Newtonian scope design dilemma

[markse68]

I had a few issues with my ap scope which turned out to be due to astigmatism caused by the secondary mirror. So I have been looking at buying a new secondary and have ordered what I have come to realise is the wrong size- doh!

But in looking closer at my scope I have come across a bigger dilemma to do with the general design  and in particular the limitations imposed by my coma corrector- an RCC1 from Baader. This has a 72mm long barrel that sits inside the focuser and requires a 91.5mm back focus to the CDD plane. So the focal plane of my scope needs to be a long way outside the OTA to prevent the CC from intruding into the primary light path. BUT the inside diameter of the CC is only 1.81" so that restricts the light cone that can reach the CCD without being hard vignetted.

My camera has a 1" diagonal sensor so the fully illuminated field would ideally be 25mm diameter. But this doesn't seem to be remotely possible with this arrangement. Either it's designed for smaller sensors or slower scopes (but it claims to be optimised for f4)

My dilemma is which size secondary to use or whether to bother at all- maybe I should just get a shorter CC... 

Here are a couple of Secondary options put through Newt for Web with the correct dimensions for the OTA, focus plane height etc.

 

2.75 seems to be the optimal for no vignetting of the 100% illuminated image circle. But that circle is only 0.38" dia (9.65mm) which seems way too small for my sensor

 

3.1 gives a larger potential 100% illuminated circle of 0.89" (22.6mm) which would be almost perfect for my sensor BUT it is getting vignetted by the end of the CC

Does that mean that there is no advantage to increasing the size from 2.75 to 3.1"? The 3.1" would increase the central obstruction from 34% to 39% also. 

I could probably calibrate out the vignetting of the 2.75" mirror- it is what was fitted in there up till now and wasn't even optimised for minimum focuser height so would have been even worse, but there's not much point in having a "fast" scope if you then don't get 100% illumination because of too small secondary and/or vignetting.

Not sure what to do,

Mark

Edited September 30, 2021 by markse68

[markse68]

ps does anyone happen to know how much the Baader RCC1 shifts the focal plane? I’ll measure it when i can but wonder if anyone knows as it’ll affect these calculations i guess.

Thanks,

Mark

[Craig a]

From what I could find out it shifts it out by approx 10mm 

[markse68]

Thanks Craig- good to know. I suppose it might be different for different  focal ratios? I’m planning to try to measure it tomorrow night on my f4.5

Mark

Edited October 2, 2021 by markse68

[markse68]

I just confirmed that for my f4.5 scope the offset of the focal plane caused by the Baader RCC1 is approx 4-5mm outward. I used a jerry rigged adjustable focusing screen using a height gauge aiming at Jupiter. Difficult to be super accurate without a high powered magnifier on the focusing screen but should be close to that. It’s what i read Baader used to claim as well though i can’t find that in their product description anymore. I had hoped it would be more as it would have made a smaller mirror justifiable.

Mark

(I also found out my primary is f4.4 not 4.5 as i had thought)

Edited October 4, 2021 by markse68