Again TS 152 F5.9- does it worth over ST 120 achro? I am using bino or ultrawides

[Louis D]

One more update on the KUO 152 and filtering.  Santa brought me high transmissivity SP625, SP650, and LP470 filters, where SP means Short Pass (cyan in this case) and LP means Long Pass (yellow in this case).  The LP470 exactly removes all violet fringing, unlike my Hirsch Light Yellow #12A which has roughly the same cutoff frequency, but trails off a bit, allowing some dim violet to get through.  The SP650 helps some with cutting unfocused red light while causing very little color shift.  The SP625 almost completely eliminates unfocused red without causing a drastic cyan color like my SP600.

So, I spent some time last night on Jupiter, the Orion Nebula, and Orion's Belt (Collinder 70) with the LP470 ahead of the diagonal and swapping in the SP625 and SP650.  For dim objects, the SP650 combination was good enough, but for bright objects, the SP625 was noticeably better.  With the LP470/SP625 combination, I could sharply see Io approaching Jupiter and then merging with it.  I still couldn't make out that it was in front of it, though.  Without the filter combo, Io was lost in the unfocused light flares.  I then added the Zhumell Urban Sky (Moon & Skyglow) filter (roughly the same as the Baader Neodymium), but I didn't see any appreciable new details, although it did slightly dim Jupiter making it a bit easier to view.

Before coming in for the night, I removed the LP470 from the diagonal and moved it in and out of the FOV above the eyepiece.  Even combined with the SP filters, there was no appreciable dimming of the Orion Nebula with the filters thanks to their 95%+ transmission each.  However, the Trapezium was easier to resolve at low powers with the filters than without again because of their suppression of unfocused light flares.

I even compared the various LP/SP/M&SG combinations to the Baader SemiAPO filter.  It was no contest.  The BSAPO was doing almost nothing to sharpen up the image compared to the various LP/SP combinations.  Adding the M&SG to mimic the BSAPO frequency notches due to its Neodymium substrate didn't really add anything to the LP/SP combinations.  I think Baader did that just to remove some excess yellow to compensate for the violet suppression.  If Baader were actually serious about their BSAPO mimicking an ED scope, they need to suppress unfocused red light along with the unfocused violet light, but they don't.  For the LP470/SP650 combination, the M&SG does result in a more neutral color for Jupiter.  However, with the LP470/SP625 combination, it simply shifted the color more cyan than the yellow-green it already was.  I preferred the yellow-green cast to the cyan cast.

Overall, with the LP470 and SP625 in place, the KUO 152 was much more satisfying to use than without.  The light yellow-green color cast was hardly noticeable after a while.  I actually began to enjoy sweeping the star rich regions of the Orion constellation with it.  Once the moon is back in evening skies, I'll have to try out the new filters on it as well.

[Gonariu]

A few years ago I found this nice review of the Sky-Watcher achromatic (blue tube) written by an American of which I put the link: http://affordable-astronomy.atspace.com/startravel150/page1.htm

[michael.h.f.wilkinson]

If you want a 6"wide field scope, you could try to hunt down a Meade SN-6 6"F/5 Schmidt-Newtonian (or go for a rather more expensive ES 6" F/4.8 Mak-Newt, which is still cheaper that a 6"ED or APO frac). I found mine for just EUR 165 for the OTA. Brilliant refractor-like performance without diffraction spikes or noticeable coma. With my 31mm Nagler I get 24.5x magnification and 3.34 deg true FOV.  The OTA is also fairly light (5.7 kg) and cools down quickly.

[Louis D]

23 minutes ago, michael.h.f.wilkinson said:

If you want a 6"wide field scope, you could try to hunt down a Meade SN-6 6"F/5 Schmidt-Newtonian (or go for a rather more expensive ES 6" F/4.8 Mak-Newt, which is still cheaper that a 6"ED or APO frac). I found mine for just EUR 165 for the OTA. Brilliant refractor-like performance without diffraction spikes or noticeable coma. With my 31mm Nagler I get 24.5x magnification and 3.34 deg true FOV.  The OTA is also fairly light (5.7 kg) and cools down quickly.

