Hi,
I'm new to binoviewers and want to make my first purchase. I've spent the last few weeks doing some research, and I feel like I've come full circle and don't know where to go.
I'd like a pair that would perform well with my Lunt 80mm H alpha as well as my TSA 120mm refractor. Main targets would be the usual suspects- moon, sun, planets, brighter DSOs
I prefer to buy quality so I'm okay with the higher price tag. I took a look at the denkmeier, but was overwhelmed as to what's needed for solar observing.
Lunt recommends the tecnoskys, which are linear and can reach focus in any telescope (which is nice).
I've heard about glare issues with some of these Chinese sourced bins (tecnosky, Orion, William).
Any thoughts or recommendations would be really helpful!
Thank you
There real challenge with the Lunt 80 is not loosing aperture. Getting it to focus is not that hard.
For the Lunt 80, if you don't have the BF-1800, the Linear is probably the best overall choice because upgrading from the BF-1200 to the BF-1800 adds a lot to the cost. If you use the BF-1200, you have about 50mm of inward focuser travel before the system starts to lose aperture. The BF-1800 allows much more inward focus before losing aperture.
Even with the BF-1200, if you rack the focuser in more than about 10mm, the fully illuminated circle grows smaller than the solar disk, but losing a little aperture on the limb is not noticeable at all. At about 50mm though, even the very center of the full disk is not fully illuminated, so you are seeing the sun at very a lightly less than the brightness and resolution of a full 80mm, and at the edge of the disk, you are seeing it with a brightness and resolution slightly smaller than 75mm. If you were to go another 42mm in, these figures would now be a bit less than 73mm and a little less than 66mm. The goal would be to reach focus with the minimum amount of focuser travel possible.
If you have the BF-1800, then you can reach focus with a Maxbright II but you will need the 1.7X GPC (which actually only gives 1.5x) and a T2 connector. You could also use an Orion or Celestron BV and reach focus, but you would need to convert it to T2 and you would need a 7mm T2 extension.
This will still only illuminate the very center of the solar disk, but the illumination falloff is rather minor, and if you want to study a detail at the full aperture of the scope, you would just put it in the center of the field. To do this though, you must use the BF-1800. You could also use a Celestron BV and convert it to T2, but you would need the T2 extension and GPC. I use the MB II and a 1.7x GPC. Again, only a small circle at the center of the disk is fully illuminated, but the aperture loss at the very edge of the field is quite small and virtually impossible to see.
Likewise, if you use a Celestron or other BV, you can use a 2x Binoviewer barlow, or just the Barlow element from a 2x Barlow, but these will usually give more like 2.6x. Even 2.6X is OK though because you can still get the entire solar disk into the field of most low power eyepieces.
Another option is to use the Televue 1.25" 2.5x Powermate. When used with nonviewers, this will give about 2.2x rather that 2.5X. This will actually work with the BF-1200. The entire disk will not be illuminated, but it will come pretty close.
If you have the BF-1200 though, the Linear is the easiest choice because it can work in all other telescopes as well, or you really need to be looking at something that gives at least 2x or you will lose aperture, Now a bit of aperture loss is not the end of the world, but solar telescope aperture is the most expensive aperture on the planet. If you have a BF-1200, you don't have much inward focuser travel to lose.
Remember, with the Lunt 80, the sun is pretty small at the focal plane, and you should not be worried about using a very poweful GPC.
(Here are the figures. The solar disk is about 5.13mm at the focal plane. Even if you wanted to have enough to frame all of the prominences, you would only need maybe 6mm. This means that at 2.6x, the sun would be 15.6mm at the focal plane, so it would fit into the field of view of a 20mm plossl. Now the power would be high, but the point is that even if you have to use a 2.6x GPC, you could still easily see the full disk. A pair of 25mm Plossls would give 58x, which is a pretty useful power on the Lunt 80.)
There are a bunch of ways to do it though, but your path forward should consider the BF size. You have more options if you are using the BF-1800.
Edited by Eddgie, 16 February 2024 - 09:49 AM.
