Here's the info on the LS230 double stack:
He's not wanting to keep both scopes as he wants to give me the SM90 when the new one arrives so the Lunt has to be good at everything. (Emphasis added).
That's impossible, so he's likely to be disappointed, as Andy Lunt stated for even the single stacked LS152 the design purpose is not to provide optimum full-disc views, but optimum high-resolution close up views:
"It is my opinion that the smaller systems require a larger sweetspot at the slight compromise of bandpass. Given that the smaller aperture results in a minimal effect by seeing conditions and air turbulance, I like to see a fairly bright image that provides an nice overall Full solar image with a balance of proms and surface (typcial to <0.7Angstroms).
However, as the aperture increases we are faced with a few more decisions.
Increased aperture, in my opinion, allows the user the advantage of light due to image scale. It allows the use of higher magnifications under ideal conditions to see within the details of the Sun without significant loss of brightness.
Increase in aperture has the disadvantage of being effected by poor seeing conditions.. However, I would prefer to know that while my system may not have ultimate performance under less than average conditions, it will provide the best resolution and contrast under ideal conditions, AT HIGH MAGNIFICATION. If it is not your intent to use the systems at high magnification, you do not need a large aperture scope.
Under low magnification the user will note a sweetspot in the eyepiece. If the Sun is placed in the center and an eyepiece is chosen that allows the Sun to be about 40% of the field of view, you should see uniform detail around the entire disk. In some cases the sweetspot maybe slightly off center. This is an unavoidable result of tilted optics and mechanical tolerance holding those optics.
With the Sun at about 40% of view, you will have an dark area around the Sun equal to about a Sun radius..
Moving the Sun around in the field of view you will note that the proms will drop off quickly as the image moves to the edge of the field of view. This is due to the off axis rays becoming greater than the acceptance angle of the Etalon. This is NOT the result of a poorly tuned Etalon (however, you can re-tune to get this detail back at the loss of the center (basically de-tuning the Etalon to match that angle)), or the result of mis-aligned optics.
Note that when you magnify the image you are using a much smaller area of the field of view and will note that the sweetspot quickly disappears as the heart of the system is utilized.
Note: the narrower the bandpass of the Etalon (higher R for narrower bandpass) the faster, or more pronounced the sweetspot.
The image at top (had to get there eventually represents the ideal internal system. We try to provide what we refer to as a slight “bird” shape. The Sun would sit within this area provided that the magnifiaction was set such that the Sun was 40-50% the field of view. The center comes down very slighly in order “push” out the sweetspot.
The narrower the bandpass of the Etalon, the narrower it’s acceptance angle. Therefore, lower bandpass systems will deteriorate faster at the edges.
The above deterioration can be offset by placing a wider bandpass Etalon in the system. ie: if we placed a <.08A Etalon in the 152T, we could increase the sweetspot to 70-80%. Hmmmm, I designed the 152T for high resolution, high magnification viewing. Research of the active areas, and NOT for a quick overview of the entire Sun. That can be done with a 60/PT or a 100/PT. I believe that I would be doing a dis-service to the customer if I provided a systems that worked okay at low mag by simply showing a uniform, if less contrasty Sun, but showed very little detail close up. There’s a reason the University of Hawaii moved the 152T to the top of the volcano and typically use it at 100-200X."
For the 230, the field angel magnification for the first etalon in series is roughly the aperture divided by the etalon diameter 230 / 80 = 2.9, quite high for full disc contrast uniformity to be maintained (actually it can't be), especially because it has a narrower FWHM of 0.5 A. Therefore if he likes to have good full-disc views, he should not give away the SM90 DS, and use the LS230 (or LS152) for times when seeing allows good close up views, which likely will be fewer unless he lives near the coast with a stable marine layer nearby to preserve better than average daytime seeing.
Most of the time he'll likely be using the 90...