TeleVue Paracorr

Purchased the Paracorr for my 8" GSO Newtonian F/4 imaging scope. The paracorr is the cats meow in astrophotography, nothing comes close. Corner to corner my images are flat, even at F/4.Words of advice: If you are using a GSO Newtonian, you will likely be in a similar situation as me. The paracorr must be backed out of the focus tube at least 20-30mm to properly achieve focus. Other newtonians don't have the same issue. It's not a deal breaker, but it does create a bit of leverage against the focuser that causes a mild amount of sensor tilt at certain angles.

This always stays in my f/4.6 14" dobsonian. I couldn't see buying anything except the TV Paracorr

Clean pin point stars.. a must for anyone pushing an F5 or below.

Works well but not much of a difference in a 4.5 to 4.6 f ratio dobs. My type I appears to be sufficient.

What's a coma corrector for?Newtonian reflecting telescopes have parabolic mirrors which create an optical aberration known as "coma." This has the effect of causing stars away from the field centre to look like little outward-going comets, rather than sharp, round, points. This happens because different parts of the mirror do not reflect light to quite the same location. It is not a "defect" in the mirror, it's just how parabolic mirrors work. Coma is more obvious further from the field centre and it occurs more in mirrors which have a faster focal ratio (a more curved mirror surface, yielding a shorter focal length). Coma correctors remove the aberration by adding inverse coma, so no net coma is seen at the eyepiece. This means that the corrector needs to be placed accurately and the telescope has to be particularly well collimated.Who needs a coma-corrector?I first used my Paracorr-2 on a 12" f/5. I bought it because it was on sale and because I was expecting delivery of an f/4 in the near future. I must say that I was surprised how much the lower power views were cleaned up by the corrector. I had no idea how much coma was present until it was gone! Open clusters with a 30 mm Explore Scientific 82 degree now looked gloriously sharp, as did the views through a 27 mm Panoptic. With eyepieces 19 mm and shorter, my 12" f/5 showed little or no obvious benefit. This is as expected, as only the central portion of the image plane is being observed. With my f/4, I find the corrector to be vital. At both f/5 and f/4, the Paracorr never left my focuser. What more can I say?Will the Paracorr work for you?Aberration at the field edges can be due to both coma and astigmatism. As discussed above, coma is due to the primary mirror. Astigmatism, on the other hand, arises from the eyepiece itself. Astigmatism is worse in cheaper wide-field eyepieces which are being used in faster telescopes. By "faster" I mean f/5 and shorter. Astigmatism manifests itself as stars which look like little crosses, whereas coma is little comets. If your eyepieces aren't well corrected for faster focal ratios, then adding a coma corrector will get rid of coma but the astigmatism will remain. This needs to be kept in mind. Also, the buyer needs to remember that this corrector has a 1.15 barlowing effect so the telescope's focal length will become 15% longer. Whether you "need" it at f/5 is unclear, some people do and others don't. For me the answer is "yes."The Paracorr 2As for the details of this corrector, I find it it to be very well machined, the optical surfaces were free from dust and finger prints. The compression rings work well. The box is tough and I store it in the box when not in use. The box lives in my observing case. The tunable top turns easily and locks easily. The included 2" to 1.25" adaptor is of high quality. The unit is threaded to accept 2" filters. Yes, it's expensive but you can tell it's high quality. Furthermore, Televue will repair it for you should you damage it (you pay, of course). Still, not a lot of manufacturers will do that.Different eyepieces need to be set to an optimal distance from the corrector lens in order to get rid of the most coma. This is what the tunable top is for. This is easy to do, particularly since most Televue eyepieces can be set to just one value. It's easy to figure out the correct setting for other eyepieces (there are guides on the web, I won't discuss it here). What I've found, however, is that even without dialling in the corrector, almost all of the coma is removed at whatever setting you're at. In practice, the Paracorr sit in the focuser and you can forget about it. Nice!Why Paracorr 2?There are some differences between this unit and the previous paracorr. Basically, the new unit is designed for telescopes faster than f/4. It also allows the 31 mm Nagler and 21 mm Ethos to come into focus for all observers. I believe very myopic observers had trouble with this previously.