Coronado Rich View Tuning

[Merlin66]

Just a whinge.......
Why the devil did Coronado/ Meade ever go for the Rich View Tuning!!!!
I've had a few filters "for repair" where the Rich View system "didn't work" what a PITA!!!
I thought I knew enough about Ha filters to come to terms with this Rich View tuning but it definitely, to me anyway, seems a very hit or miss system.
Go back to the original tilt-tune external etalon anyday.
(Or is there something significant I'm missing here???)

[Montana]

Can you tell me how rich view tuning works? I thought it was just a posh word for tilt tuning?
Alexandra

[Merlin66]

Alexandra,
The Rich View tuning "seems" to work by the compression of the etalon spacers - whereas Lunt with their "pressure tuning" alters the gap by changing the air pressure and hence the effective optical spacing, the Rich Tuning seems to be a mechanical arrangement where the actual spacing is physically altered. To me it doesn't seem to be very reliable.

[Bob Yoesle]

"Rich view tuning" - involving mechanical compression of the spacers and therefore the gap spacing - was patented by Andy Lunt et.al. in 2005 at Coronado, Tucson, just after Meade purchased Coronado filters. All the Coronado patents, including the central spacer patent therefore went to Meade:

https://www.google.com/patents/US20050078906

From what I have seen, air pressure tuning (which effects a change in air density and thereby refractive index versus spacing) is a bit more reliable and uniform. However, for a front mounted etalon, having the ability to just remove the ghosts from the field of view and then fine-tune the etalon by mechanical pressure would seem to have advantages over just tilting alone. Lunt appears to have abandoned the idea of making front mounted etalons with air pressure tuning, which would require sealing the etalon between two optical windows versus the the collimator/refocusing optics used for internal etalons.

[Merlin66]

Bob,
It would be interesting to see a Poll of Rich View users who actually get the mechanism to "fine tune" for them......
I'm quite happy with the original front tilt tuned etalon........

[Bob Yoesle]

Hi Merlin, for single stacked use tilt tuning can work very well if the CWL is not too far off the H alpha line, and I have had good results as well.

But for use as a double stacking etalon, the theoretical ability to re-center the wavelength on the H alpha line after using tilt to remove ghost images seems like it would be an attractive feature for achieving the maximum benefit. I observe that with double stacking, the best contrast result when minimal tilt is used, and often some partial overlapping of the images gives spectacular contrast if you can ignore the overlapped portions.

If air pressure tuning was available for front etalons, I'm sure it would prove to be advantageous. Unfortunately, it doesn't appear to be commercially viable. I have no direct experience with rich View tuning, so would defer to those who actually have had the pleasure - or displeasure - of using it. But from what I can determine, air pressure tuning seems like it would be far more uniform a way of tuning an etalon verses mechanical.

B-)

[Merlin66]

Bob,
You raise an interesting aspect of the DS problem....
I used for many years a double stacked external SM60 pair on an ED80. Both filters had their original T-Max tilt tuning base plate.

As we know the tilt tuned etalon is designed to pass a CWL in the red wing and tilting the etalon brings the CWL down to and through the Ha CWL.
This effectively means that if both etalons were identical (or very similar) they would both need a similar amount of tilt to be sitting on the Ha CWL.
By inference this says that if both etalons are rigidly fixed to each other (no effective T max tilt between them) then they should both be "on band" and give an ideal DS outcome.

In my case I had the front element rotated by 180 deg from the rear and a "compensation tilt" applied via the T Max. This arrangement seemed to work well for me - a good double stack resolution and minimal if any reflections. (I can't honestly remember ever being bothered by them)

To me this is the only way of providing a tilt to the front DS etalon (ghost removal) without compromising the CWL bandwidth.
If a random tilt is used, then the CWL will be shifted into the blue(??) and a Rich Tuner/ Lunt Pressure tuner will not be effective in bring the CWL back into the Ha or red wing.
Thoughts/ comments???

[Bob Yoesle]

Hi Ken,

Ideally a double stacked etalon system would have one etalon exactly on-band with little or no tilt (the "primary" etalon" - either internal or external closest to the objective). The "secondary" etalon - usually placed on the front of the telescope ahead of the primary etalon - would be exactly on-band with just enough tilt to remove any retro-reflections to just outside the field of view.

