A superb combination for DEEP photospheric Solar obsvering: TS 152RFT w/Lacerta Lac-2s Wedge and 589nm D-line filter!!!

[Gemineyes]

I was not entirely certain exactly WHERE to put this - it almost counts as a "full set up" in a way, but it is solar related, so I am posting here...

The Gear:

• TS Optics 152mm f/5.9RFT Doublet Fraunhofer Achromat (f=900mm)
• Lacerta Lac-2S, Brewster-angle Solar wedge* (modified and will have details below)
• Omega Optical 34mm OD, 589nm, FWHM 8nm D-line optical filter, with OD3+ blocking to >2um, 68%T in custom-made cell, filter was a one of surplus.
• B+W 46mm polarizing filter, LINEAR) - mates to my custom cell above
• Stellarvue M2C mount on Oberwerk TR3 Walnut tripod. 

Setup below:

Rear closeup of Lacerta modified Wedge assembly on RFT:

 

Okay - so far nothing outrageous-looking or world shattering, but it is a fun setup, and is read in less than 4 minutes from toting out to viewing! The system tracks smoothly, and is fun to use, not even just on Solar applications...

Nonetheless, the reason for this post is to share ideas about the "receiving end", that is the wedge and filter system, and how well it works, why it does with THIS scope especially, and what may be possible to find and use additional commercially-available filters of this type (D-line, 589nm C.W.) to significant advantage on both visual and perhaps AP as well. AFAIK, not many are observing in D-line light. Some interesting "hybrid" details can be seen (imagine a Baader 538 CF+Ca-K to an extent - combined).

 So... what's the big deal, then?! WELL...

 

The 152RFT is a rare bird - very nice scope for an achromat, and shines very well in RFT deployment. What is unique about this optic is its color curves:

Courtesy of TS Optics' website for the 152mm f/5.9 RFT

They picked odd colors to represent each Fraunhofer line, but the central dark red-brown 0.588um line (587.6nm Helium D3-line) is the one of note; it is the optimal monochromatic Strehl wavelength as the optical doublet is optimized for 588nm light. Focal error is extremely low at this line! This gives it an interesting general view and reduced apparent chromatic aberration performance, but I will not get into that since we are only concerning the nifty D3 line as the one to use here in this setup.

So... while this was "accidental" in a way, it was also by design that the pairing of these two systems led to an amazing result: diffraction-limited photospheric observing of solar details with a fairly inexpensive and short f-ratio Achromat! I say "accidental" because I mounted the 34mm filter about 16 years ago, when I got it as a surplus "one of", likely a cancelled order that sat in stock. Omega had this available, and I had always played with using this with my Baader wedge on my Apos, which also works to great effect, but the Lacerta Wedge I recently procured optimizes this system better - more on that shortly. 

I have since found a possibly even better option that I am in the work of procuring and assembling on the D-line filter side. I'll mention that when I get those, but will also cover this below after the main topic. Stay tuned for that! (sorry, just don't want to go too far off topic just yet!)

To utilize narrower (< 10nm FWHM) bandwidth filters most effectively, one ideally needs a wedge with refractor, or have the ability with a ND3.8 full aperture filter with additional ND filtering, as they can then "tune" the light throughput to what is needed. Getting an optimal combination of brightness and contrast is critical for ultrafine details to be seen visually. The wedge makes this very easy!

Before I continue, I wish to initially thank Victor Boesen's great review of the Lacerta Lac-2s wedge, as I'd had my eye on it for a while, and picked one up this summer! Vic's post is here: 

 

Okay - on with this dissertation...

I have used this 589nm line filter in both my Baader Mk1 wedge and APM 2" Wedge as well. It does work very well, but I found that by using it in the Lacerta wedge, I got JUST a bit more detail. Why? I think it is a combination of the uni-polarized light and also the wider brightness range capability of the Brewster angle wedge. I have had my Baader Mk1 the longest and it is superb as well, but IMO, this Lac-2s wedge just edges it out in general so far from testing in my APOs and other scopes. Not a huge difference, but when it counted, it seemed a bit deeper in full-spectrum WL depth of plage and granulation. 

So, as promised, what mods did I do to the Lacerta? I did not like the stock EP system using screws, and it was also shallow, too much so for use with my systems of filtering employed in a variety of solar work, so I followed Victor's suggestion of a clicklock and added a 15mm TS Optics M54 extension tube, along with using a Baader ND3.0 mounted inside the tube with a M54m-M48f, 4mm adapter ring (ZWO) which allowed the ND filter to be in the wedge all the time for safety. The additional ~12mm of depth allows longer filter clearances for various stacks and the ZWO ring acts as a stop to prevent hitting the ND3.0 at the base of the tube assembly. Works great!

The cost of this is a bit more of an "in-focal" requirement, but so far, ALL my refractors work with the Lac-2s. This also added around $180CDN of extra cost, but if I am doing this kind of stuff to the best I can, it is worth the extra expense - so far it has proven so! Even the Lac-2s plus add-ons (not counting filter aforementioned), is still less than the current Visual Baader wedge by at least $150-200CDN when no sales are on. Say - 100 to ~120 quid?

