Changing from Solar filters to solar wedge.

[Louis D]

On 21/11/2023 at 19:51, Gemineyes said:

my Stellarvue Chroma filter (MV2, which I designed for Vic Maris)

From your Stellarview filter review: I employed a special approach and sent this curve to Vic

Can you post what that curve was that you suggested?  I'm wondering how similar it looks to the Baader Semi APO curve in dark blue below:

I have that one, and it does a good job of avoiding adding a yellow cast to the image.  However, it doesn't suppress all the unfocused violet and none of the unfocused red.

I've found a Hirsch #12A Light Yellow and a Hirsch #82B Light Blue (Cyan) combine for a yellow-green view that suppresses pretty much all of the unfocused violet and red and really sharpens up planetary views without darkening the image too much like a #11 Green or especially a #56 Green does.  I recently picked up an X0 Yellow-Green filter, and it looks promising as well.

[Gemineyes]

Sorry - I must have missed this last post Louis!

I'd rather not share that curve - it was a very difficult one to do up, and is NOT anywhere close to the SA or Baader filters. 

However, I am currently working on a new one with a supplier and it will be pretty color balanced, and should curtail most chroma for better-corrected achromats.  My Original SV curve is VERY costly (so I am told) to make as it is designed to allow max might whilst curtailing a good chunk of chroma, and retaining excellent color balance. 

One issue NOW is most filter manufacturers have difficulty matching this original curve, so I am trying to do a best fit scenario - if one cuts too much spectrum it does not help the smaller apertures much as the light drop basically limits magnification to avoid excessive dimming. However, if this curve CAN do a good overall balance, then winning for the most part. This new one will cut the deep red, and cut on around 465nm. As of early August - we revamped it. It is not my ideal situation, but I wanted to get an affordable unit out there which does a decent job. 

I am still working on some other options, but still in work. FYI, the SA filter I find does pretty well in my wedges with my ST120 and TS-152 f/5.9; some residual chroma, but only off axis - solar disk has a super thin blue ring, maybe 3% of total area (est.); I can null it by being on axis for the area, and the SA filter also acts as an edge filter, so some solar details pop out even more, due to the spectral response of the SA filter. Leaves a very mild, pale straw color to the sun - not objectionable (to me), and very sharp details visible in BOTH scopes. 

I now use for planetary and general use the APM 2" Prism diagonal, which when added to my "chromatic stack" REALLY reduces CA to ALMOST null in the ST120. I was very impressed - I am in my late 50s now, so it IS possible my blue sensitivity is a bit less than younger observers.

In my TS152, I use a APM "blue" mirror diagonal, which has SOME blue-violet reduction, and then add my filter stack to it. Not 100% chroma free, but VERY good, with excellent light throughput. 

Achromats can be VERY useable for solar system objects, and I think the big issue is expectations. I have used achromats for years to very good effect. In fact some of my best results were with them. My Apos of course do better, and have less seeing sensitivity, but when conditions are steady, both do very well. 

Stay tuned, I am nearly done that new filter curve. Again, not the SV approach but working within the manufacturer's limitations. 

 

Darren

[Louis D]

7 hours ago, Gemineyes said:

This new one will cut the deep red, and cut on around 465nm. As of early August - we revamped it. It is not my ideal situation, but I wanted to get an affordable unit out there which does a decent job. 

I picked up a Midopt LP470 filter which seems to be just about the Goldilocks shade of yellow.  It seems to cut all of the violet fringing without digging into the blue spectrum much at all.  It also has a very high transmission above 470nm despite not being an interference filter.   When paired with their SP625, most of  the unfocused red is also taken care of.  Unfortunately, that SP filter cuts some blue-green, so not a perfect solution, but it is close.  The combo results in a very pale green color cast.  Adding an M&SG filter to cut yellow doesn't really help much, so I just go with the LP470 + SP625 combo.  There is still a very small amount of red fringing, but it is difficult to detect.

I also tried a Chinese SP600 (in an unmounted square held above the eyepiece) with the LP470, but it cuts too much red and orange-red to be acceptable for general use.  There is literally zero red fringing, though.

[Gemineyes]

Did some solar today with my new (as of 8 weeks now!) Lacerta Lac-2S with my 152mm f/5.9 RFT on the sun:

Ran initially my 16mm TS UWAN (56X in the RFT), then 13mm UWAN (69X), and then my APM 15mm UFF (60X) when seeing dropped...

Initially, seeing was near 1 arcsecond or better, until later in the morning, when it went variable, between >3" to ~1-1.5" later. 

Solar Filter chain: 

1. Lacerta LAC-2S Wedge (OD 1.07) 
2. Baader ND 3.0 ND filter - integrated in wedge housing
3. Player One 2" UV/IR Cut (additional OD 4.0 outside of Vis - for NIR safety!)
4. Baader SA filter 2"
5. 1.25" linear polarizer(s) (on EPs) 

That DOES sound like a LOT of filters... the clearance of the Lac-2s only gives my ~12mm beyond the 1.25" insert, so it won't allow my SV stack easily without risking hitting it on the topside with the polarizer. This wedge has a lot of in focus requirement, and even in the 152, I had only ~6-7mm max to spare!