Not sure who you're addressing, but I also have a 6" f/5 GSO Newtonian that makes for a fine, sharp wide field scope with no false color, cool down, or dewing issues.  I think I picked up my lightly used copy with the GSO Linear Focuser, GSO coma corrector, 8x50 finder, and laser collimator for about $300 shipped.  At 13 pounds, it's also considerably lighter than the KUO 152 refractor.  The spider vane diffraction spikes don't bother me at all.  I also like having the eyepiece up nice and high so I don't have to extend the tripod's legs, leading to a more stable mount.  While the Schmidt-Newt and Mak-Newt sound interesting, I'll stick with the simplicity of a pure Newt for now.

[Louis D]

3 hours ago, Gonariu said:

A few years ago I found this nice review of the Sky-Watcher achromatic (blue tube) written by an American of which I put the link: http://affordable-astronomy.atspace.com/startravel150/page1.htm

The reviewer totally missed that the residual false color with the minus-violet filter was coming from the red end of the spectrum.  That was what was causing some issues with planetary observing.  Literally no commercial false color filters include far red filtering for unknown reasons.  Sure, it's not obvious with no filtering, but once the violet fringing is minimized with a minus-violet filter, it is blatantly obvious as a bright red rim around every bright to dark edge on bright objects.

It's not very obvious with scotopic vision due to the response curve of the rods in our eyes, so DSO observing is generally not impacted all that much by it.  In my experience, it's the red light above 625nm that really contributes to red flaring in achromats, and scotopic vision pretty much doesn't pick up anything above 600nm as shown below:

Thus, little to no red flaring on dim objects once dark adapted.  However, photopic vision really picks up on red above 600nm on bright objects like planets and the moon, so some sort of short pass filtering is needed for fast achromats when observing such objects.  Weirdly enough, no astro vendors sell such a useful, even necessary, filter.  I've had to retask industrial and photographic filters for astronomy use as a result.  The closest astro filter is the long out of production Hirsch Light Blue #82B which is really a light cyan with a gradual short pass cutoff around 625nm and fairly high transmission.  I happened to find one on ebay last year.  I'd never heard of it until then.

[Louis D]

I just confirmed this morning upon observing Venus using my ST80 (which has about the same color correction as the KUO 152) that the LP470 cuts all violet fringing while being slightly less yellow than the Yellow K2 filter.  In comparison, the Hirsch Light Yellow #12A does leak a tiny bit of violet, but you really have to look for it.  Either it or the LP470 make an excellent minus violet filter by keeping as much blue as possible.

At the red end of the spectrum, I confirmed that the SP650 helps cut spurious red a little bit while the SP625 cuts nearly all spurious red.  Whatever red fringing was left was nearly undetectable.

The resulting color of combining the LP470 and SP625 is a light yellow with slight blue-green tones.  It's obvious, but not horribly distracting.  Now, how to convince a commercial company to market a dielectric 470nm to 625nm passband filter for fast achromats?  It won't turn them into semi-APO EDs, but it will sharpen them up enough to use as acceptable planetary scopes while still passing as much of the well focused spectrum as possible.

[Ags]

Astronomik L3? Maybe not aggressive enough for visual use - it seems to pass 430-570.

[Mr Spock]

I have the 8" f4 now with coma corrector. Fits to an EQ5 perfectly and is about the same length as my 4". With my 30mm UFF it gives 2.4° and exit pupil a shade under 7.

No false colour at all of course 

[Louis D]

13 hours ago, Ags said:

Astronomik L3? Maybe not aggressive enough for visual use - it seems to pass 430-570.

It would hardly be noticeable to the human eye:

It appears to cut on at 420nm and cut off at 680nm.  Everything below 460nm is bright violet, so it wouldn't be doing very much.  That, and the human eye can barely see down to 420nm, so the violet clipping would be imperceptible.

