Mark Petersen:
AstroShed:
Torben van Hees:
Tyrel Smith:
I received a note from Tak America recently that addressed this. They claimed that they have recently learned that shooting with the dew shield retracted seems to eliminate these dark wedges. I haven’t owned the scope in years so can’t confirm. I also had heard the aperture vignetting explanation, which seems legit.
AstroShed:
Tak are a law unto themselve, I had the 85 and it was dire in all respects, and it’s the only scope in the world that I know of where you have to now use 2 flatteners with it….absolute joke, we buy a premium Petzval 4 element design to get away from back focus issues, then because of the poor optics and the fact they can’t cope with modern small pixel cameras, they decide to add another flattener with a dedicated 56.2mm back focus into the mix, well at least they supply it with the scope now, they didn’t when I got mine
The lighthouse beam effect is caused by a rubber seal on the front element that goes around the glass, and if you are lucky you get one that fits, if not like most of them you get one with a tiny gap between the two ends of the rubber, as they don’t meet, this causes the effect….also if it’s fine when you get the scope, over time the rubber shrinks back and causes the issue.
poor QC from a premium manufacturer, I will never buy another one…☹️
First I read about these two explanations (dew cap and rubber rings). I can't see any gap in the rings on mine (actually, I can't really make out any rubber rings, really). If the dew cap was undersized, that would be a major blunder. When I get mine out the next time, that will be easy to check.
I have to disagree with Stewart William. Takahashi publishes their optical design specs, so it's easy to check if they fit your quality requirements. The FSQ-85 without the flattener is slightly better in field correction but has much better on-axis sharpness than the usual FPL-53 triplets with a 0.8x reducer/flattener or the so-called "flat-field astrographs" that are currently flooding the market. The off-axis aberrations are more visible because these other designs introduce spherical aberration which leads to big but round stars. The 1.01x flattener fixes these off-axis aberrations in the FSQ and makes the scope far better corrected than the usual fare: Excellent to APS-C and quite good to full frame even with small pixel cameras.
Well you are entitled to your opinion, but how many other scopes are out there that require 2 flatteners to work, I will tell you none, I swapped my FSQ85 for an Esprit 100 and the difference is huge, the Esprit being far better and corrected than the tak ever was, with or without the second flattener, and if you are happy with stars like this in the corner of your images with a modern small pixel camera, then good luck
This looks like a collimation issue. I had a FSQ-106EDX3 that developed a collimation problem and stars looked exactly like this on the edge of the frame. I sent it to Tak USA and they recollimated it and it was fine afterwards. Nice round stars.
The FSQ 85 does 'suffer' aperture vignetting, often known as cat eye bokeh for lens users or inverse dark lighthouse beams for the rest of us. Its a part of the design.
It does not happen because of a gap in the rubber seal anywhere inside the scope. Retracting the dew shield is one way to test, I will be curious if there is a benefit since the field stop is close to the third lens (1st of the two back doublet correctors). But I am going to try it out on a couple of extremely defocused star fields to see how (if) the edge stars change in almond/cat eye shape as the dew shield is extended - just for fun.
In 9um pixel cameras, issues were hidden a little more, but it shows more readily when sensors are large, pixels are small and especially when they are backlit, since it is a planar surface where light hits and diffraction from microlenses (the classic short stubby newtonian-like spikes from the older ccds) are not there to cover the effect.
But, the astigmatism in the above image is absolutely a collimation problem in that Tak. You can 'hide' it with 9 um pixels, but I can get absolutely perfectly round stars on the imx 571 with 1.01x flattener and also with the 5.94 um imx 410 full frame also with the flattener. They are the best color corrected stars of any refractor I have used, but the aperture vignetting is annoying, must admit.
Modern petzvals liek the askars are not formal petzvals anyway, but an optical design to flatten the field. I like them scopes also. True that they also use SA to balance on-axis sharpness with flatness across the field. The FRA300 is one of the best examples where this balance is really good, but its color correction does not come close to the FSQ 85. I have both, and how much it bothers anyone is in the weeds, personal preference stuff really in my opinion. The PHQ line take an approach using SA and flat fielding, with lower curvature to slow the system down and deal with lateral CA and star size a bit better, but its the easier route to do this.
The quad design of the FSQ 85, for me, gives the benefits of excellent CA, SA and aberration control for a fast aperture that is maybe matched but not beaten by any at that aperture of focal length, and the flat field for small chips, or APS-c with huge pixels is a bonus prize, but not why I bought one. The new flattener is just excellent and with a nicely collimated scope, works a treat and like my epsilon, its backfocus distance is much less of a guessing and trial game compared to generic flatteners. But, it all works out eventually and then the system is set.
Pity the example above has given a bad taste - its lenses were out of whack. for some, the need to be flat field out of the box up to APS-C minimum is a modern expectation, and I do agree that the Tak design could be updated - but the slow front doublet with a pair of correcting lenses is best used to correct aberrations for color and sharpness more so than a large flat field. Honestly don't know if there is a modified petzval design that can take the best of both worlds. The AP 110GTX needs a quad reducer to do something similar. The new VSD90ss look interesting, but it too has the aperture vignetting - field stops and where they end up etc.
I keep FSQ stars gently stretched as a broadband imager, where OSC imaging with modern sensors is the place where you get to see this effect the most and the stars are not narrowband-suppressed in size and intensity. All in all, I just love the look of the FSQ85, its my little scientific instrument (the new version, with the silver hardware). I have gawdy fast reflectors as workhorses, and see past the 'flaws' of the FSQ cause it just so nice of a lump of glass, cream enamel, and baby blue casting - silly I know...