Running mid-morning at 69X, the sun looked AMAZING! Very pale cream tint to it, and very clean view. 

Only a hint of fringe on the limb edge until positioning the eye in the EP to null it. Thin, maybe (!!) 1-1.5% pale yellow (max) inside fringe, or faint blue (<1%) outside limb, depending upon eye position. Inside the disk - TONS of granulation, pores, facular plage well into the 40% of disk, as we do not have as much spot active regions currently compared to say 2-3 weeks ago. All are still mostly on the S hemisphere. 

Best thing was several "chains" of pores forming outside of several ARs - one sitting in the mid-disk, had a run of 5-6 small dark pores, which showed very well. Another highlight was the training spot on AR 3825:

 

This spot had a small bright patch - I have seen this in my APO scopes and big binoculars, so I MADE sure that this was not OOF color fringing. When centered in the view, still visible over a wide range of eye placement, so NO, not a fringe. Looked a bit brownish-tan with a hint of mauve. Turns out when I checked later the HMI view on Helioviewer shows a brightening region:

Very nice indeed! This was seen around 11:10-11:30 MDT (North America), so 17:23 was in the window.

No other regions I could see showed this sort of regional "heating" of the umbra, and it was definitely there. 

Very enjoyable solar session, with the Lacerta trial as FL on the RFT... 2-3h total observing time (16:45-17:55, then 19:50-21:20UT). 

This (not very good, sorry!) approximate fringe image of the sun is not quite representative of the view but think of approx. 2/3 this fringing and more "pure yellow/deep blue:

Yellow is pale and ~1.5x larger than the pale, deep blue fringe on the outside. All but disappears when eye positioned for best view. Gone then. 

NO fringing in main spot areas or disk - fringing is most likely due to field curvature and residual CA, not filtered by the SA filter. When looking at a region centered or even obliquely centered, no fringing at all. Clean detail! FYI, My Euro EMC (Baader AS) filter with my nominal planetary filter stack does not show this almost gone. But in the wedge, we had limited filter spacing. 

Darren

 

 

Edited September 16 by Gemineyes
Spelling correction

[Gemineyes]

Louis, 

Another way to reduce the red OOF portion is to use a good UV/IR cut filter: the Astronomik L-3 is a good one, as is the Baader CMOS unit. Both should cut the red around 670nm, and reduce the CA.

But the SA DOES cut already around 670 or so, so by adding this, you should shift the curve about 5-8nm or so blue on the red cutoff end, by stacking these... 

Basically, the SV CA filter I developed has a roll on around 450nm and cuts around 690-700nm - I use a UV/IR cut with it on my bigger Achromats to drop the red fringe to almost nil. In the middle, it has a partial reduction (between ~520-580nm) in the ~25%T regime. It is a shallow camel hump in this range sort of. So basically, it passes 80-85% of the overall light (integrated across visual) acting like a 22%T neutral density filter, but color balance is retained. It apparently is VERY hard to do and so far my attempts at getting someone to replicate it has been harder than I thought...

It was a unique and better approach than slashing light like the SA does, though it works pretty well also, I must say. My filter has a smoother roll-on and reduction portion, so it "behaves" more like a Wratten style "FLD" filter in that regard, but it is actually an interference type. 

 At the time the SV achromats were being made, in the early 2000s, these did a superb job and worked well. Most were <102mm - mostly 90mm f/9, 80mm f/9 and 80mm f/6 scopes. So already less CA than larger apertured scopes.

Hope this helps for now. I likely WILL post this curve in the near future, if I cannot get a supplier to do one. It is a VERY good curve. To adapt to larger achromats, it needs a bit more blue reduction, and SLIGHTLY deeper mid-visual drop, with around a 670nm cut off. That way it passes relevant nebular bands and still does well for planetary/lunar. 

 

Darren

Edited September 18 by Gemineyes
addition

[Louis D]

16 hours ago, Gemineyes said:

Basically, the SV CA filter I developed has a roll on around 450nm and cuts around 690-700nm - I use a UV/IR cut with it on my bigger Achromats to drop the red fringe to almost nil. In the middle, it has a partial reduction (between ~520-580nm) in the ~25%T regime. It is a shallow camel hump in this range sort of. So basically, it passes 80-85% of the overall light (integrated across visual) acting like a 22%T neutral density filter, but color balance is retained. It apparently is VERY hard to do and so far my attempts at getting someone to replicate it has been harder than I thought...

You might have to resort to a light dye to get the mid-frequency attenuation you want.  I've got a Tiffen CC50B filter that appears violet to the eye that might be close to what you need.  It actually looks quite similar to a M&SG filter to the eye, but gets there in a totally different manner spectra-wise.  When paired with a light yellow filter, either gets you back to a muddy gray.  The CC50B cuts green and yellow the most, red a bit, and blue/violet very little.