As it turns out, the extra space was PERFECT to fit my custom filter cell stack with B+W Linear Polarizing filter and D-line filter. I have enough extra depth when using a 1.25" Baader CL insert adapter with 5mm M48 tube assembly to allow 90% of my 1.25" eyepieces to clear with at least 1.5mm gap or more, including Nagler T6s, My UWAN TS Optics EPs, My APM UFF EPs, and ES82s as well. I mounted my D-line filter inside a 37mm filter cell, which was available from an older camera filter set. I then used graphite-epoxy to carefully affix this (back in late 2007) using a He D-line laser I had at the time to align the blank to the cell. I then used step up rings to mate to this 37mm cell, to allow use in 2" accessories. At THAT time, I was using my B+W with step rings to fit on my diagonals, so I mated a 46mm ring permanently on the front end of this filter. It was planned to use with a wedge, so no worries.

Now jump nearly 18 years, and here's this almost forgotten D-line filter... now ready for use in the RFT! 

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First light with the combination was yesterday, September 17, 2024, near mid-day MDT in Canada (Edmonton). I had VERY good to excellent, even sub-arcsecond seeing at times! I thought after 30 years of solar (including H-alpha with a Baader SS0.22A setup I can use) that I was near the limit of what I could get out of my 120mm APM-SD Binoculars or scopes on full or slightly-reduced (for CA control) WL continuum observing, but I was in error! This combination was a whole new level of ultrafine detailed observing: (from CN post I did yesterday to save typing again and CTS lol)

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<<Initial portion removed for both brevity and repeat content >>

...It is sort of a perfect storm here; optically, the RFT doublet is optimized coincidently to the Fraunhofer d-line, one of the quirks of this optic. It has best monochromatic Strehl at 589nm. As my "luck"  would have it, mates ideally with this filter in the chain. Atmospherically, we had sub-arcsec seeing with the sun near apex and just a light wind, which really steadied the view today. Sol was between 32-36 degrees during my observational window of Sept 17 18:20-19:50 UT. (lunch break basically) If I had NO work to do today, I'd still be out - almost missed my 1pm Teams meeting!

So... how did it do, and what was visible?! Let's refer to today's active region HMI view: 

 

I am still honestly reeling from the amount of details - not many times do I get SO much at once that it is hard to fully process, but here are the highlights:

First, imaging taking the sun, have it like a ball of yellow colored dough, and rolling it in ultrafine ground pepper! That is sort of the level of granular detail seen today all over the disk - exc. where facular plague and other details were seen over this. Granular cells were alternating DARK and light (in sodium yellow tint of course), which is VERY easy for our eyes to see optically, contrast-wise, one of the better combinations of tints for this. I could NOT not see this even when not specifically looking. It was SO obvious! Seeing had me well below an arc-sec for resolution. I will confirm this later, but I am 99% certain of it (talking like in the 0.85" regime, or possibly slightly lower at best - pretty much near the optical limit or very close for this 152mm optic).

Facular Plage details were DEEP - about 2x deeper in estimation than seen in a Baader 538nm CF. Structural details in granulation, over most of the disk, including the mid-disk portions, had excellent visibility. On spot penumbrae, most of the active regions had fine featherlike appearance (esp. the mid spot group, AR 3825), and excellent rendition of fine structure within them, even to >90% of the disk view. Subtle shading and granular depth (read: temperature and size) variances could easily be made out, within and around the plages too. Image sort of a "cross" between a Baader CF and a good Ca-K image, but with yellow and dark tinting instead of green or blue-violet. Best way I can sort of describe this! 

AR 3828: Besides being amazing, the primary spot had some discernable structure, seen yesterday evening in this scope in white light as it was splitting down the "middle"; it showed last evening as a partial split with a light bridge - The split at the time today has not quite fully bifurcated the spot yet. penumbrae were "featherlike" surrounding this spot, and superbly deep plage seen throughout this area into AR 3825; also, it was very nice between it and AR 3827, including some small pores dotted about as well between them. WOW. 

 AR 3827's penumbral region also was very "featherlike", with ultrafine filamentary details seen in it. read - VERY fine! I even saw a few porelike specks on the S leading edge, just within it, for a bit anyway, before they either merged with the adjacent filament (thickening), or stabilized to nominal average temp... Some variances in tint were seen in AR 3827, 3828 (initiation of another light bridge I suspect), and AR 3825. These showed as spotlike variances in the umbral coloration - since I was basically at one wavelength, this would be a temperature variance within the spot umbra. I have seen these in color before, using my SV/TMB 105 APO, and my APM 120SD Binoculars, showing as a sort of deep mauve-taupe sort of color. That was a few weeks back, and in monochrome, showed as a paler tan-yellow tint, brighter than the deeper umbra region, but within it. This was NOT OOF color, or seeing - seeing was solid. Even when  few times the seeing varied a touch to maybe just above 1 arcsec for a few seconds, this structural feature seen held well, but looked better when stable seeing returned. 

