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Vixen FL55SS: an ongoing "review"

astrophotography equipment refractor
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#1 Hesiod

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Posted 17 February 2019 - 01:55 PM

I ended up purchasing the tiny Vixen refractor instead of a Canon EF300 for a number of reasons, some of which could even sound reasonable.

Since had troubles at finding informations about it (there are some Japanese reviews, but I can not read the language and google translator is not very reliable...), thought that could be useful to add my little contribution for those who can understand my "pidgin" English.

So, done with the foreword, can start listing what it is included in the box, and what instead I needed.

The fl55 box contain the refractor and a dovetail bar, Vixen-type, so for the purpose of astrophotography it is almost mandatory to purchase one of the "kits" (I chose reducer+flattener); I also puchased an adapter ring for Canon DSLRs, which acts also as angle adjuster: a brilliant solution for small telescopes.

The kit contains the optical elements and an extension tube replacing the one provided with the refractor; the tube ensures that the distances between flattener, reducer and sensor are the right ones; the optical groups are screwed at its ends, and the tube is the screwed to the refractor's drawtube.

Since the flattener protrudes in the drawtube, care is required to avoid scratching the dark matte paint.

Overall think that it is not a bad design (especially if compared to what has been done for my FC100DF), but there are still a few rough spots.

The dovetail bar can be unscrewed, freeing a couple of 1/8" threaded holes and a single 3/8" one; however take note that, with the photographic kit and the eos1100 body, the center of weight is located a couple of centimeters behind the focuser's knobs, so to balance the rig a rather long plate with eccentric screws is required (with my setup and just the flattener the center of weight falls ca 9cm behind the rearmost 1/8" screw); this is a most annoying aspect, since I had to unscrew a knob to be able to balance the refractor on the Astrotrac mount.

Since it is almost full Moon I focused on star testing the refractor, leaving to more favorable times the evaluation of its widefield astrophotographic performances.

To star test the fl55 had to unscrew the eyepiece holder and screw the stock extension tube: an inferior option if compared to Takahashi's quick release system.
I chose Polaris as a target due to its negligible apparent motion and peformed the test though a 4mm Abbe eyepiece and a Baader Solar Continuum filter (green filter peaked at 540nm); later added a 2x and 3x Barlow lens and last removed the extension tube and replied the test with a Baader prism and a TAL mirror stardiagonal to identify the most fitting one.

At prime focus the refractor provided a very good diffraction pattern; then applied a 1.5mm defocus (the value suggested by Suiter for f/6 telescopes, but also the size of my smallest Allen wrench, so I chose this for better reproducibilty) and observed slightly asymmetrical patterns.
I had estimate a value better than 1/6th wavelenght and maybe close to 1/8th wavelelnght for low-order spherical aberration, with patterns consistent with overcorrection.

The FL55SS showed an excellent correction of chromatic aberration, at the same level of the FC100/740 refractor.

At last aimed the little telescope to the Moon, observing our companion at 100x (3x lens, 9mm eyepiece) and 150x (3x lens, 6mm eyepiece).
At 55mm the resolving power is very low, but the views were enjoyable nonetheless.

As it could be guessed, the small refractor performed slightly better with the mirror stardiagonal; therefore, even if do not plan to use it often for stargazing, in such cases will use the 1.25" mirror.


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#2 Hesiod

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Posted 18 February 2019 - 11:50 AM

Here the first photographic tests, performed by day

 

gallery_215679_8115_1860500.jpg

 

This picture was taken with the field flattener


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#3 Hesiod

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Posted 18 February 2019 - 11:51 AM

This one with field flattener and focal reducer, keeping the same parameters of the previous image to show the effect of speed gain

 

gallery_215679_8115_1519388.jpg


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#4 Hesiod

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Posted 18 February 2019 - 11:52 AM

And last, this was shot without any of the optional optical groups: the camera was focused in the middle of the field, field curvature can be easily noticed

 

gallery_215679_8115_251100.jpg



#5 Tyson M

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Posted 18 February 2019 - 01:40 PM

Nice series of comparative shots, with this seemingly unknown scope!  

 

That flattener and reducer work great.



#6 Hesiod

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Posted 18 February 2019 - 02:44 PM

Yes, even if I envision an inconvenience: the system works flawlessly for Canon cameras; a similar adapter should be available for Nikon's, but as for other models of cameras, I suppose the only option is to resort to an adapter to Canon lenses, hoping it does not change too much the distance between sensor and flattener/reducer.

 

Also, I think that a frontal thread for filters could have been a nice touch


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#7 Hesiod

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Posted 19 February 2019 - 04:10 PM

I took advantage of the proximity of the Moon for a low-resolution picture

 

gallery_215679_8115_53437.jpg

 

Since it is allowed by the very short focal lenght, I chose to use the zwo asi120 planetary camera instead of the canon, for better sampling and to resort to "lucky imaging"


Edited by Hesiod, 19 February 2019 - 04:10 PM.

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#8 Hesiod

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Posted 19 February 2019 - 04:42 PM

I have performed a Ronchi test with a 10 lines/mm grid, with excellent results: at 5 stripes/field these were straight, and showed minor bending at 3/field.

By performing the Ronchi test acknoweledged a mistake in my notes (I had flipped topside down the paperbangbang.gif): the small refractor is slightly UNDERcorrected and not, as previously wrote in post #1, slightly  OVERcorrected.

My bad, and apologize to everyone for the mistake


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#9 Hesiod

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Posted 23 February 2019 - 06:52 PM

Here a picture taken with the field flattener (just converted into .jpg to be uploaded, can provide the .raw if requested)

 

gallery_215679_8115_3227407.jpg

 

On aps-c sized sensor the field flattener does a good job


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#10 dUbeni

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Posted 25 February 2019 - 06:34 PM

Hi Alberto

I just received mine today, I'm not an astro-photographer, but I'm impressed with views I got so far, yes it has some field curvature, but it's no noticeable observing star fields, I see a bit of coma on brighter stars near the edge of field (expected), it doesn't bother me. Unfortunately I think the field flattener is only applicable to astro-photography, have you or can you try it with an eyepiece?

 

CS

Bernardo



#11 Hesiod

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Posted 26 February 2019 - 08:07 AM

Hi Bernardo,

to use the field flattener with a stardiagonal you would need a custom, shorter adapter (with the flattener you receive a tube similar to the one provided with the OTA for straight observing, but with a double thread on the front end where can screw the flattener); as for straight observing will do some experiments next week

#12 nicknacknock

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Posted 26 February 2019 - 09:59 AM

I use with my Borg 55FL this flattener, but I use this scope with only one eyepiece, so I fixed the spacing appropriately. Visually, the FOV is now flat edge to edge with a Pan 24 with 68° AFOV. I have switched recently to a 2" diagonal and wider AFOV eyepieces and have yet to test performance with a 82° AFOV eyepiece though so can't comment on possible performance with wider AFOVs.

 

If you are to do this with  various eyepieces, parfocalizing rings and extensions would have to be used once you play with each eyepiece and find the optimal distance.


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#13 dUbeni

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Posted 26 February 2019 - 11:25 AM

Thank you Alberto and Nick.

Alberto your experiments will be appreciated for sure.

Nick your idea of having a 24 Pan with a permanent flattener is tempting.

 

thanx.gif  

CS

Bernardo

 

(edit) Nick what is the focal length of your Borg 55FL? How well does it work?


Edited by dUbeni, 26 February 2019 - 11:30 AM.


#14 nicknacknock

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Posted 27 February 2019 - 12:30 AM

Hi Bernardo,

 

Focal length of Borg 55FL is 250mm. For visual (I need to stress that as the eyes have some measure of accommodating field curvature Vs a camera) it provides a flat field.

 

It was a relief to be able to observe through it without needing to refocus, as initially I had to pick where to focus - center or periphery of FOV. Again, I stress, a one-trick pony. I never tested it with more eyepieces to determine the spacing required for a full set of eyepieces, but with the Pan 24 it was perfect.

 

 

 

 

Attached Thumbnails

  • 2018-04-06 18.44.56.jpg


#15 dUbeni

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Posted 27 February 2019 - 01:57 PM

Thank you Nick, very helpful, yours has an even shorter focal ratio. I didn't feel the need to refocus, but I only observed for a couple of hours and tried several accessories so I do need more time with it. It's cloudy for now.

Beautiful set up you have, do you like the filter slider?

