John there are a few reasons I like f/7:
- f/7 maximises the useful range of magnifications possible from eyepieces to about 10:1 between highest and lowest, without using Barlows. Most f/5 scopes are limited to a much lesser range and with f/4 scopes its more like 3:1. Similarly at long f/ratios the range is limited - the lowest useful power of the MK91 is about 100X and it is useful past 600, but that's still only a 6:1 range.
FWIW The Crago 16" provides a perfect example of this.
- f/7 isn't going to challenge low-glass eyepieces selected to give optimal planetary views - I am not a fan of ultrawides, nor eyepieces with far too many elements (Al Nagler I am looking at you). I'm sure I could design the ultimate 3mm 200 degree fishbowl eyepiece the size of a housebrick with 20 elements - and it will be the ONLY eyepiece you'd ever need, but...the image quality won't be great.
- the secondary obstruction is small enough to be neglible even allowing for a slightly oversized secondary to minimise vignetting off-axis at low power. At f/5 this isn't the case and forget f/4 (I've been there done that). Conversely going to f/8 or f/9 doesn't achieve much either (have seen some really long old-school newtonians).
- at f/5 (as indicated in your post in another thread) the compromise between backfocus and secondary mirror obstruction becomes a real issue. At f/7 it isn't an issue, ie. plenty of backfocus can be had without a whopping secondary that's going to hurt image quality.
- in a lightweight 14" f/7 dob the eyepiece height suits me just fine. Shorter dobs are frankly a pain in the back, and pointless as I end up putting a stool under them to raise the eyepiece up.
- the cost of a few inches longer struts is nothing...
And that's aside from the issues with figuring the mirror precisely.
The last issue I have with all newtonians is diffraction spikes from spider vanes. Sure, most of you think it doesn't matter. But actually it does, as I found out when comparing the view of a tight double (separation 0.6") - the MK91 resolved the pair but the Mewlon could not thanks to the spikes. A design with 1 vane or two curved vanes could be better, I guess.
On another note I have had a view of the planets through the 12" f/23 tri-schiefspiegler made by Barry Adcock long ago. That was in a league of its own. This showed just how good an unobstructed, perfectly achromatic scope could be. Newtonians simply don't come close, but the tri-schief was far from portable...
Last edited by Wavytone; 11-07-2019 at 10:06 PM.
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