 

The biggest HUGE highlight for me was AR3825: The penumbral area was SO detailed! Again, ultra-featherlike details prevailed. I witnessed what I can only call the "bear claw"; this was an extension of 4-5 darker and longer filaments (better than HV movie shows!) beyond the main border of the penumbra of the main spot, facing east and slightly below (reversed in wedge). I've marked this on a Helioviewer snapshot which does not show this well - it was WAY better in 589 D-line light: 

 

Here's a movie of this over 6h, most of the structure was in my window I think: https://helioviewer.org/?movieId=9kKW5

You can see in the above snap a bit of this "claw-like" structure - but it was WAY more detailed and changed in the range of 5-10 minute timeframe. VERY neat to witness this!

So... in a nutshell, I have a new solar observing favorite: my 152RFT w/Lacerta wedge and 589 d-line filter. The Lacerta Wedge gives a wider brightness range than my Baader or APM wedges, and I suspect part of the reason WHY this was so good, as well as having full polarization in the light coming off the wedge surface. If time had allowed, I wanted badly to go to 100X. I just was having a ball at ~70X, and time was precious. Will d more with this as conditions allow. This level of detail reminded me of a "live" movie or fast changing photograph. I will look into adapting my camera to shoot this sometime, but not a priority - I just got this running, and it is NOT on a tracked mount - that's on my next list for a mount to handle this 23lb. scope. 

*the mods to the Lacerta Lac-2s are basically removal of the stock EP/cam mount, which has M54 threads to mate to the wedge body. I added a 15mm extension tube, and a ZWO 4mm thick M54m-M48f ring to mount my 48mm Baader ND3.0 in the unit similar to the Baader wedges. I then use a clicklock and CL 1.25" adapter, and used 3mm extension to the d-line housing to clear any 1.25" EPs I put into the upper section - will fit APM UFFs, TS UWANs, Nagler T6s also. The ring acts as a "stop" to prevent any filters from hitting the ND3.0 filter at the wedge entrance. So I know when I am full length for the 2" filtering. 

**the d-line filter was a surplus (one-of, probably a cancelled order) It is an Omega Optical interference filter, 34mm x 2mm, in 37mm OD ring assembly mount (round). FWHM 8nm, outside of passband blocking of OD >3.0 over 400-1200nm, just ~1/9.2 wave (1/10 wave at 632nm) at 589nm. Mounted 37mm ring Inside a cell (43mm) using graphite epoxy and used step up rings to mate to 48mm (m) and 46mm (f - to mate to B+W Pol filter). Checked alignment during mounting using a He laser I had in 2008 when I got it. 

I am looking at options for other d-line filtering and will update when information presents itself. Also, I may look into a He-line Quark down the road - that might be FUN! The 589nm allowed the BEST this scope can deliver and it did so in SPADES! 

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Before I conclude, I have found a possible even better solution for 589nm. Knight Optical in the UK had only 2 left in stock (sorry bagged both!) of a 589.6nm, FWHM 3nm @ 55%T (UV/VIS Blocked OD5.0 to 1.2um), 25.4mm OD mounted, which will have to be custom-fit into 1.25" filter cell blanks. They likely carry these as stock items, I'll verify this - and here's the passband info for the 589DIN25 filters, below:

This acts like an edge filter for the He -D3 (d) line, and passes the Na-D1/D2 doublet as well. May be VERY interesting! I have plenty of light to play with yet in the Lac-2s so should be a hoot! I got two as I'd like to try these in my APM 120s later as well, andwould like having two for EP-mounting for change outs more easily. Stay tuned on those results in a few weeks after they arrive and I figure things out there!

In the meantime, there are also 10nm FWHM 588-590nm filters out there, and these should also be applicable to this use. Just make sure they are at least OD3 or better in NIR for safety.  

Clear skies, and good solar - I hope this post was informative... 

 

Darren

 

 

 

[Gemineyes]

Addendum:

I was parousing the CN posts on the sun today, and many were saying the sun was quiet! However, at lunch again today, before posting this above thread topic, I was out with the 152RFT "D" scope at 69.3X (13mm TS UWAN) and there was action:

I had my TS 152RFT w/Lacerta Wedge and 589nm D-line filter and it was NOT quiet! There was a lot of plage on the eastern (coming into view) side starting to show mid disk, and also AR 3828, the new bigger spot (at the lowest part of your drawing), had a nice light bridge with a filament showing extended past the main penumbral "fan" - Also, seen was some localized heating (brightening) within the umbrae near AR 3827 and 3825's trailing spot:

(FYI the level of plage is AT LEAST as good in the 152RFT/D-wedge BTW...)

Micropores within the penumbral fans, and tons of pores and small spots (counted >11) off AR2825 lower section forming an arc, and above and between the main spots a whole TON of pores and small spots - several came and went within 1h...

Also, the AR3828's penumbral fan was changing shape over the 1h period I was viewing, as was AR3827's. It was subtle, but I was at ~70X in my 152, so probably had larger image scale available. Great seeing today for most of it too, in the arcsec regime at times. 

So, nope! Not quiet at all!

 

The sun never sleeps...

 

Cheers, 

 

Darren

Edited September 19 by Gemineyes