Due to variations in spacer thicknesses, and the mechanical alignment of the etalon it its cell (e.g. "potting" with silicone and foam), no two etalons are exactly alike. I have seen some Meade Coronado etalons (possibly intended to be used a secondary DS etalons) that had such a high CWL and required so much tilt to come on band that they were essentially useless due to the banding created by the excessive tilt needed to come on-band. These etalons might be good candidates for mechanical pressure tuning. However, if the tilt needed to remove the reflections indeed moved the CWL to the blue wing, then as you note the mechanical pressure "Rich View" tuning would make it impossible to get these etalons on band. The problem is no one would know until you actually tied it - no commercial filter maker supplies you with the actual individual etalon specification or measurement, and QA seems to be hit or miss.

However, positive air pressure tuning (Lunt) would actually shift the CWL to the red, and would be the ideal method of tuning in the case of the etalon being tilted to the blue wing to remove reflections. Using an air pump that could provide both a partial vacuum or positive pressure could be used - if tilting to remove reflections left the etalon slightly off-band to the red, a bit of partial vacuum would bring it on-band, and if the tilt resulted in the CWL being in the blue wing, adding air pressure would shift it red-ward to bring the CWL on-band.

The air pressure tuned Lunt etalons are designed with a CWL well below the H alpha line (the opposite of tilt tuned etalons), therefore only positive air pressure is used for tuning. I assume the secondary DS II modules employ a slight amount of etalon tilt (sometimes apparently insufficient) in an attempt to minimize reflections, with the etalon still requiring additional pressure to come on band.

[Merlin66]

I just reminded myself (!!) that an identical study of the SM60 tilt etalon was conducted back in 2008 by Aristidis Voulgaris.....
He found the FWHM bandpass was 0.64A (0.68A as measured in the Ha line) and the tilt 0-1.3 degrees moved the CWL from +0.46A (red wing) to -1.2A (blue wing) (based on the Ha line)
This allows a doppler shift imaging from +21 to -55 Km/s
It would be interesting to do the same study on the Rich View design.....how much "adjustment" to the CWL does it provide??

Bob,
""if tilting to remove reflections left the etalon slightly off-band to the red.."" I think there is a higher probability that a ghost tilted etalon would end up in the blue wing....
(Based on Aristidis data, the maximum "ghost" tilt would have to be <0.4 deg to keep the CWL on Ha or in the red wing)

[Bob Yoesle]

He found the FWHM bandpass was 0.64A (0.68A as measured in the Ha line) and the tilt 0-1.3 degrees moved the CWL from +0.46A (red wing) to -1.2A (blue wing) (based on the Ha line) This allows a doppler shift imaging from +21 to -55 Km/s

Hi Ken,

That is great information - also in keeping with the idea that a tilt-tuned etalon should be designed so that minimal tilt is needed to bring it on band.

Most of the early DS attempts with Tucson Coronado etalons fall in this category - the "matching" process was intended to insure that via random variation the etalon requiring the least tilt was used as the primary, and an etalon that needed more tilt was used as the secondary. This was referred to as secondary etalon "pre-tilt," and it was therefore hoped that little, if any, additional tilt would be needed to get the secondary etalon on-band. It also demonstrates that when using an etalon intended for single filter use it might end up in the blue wing following ghost removal and not able to be brought further on-band if the CWL is very close to the H alpha line (hence the need for "matching.")

While it also might be random variation, it seems the more recently purpose-made double stacking etalons are designed with a CWL which is significantly higher, and I have experienced more than one that did not come on-band even at maximum tilt. If this higher CWL is the case with the Rich View etalons, they might be tuned high enough as to remove ghosts by tilting, and still remain slightly "high" and can be brought on-band by compression of the spacers. IF that is the case then these might be superior to etalons tuned by tilt alone.

Again I have no experience with these Rich View etalons. It would indeed be nice to see what they measure out as for design CWL (zero tilt).

[Merlin66]

Bob,
I think it really comes down to how much CWL adjustment the Rich View tuner actually provides (when it works!)
(I should have tried when I had access to a "dead" SMII etalon - with no RV adjustment does the etalon still tilt to give an Ha central CWL - somehow don't think so....)