 

CS

Bernardo



#16 nicknacknock

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Posted 28 February 2019 - 12:45 AM

Bernardo,

 

Filter slider is a useful accessory to avoid the ol' remove eyepiece-screw on filter-insert eyepiece process. I got a second Baader 2" BBHS diagonal so now off to spend even more to order the filter changer system for that too!



#17 Hesiod

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Posted 01 March 2019 - 04:35 AM

I did a brief "visual" test with the flattener: it seems to work (obviously without stardiagonal) but would not go as far as to recommend such solution.
In truth, had been interested in a "visual" small refractor would have purchased the foa60 (or a good binos)

#18 dUbeni

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Posted 01 March 2019 - 07:17 PM

Thank you Alberto, for experimenting it on visual mode, bow.gif I was curious because flatteners are mostly designed for photography. Two questions: why without diagonal, not enough backfocus? Did with you try several eyepieces or just one?

Anyway I didn't feel the need to refocus stars on the outer field, probably because my eyes accommodate FC better in the dark.wink.gif

  

I will get a flattener sooner or later and try some photography with a Canon 600D (T3i). This telescope hopefully has short enough FL to begin astro-photography.

The reason why I didn't choose a FOA 60 is that I would never get the immense fields this little thing provides. Binos give me a pain in the neck unless I'm lying down, and are hard to hold steady with the exception of the Vixen SG 2.1x42, which is a wonderful companion every session. watching.gif

FL55SS is also a great grab & go.

 

CS

Bernardo



#19 dUbeni

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Posted 01 March 2019 - 07:28 PM

Hi Nick! I don't want to hijack Alberto's topic, but I've been several times at Baader's site and found it confusing regarding how to attach UFC into the telescope train, yet when I look at your photo it looks simple enough. what part do you have to attach to Baader ClickLock and diagonal?

 

CS

Bernardo 



#20 nicknacknock

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Posted 02 March 2019 - 05:45 AM

Bernardo,

Will send you a PM now.

#21 Hesiod

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Posted 02 March 2019 - 08:20 AM

To use the field flattener you have to screw it on the objective end of a modified extension tube, provided with the optical group, as long as the stock one you can employ for straight observing.

I think that the purpose of the tube is to place the flattener at the right distance from DSLR sensors, but obviously it prevents a stardiagonal to come to focus.

If want to use the stardiagonal you should need a custom extension tube.

I have tried just with a 20mm/70° to evaluate the behaviour across a tfov as wide as possible

 

The FL55ss is a very "easy" astrograph, and guess you will have little troubles with the DSLR.

Under this regard it is a better option thant the FOA-60, which is very slow (and much less portable, being almost thrice as long); however the small Takahashi can use natively 2" eyepieces and therefore the "visual" tfovs will be more or less the same.



#22 dUbeni

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Posted 02 March 2019 - 07:49 PM

Thanks Alberto, you satisfied my curiosity grin.gif

Tonight I sketched the Hyades with the 2" APM UFF (ultra flat field), 7.2º @ 10x, and took a look at Mirfak and the Perseus cluster Mel 20, I was never able to see these as a whole comfortably. Now I feel the need to go into dark skies and really explore these rich fields.

 

CS

Bernardo



#23 Hesiod

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Posted 04 March 2019 - 03:10 PM

Here a picture of the FL55ss placed on a table-top tripod

gallery_215679_8115_2418226.jpg

The whole telescope is around 2.5kg, its largest piece 30cm long.

 

By removing the dovetail bar ca 300g can be gained; the quick release plate grants the possibility to balance the tube

gallery_215679_8115_291195.jpg


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#24 Hesiod

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Posted 06 March 2019 - 12:38 PM

Here a single sub (again, PM if want the .raw) taken with the FL55ss flattened and reduced

 

gallery_215679_8115_220680.jpg

 

Results are worse than with the flattener alone, but quite decents nonetheless

The image (a single unguided pose of 120") was taken by placing the diminutive refractor on a Polarie star tracker.


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#25 dUbeni

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Posted 07 March 2019 - 05:40 AM

Hi Alberto

Although you say the results are worse, it seems to have less noise. It's a shot of M44 right?

Your experiments are inspiring me to give a go at astro photography.

Last night I made a sketch of Alpha Perseus moving cluster, it's really nice to fit all those stars in the field of view.

By the way the the telescope alone, without any focuser accessories weights 1Kg (1006 g - 2.2 lb).

 

CS

Bernardo




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