30” scope

[Hakann]

A technical question for the above idea.

If I like to use a 3.5” diagonal.
Just a thought/question on that.

I can get a Pyrex 30" core here in Sweden maybe, but is just 38 mm thick, so a 20;1 factor.
Very thin for a 30" plano ( sigatta is near 12 mm vs a f/4 )

Well, thicker more weight/more glass but...and at ex f/3.5 the FL is shorter but then a bigger diagonal is needed.
( still a ladder )
But the chance of a better parabola is better at f/4, or say, more 'easy' anyway ( I add maybe as this is not easy )

I has a Antares 1/30w at 3.5" diagonal.
On the set-up for a Ethos 4.7 or the 3.7 mm, and that small diagonal the focus pt will be 1.33" inside mirrors edge.
At a 13.62" L-distance I has it ok ( from Bartels program ) for a 3.5" diagonal ( set at 3.4" app )
That is 11 % obstruction.
-I guess if that is 11 % or 25 %, might not make a big difference but 11% is nice.

But a 4" diagonal I has the EP outside mirrors edge at half the EP's field stop, but that is a total of 81 cm2, and a 3.5" diagonal get 62 cm2, so the 4" is around 20 cm2 bigger.
But the EP will intrude in mirror, and that is around 12 cm2, so it creates less space total than a 4" diagonal.

Would this ‘intrude’ cause spikes at mid or high powers ?
It is 0.17 in FoV for the Ethos 4.7 mm and 0.14 FoV for the 3.7 mm, so object's is faint stuff and a good sky is needed.

I has test put in a credit card at the side under observing at both low and high power before, and I never notice spikes from it or see a ‘worse’ image.

Ideas on this ?

A note, as to get the EP to work I will use the EP as a 'drawtube' and a own focuser body/Crawford.