting opinion: https://reflexions.j...018/08/29/6094/
  2. That's a very good point about limited flexibility.  I was just thinking binary: bare scope IS DIFFERENT than scope+flattener IS DIFFERENT than scope+flattener+reducer.  But the relative focal lengths of those three setups are really not very different (237~312 mm for the FL55SS; 252~374 mm for the FS60-CB).  The Q module extender of the Tak (600 mm) is significantly different, but then you need a different mount, which changes the discussion entirely.
  3. Light pollution might be the elephant in the room for me, given my urban location.  I understand the first bit of what you said (how extra speed is beneficial) and the end (stacking a gazillion files is nasty), but could you explain the middle part to me ("for the single sub, is is a bitter-sweet advantage especially if do not use a LP filter")?  Edit: Do you just mean that attempting a single sub in high LP skies is fruitless anyway, regardless of speed?

Edited by Neubadger, 02 December 2020 - 10:56 AM.


#15 Neubadger

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Posted 02 December 2020 - 10:41 AM

Has anyone used an auto-guiding setup with the FL55SS?  I would not guide initially, and maybe never on a Star Adventurer, but having the future option would be nice.  The review I linked above mentioned that it would be tricky, given the configuration.



#16 Hesiod

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Posted Today, 01:29 AM

No, I meant that a faster system will hit the sky's limit sooner: usually it is good, but if the subs become too short, then they start to be named Legion...
The q-module is interesting for visual, but will cripple the telescope's performance as an astrograph, especially from light polluted sites, because if the strong impact on integration time.
As for the focuser, I think that the dual speed improvement is much overrated, but is just a personal opinion.
As for the guide, in truth I used more often the fl55 as guidescope than guiding it; its configuration makes hard to use OAG devices, but there are no issues as long as opt for a side by side setup, which I suppose being the standard for small telescopes


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