Using the Televue Coma corrector
Only eyepieces with less than 150 magnification suffer coma. The 8.8mmUW and 14mmUW will not show any coma because of the built in Barlow lens, if the stars look fuzzy it is because the seeing conditions are not good enough. I find that most nights that 100X magnification is best I can manage, only 1 out of 3 nights can I use 150X magnification and 1 out of 10 nights can I use 200X or more magnification which I would use on the planets, moon and planetary nebulas any how. We can stop down the large telescopes, with short F ratios (like 18" F4.5) to improve the image for planets and the moon. With an of axis mask 20" in diameter with a 4" hole near is edge and positioned between the vanes of the spider, this would give a F20 ratio. And for bright open clusters use a disc 10" in diameter at the center, attached to the center bolt in the spider this will remove most of the coma at low power or 100X magnification. Test on M7 with and without the mask and you should see a big improvement. The 10" center mask will also collect most of the dew, which would settle on the secondary mirror, so you can observe longer. If you look at NGC104 at 100X without the 10" center mask the coma will not be noticed if the Globular is in the center of the field, for the sky is dark around it. Generally the wider the field the more coma is seen. So why bother with wide field eyepieces and they cost 10X more in price. Orthoscopic eyepiece shows almost no coma and cost about $60 but the field is only 40 degrees. My view is wide field eyepiece cannot be justified.
So why do people buy them? (Dobsonian don't track) the wider the field the less time spent adjusting the position, with a ½ degree field eyepiece the Moon moves out of view in 2 minutes. By the time it takes to change a filter the object is gone.
So why buy a coma corrector? At $500 it had better do something. It has no benefit for eyepieces over 150X magnification because it is not needed, for wide field eyepieces around 110X magnification it removes most of the coma but for wide field eyepieces around 70X it removes half of the coma. The 10" center mask can do this better but want about the light loss I hear you say? Well the coma corrector may reduce the light getting to the eyepiece by the same amount.
The Televue coma corrector (parracore) can be used as a photographic aid as it has a thread so it can be attached to a 35mm SLR camera, which I find very useful. It also increases the magnification by 15% and also extends the focal point further back out of the focuser, which is good, because most cameras will not focus without it. Of cause the masks can do this just as well or better if you can focus the camera.
I use Televue coma corrector with assorted masks. In the end it is the skill of the operator to get the best out of the telescope, by whatever means possible.
By Greg Walton