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[RobbW]

What causes a sharp center FOV but blurry objects as you get closer to the edges? Is that something to do with the EP, primary mirror, secondary mirror, collimation, out-of-focus, or something else? I collimated my C8-SGT as best as I could, but at the Marriott star party, my center was nice and sharp but the points of light started to blur near the edges. Do I just need to fine tune the collimation or the focusing? Or is this something that can only be fixed with a field flattener of some kind? Just trying to eek out the clearest images possible. I have a T-ring and adapter I'd like to use with my Canon 40D for some prime focus astropix, but I want to make sure the entire image will come out nice and sharp. Thanks.

[Chicago Astronomer - Astro Joe]

Robb,

Slight blurring near the edges of the filed in SCTs are normal. It's just the nature of the beast.

But, there are factors to consider to minimize it...

Collimation.

A star test on inside & outside focus will show any mis-alignment at high powers. Chicago Astronomer Bill is our eagle-eyed expert in this, but I think you two did the adjustments already.

Related.....

The Corrector plate should be as close to 90 degrees to the focal plane as you can get it. Ever so slight angles will bias the image to a preferred side. Too much tension on the corrector plate retaining ring might also warp the angles.


Eyepieces.

Poor quality eyepieces will skew and degrade images.
At our next gathering, let's try out my eyepieces and see what happens, but I don't suspect here is the problem.


Diagonal.

A poor diagonal will also degrade the image.

Try to observe without it, doing a straight through view and see if anything changes.


Secondary mirror dirt.

The secondary mirror might have some haze on it, especially noticeable at the edges. But I would have thought you noticed any grime when you took it apart.

Do you notice it getting better or worse during focusing? If at any time the focus is sharp from edge to edge, then you know that sharp edge-to-edge views are possible...and the mirror is suspect. Mirror flop more precisely. Maybe that's why the heavy grease on the baffle tube?