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I have two 8" f/4 Newts. One I use exclusively for visual, the other for both visual and photo. With visual I can get a massive 3° true field of view and make for stunning rich field scopes. With photo, it is a versatile focal length for large and small chip sizes.
I don't know why people say an f/4 Newt is difficult to collimate. It isn't any more difficult than f/5, f/6 or f/8. What is more critical is the need for the OTA to hold its collimation, which is the problem with mass produced OTAs. These need to be modified so that they behave, not misalign so easily. One of my f/4 Newts is a mass produced solid tube OTA, but it has been heavily modified so it holds collimation really, really well. The other scope I built myself and was appropriately designed and made from the start. I also have other Newts that range from f/4.5 to f/5, and they all are the same when it comesto the collimation process.
A coma corrector is necessary, especially for photo as coma is significant with fast Newts, and the faster the focal ratio the more significant coma is.
There are a variety of coma correctors, and one optimized for f/4 is best for an f/4 Newt. They also have a range of properties, such as some have a magnification factor, others don't, some significantly put the focal plane further outside the tube but others bearly have a shift in focus. What they all have is the need to place the correct spacing between the camera or eyepiece to the corrector lens. Carefully investgate these and make a choicebased on facts, not Brand prejudice.
An f/4 Newt is a very fine imaging and rich field scope. Their fast focal ratio is not the problem any more than for slower ratios. But the fast focal ratio does expose the short comings of mass produced instruments a lot more than slower ones. While relatively inexpensive, you will need to invest some more money, reading and time to fix these short comings.
I love mine! They are also my grab'n'go scopes at home. Very well figured mirrors that also perform really well at high magnification as much as low. And stunning for photo.
Alex.
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