The human eye can barely see above 650nm, so having a cutoff clipping red 30nm above that is also going to be imperceptible.  I can barely detect the clipping at 650nm with my SP650 filter.  It's really the red from 620nm to 660nm that is problematic to the human eye with fast achromats.

Pretty much all of these luminance filters cater to imaging with fairly well corrected objectives.

Push the left edge to the right about 50nm and pull the right edge to the left about 50nm, and you'd be in the ballpark of where it needs to be for a fast achromat filter to reject 95% of the poorly focused light.  Are you listening Astronomik?

[Louis D]

11 hours ago, Mr Spock said:

I have the 8" f4 now with coma corrector. Fits to an EQ5 perfectly and is about the same length as my 4". With my 30mm UFF it gives 2.4° and exit pupil a shade under 7.

No false colour at all of course 

I looked a the GSO 8" f/4 available from Agena here in the states, but it puts the focuser in an awkward position for proper balancing:

I ended up choosing the 6" f/5 for size, weight, mount compatibility (DSV-2B), and price ($300 used with GSO CC, laser collimator, 50mm finder scope, etc.).

Either way, it's way better than the KUO 152 achromat in every way I've checked.  It's like night and day comparing them side by side on the dual mount.  The Newt just puts up a perfectly sharp and color free image while the frac puts up a mushy, chromatically aberrated image.  That, and the KUO was almost 3 times as expensive as the GSO.

Literally the best thing about the KUO is the V1 focuser.  It's a beast.  It just exudes quality machining and internals.  It's the only Chinese made refractor focuser I own that is buttery smooth, has ample inertia due to heavy focus knobs, is exceptionally well preloaded to handle heavy loads, doesn't slip or unwind with any load at zenith despite lacking a R&P, and has zero backlash, mushiness, or notchiness in the fine focus motion.  It feels heavier when focusing than the 2.5" focuser on my TS-Optics 90mm triplet APO.  That, and the latter unwinds under heavy load at zenith, has backlash when lifting a heavy load, and has a mushy feel in the fine focus knob.

Edited January 13 by Louis D

[Fraunhoffer]

On 25/07/2023 at 13:27, Mr Spock said:

Here's my two ready for action. The Dob is next to the shed, and the Tak can go anywhere. Especially useful as my house blocks the East so I can only see there from the top of the garden. The garden runs exactly East to West - North to the right and South to the left 

Now why didn't i think if putting both up in different ends of the garden. 🥴😆. Btw whats your dob sitting on.

[Coco]

On 24/07/2023 at 09:32, Mr Spock said:

A sensible course of action. As always for DSO the more aperture you can get, the better the view.

At the cost of size - my 12" is huge and only moves from the shed to the patio. Cracking views though and can spank any apo you'd care to name at a fraction of the price 

Agreed, 12 inch solid tube dobs rule! 😁 Needle galaxy in my 12 inch dob is spectacular so is M51, my 127mm f9.4 is now a solar scope... 

[Mr Spock]

5 hours ago, Louis D said:

I looked a the GSO 8" f/4 available from Agena here in the states, but it puts the focuser in an awkward position for proper balancing:

I'd use a longer dovetail and put the ring the other side of the focuser.
 

 

1 hour ago, Fraunhoffer said:

Btw what's your dob sitting on.

EQ platform. A must have for serious high power viewing.

[alacant]

On 13/01/2024 at 05:11, Louis D said:

it puts the focuser in an awkward position for proper balancing

Yeah. They use the f5 tube and relocate the focuser and secondary further toward the primary, but far too far, pushing the focus position around 180mm from the tube. Without being close to the aperture reinforcement, tightening the secondary spider to the point needed for collimation to hold, dents the tube. Unworkable IMHO.
The 6" f4 is even worse. Just look where the tilt-inducing focal plane is situated: