Setting the record straight about a superb piece of equipment
All images and text Copyright Mike Lockwood, 2016. May not be used without permission.
Let's get right to the point - I have seen so many incorrect facts and assumptions made about the Paracorr 2 and SIPS that I felt that a short article was necessary to straighten the facts out. I consulted with TeleVue on this short article.
Personally, I prefer a SIPS (Starlight Integrated Paracorr System) for four reasons, two optical and two mechanical:
1) I don't binoview. The SIPS usually needs to be completely removed for this. However, binoviewers also don't generally agree well with very fast telescopes.
2) For infinity-corrected vision, the Paracorr 2 in the SIPS is always perfectly "tuned" after the setup procedure is completed. I never have to worry about tuning it, no matter what eyepiece is used. This saves me time while observing. See the SIPS manual here. However, I don't wear glasses. If you do, you will want to keep them on so that you are infinity-corrected.
This paragraph is for observers who are strongly near- or far-sighted. These observers should use their glasses and long eye-relief eyepieces when observing with the SIPS. Observing without eyeglasses may induce coma because the eyepiece-to-SIPS position will vary from its coma-corrected position when the observer focuses. If you prefer observing without corrected vision, choosing a regular Paracorr 2 instead of SIPS will maintain coma correction independent of focus position.
3) The lens group is always exactly centered on the optical axis and is never tilted. The way it is machined ensures this. The tunable top Paracorr 2 unit can tilt slightly in some focusers as the thumbscrews are tightened, slightly degrading images. I have experienced this myself.
4) The SIPS unit has its mass closer to the focuser board, resulting in slightly less flexure of a focuser board. With the tunable top Paracorr there is the Paracorr and an eyepiece extended well above the focuser. When using the SIPS, the focuser has only the eyepiece in the barrel, and this results in less flexure of the focuser.
I recommend the SIPS for all telescopes, but especially below f/4.0 where the benefits will be greatest.
To be clear, the Paracorr 2 is a large improvment on the Paracorr 1. Coma correction is improved significantly, especially at lower f/#s, meaning that stars are rounder, and field curvature is greatly reduced as well, meaning that the whole field focuses at once.
There are conflicting posts online describing the SIPS and how it should be positioned. This article should serve as your guide for this process.
Here's another way to think about the SIPS while designing a telescope.... consider the cone of light as it enters the bottom SIPS lens.
Refer to my diagram below.
The diagram shows where the focal plane would be if the Paracorr was not there - 88mm above the bottom lens. Since we know this, we can easily calculate the light cone that gets us to that point, forgetting about the lenses, 15% barlow factor, etc. Again, this is *without* the Paracorr in place.
That's how you should design the scope - put the focal plane 88mm from the bottom of where the SIPS lens will be. (This value does not change with f/#.) This will let you size the secondary mirror, and you will see that the cone of light illuminating the central spot of the field easily fits within the bottom lens opening of the SIPS. More on that below.....
From the diagram, note that the locking ring (the part that has "STARLIGHT INSTRUMENTS, LLC" painted on it), which contacts the top of the mounting plate and locks the rotation of the SIPS unit after it is adjusted, adjusts from 20-40mm from the bottom lens. Let's center it at 30mm for sake of this example, leaving you 10mm of adjustment up or down to tune the SIPS during the initial setup, which should be enough unless the telescope is set up differently than most.
That means that we have:
88 mm - 30 mm = 58 mm from the focal plane to the top of the mounting plate.
The mounting plate is 6mm thick, so the focal plane should be:
58 mm + 6 mm = 64 mm from the top of the focuser board.
So, that's your rule - set the scope up to put the focal plane about 64 mm (~2.5") above the top surface of your focuser board, and then install and tune the SIPS as per the SIPS instructions.
Sanity check: 64 mm is about 10 mm above the height of the racked-in 1.5"-travel FeatherTouch focuser that the SIPS uses. I always advise the focal plane to be placed about 0.25" to 0.5" from the top of a racked-in focuser, and 10 mm is about 0.4", so these placement methods agree fairly well.
Finally, since many people have made completely erroneous claims about the size of the Paracorr 2 illuminated field, I asked TeleVue if they could give me some accurate numbers. Paul calculated the fully illuminated field for the Paracorr 2, assuming a 60" focal length:
f/2.5 - 14.0 mm f/4.0 - 27.9 mm
f/3.0 - 22.4 mm f/4.5 - 29.8 mm
f/3.5 - 26.0 mm f/5.0 - 31.6 mm
So, as you can see, that's large enough for comfortable visual use down to f/2.5, and because our vision is not sensitive to a a smooth falloff in illumination, we probably won't notice that at low powers. I have never actually noticed illumination falloff in my 20 F/3 or my 14.5" F/2.55. Honestly, with high-quality optics most people don't use enough power when they observe, and never get to see the detail and contrast that they should.
Go forth and set up your SIPS and enjoy the best coma correction that you can buy (for a reasonable price) for a fast Newtonian.
-Mike Lockwood, Lockwood Custom Optics