Quote:
Originally Posted by astro744
Please confirm the 42mm is the minor axis of the secondary, i.e. narrowest width. It is an odd size for a US company like Coulter to use. Sizes used to be as follows in inches:
1, 1.3, 1.52, 1.83, 2.14, 2.6, 3.1, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12
Multiply each by 25.4 to get mm. If 42mm is the major axis then first divide by 1.414.
That being said there is only so small you can go with 6" f5 as there are physical restriction of tube diameter, focuser height and desired edge of field illumination. I have an Astro-Optical 6" f5.5 "Super Short" and they used a 1.83" secondary for it. GSO 6" f5 use a 50mm secondary and Teleskop-Express use 63mm but it is optimized for astrophotography. The detrimental effects of an oversized secondary are often overstated so don't be too concerned with 42mm as it is on the smaller end for a 6" f5 but it does depend on what focuser and tube you have. SCTs have far more obstruction yet still provide good planetary views. Ideally 25% and under is optimal but at 28% I wouldn't worry about it if the secondary is good.
Now after saying all that the modern offerings for Asia are metric and not a direct swap with existing secondary mirrors especially if they are held in place rather than glued.. It may not be that simple to simply change the secondary.
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You are correct about the secondary - my error. It is actually 46mm (1.83 inches in oldspeak). I am in the process of doing all the math at present. Thanks for the info.