Reduce mirror focal length

[Cami]

Good morning
I have a 200mm Newtonian telescope. F/6 , built by me , how could I get it to F / 5
no longer having the tool ?
Thank you

[Giulio TiberinI]

Hi Cami
To bring a 200F6 to an F5 focal ratio you would have to redo the entire mirror figure ... which is a great job that frankly you have to judge if it is convenient to do. (For example, I have never refigured my dobson 130F7 from an invicta backpack weighing 3kg, to bring it to a more easily balanced F5)

And to reshape the mirror you would unfortunately have to redo the tool to deepen your current parable of 42 cents mm, taking it from your current 2.08mm F6 arrow to a new depth of an F5 which is 2.5mm.

As a tool you could make a hard plaster "for molds" like a thick dentist 3 or 4 cm (purchasable in supply stores for dental technicians) casting chalk on your current mirror (after you have protected the surface with a polyethylene sheet or parchment paper, and surrounded its circumference with a cardboard tape to form a sufficiently high gypsum containment dam).
Then on the curved and dry surface of the new plaster utensil you should glue with epoxy (which in the meantime waterproofs it) about 15x15mm stoneware tiles; spaced asymmetrically but never in contact; or glue irregular pieces of glass of similar size in the same way, or even drowning hexagonal nuts in the plaster cast (with the hole facing the mirror) for M8 or M10 steel bolts (for key from 13 o da 17mm) arranging them spaced on the baking paper, and then pouring the chalk on it, making sure that the holes in the nuts are filled, and not to move. But to avoid unwanted movement it is best to secure them in place to the parchment paper with a small drop of hot glue, just so they can't pile up in the plaster.

This is the only way because I don't think there is a focal reducer to put on the eyepiece that reduces it by such a small ratio = 0,83 how is the relationship 1000/1200 (where 1000mm is the focal length of the F5 and 1200 that of the F6.

Focal reducers are positive doublets that work in reverse of barlows, which are negative doublets that increase the focal length.
There are therefore reducers for larger jumps, and at affordable prices only starting with a halving of the focal length (i.e. 0.5x)

My question is ... why would you want to change the focal length of your 200F6 ?

[Cami]

I need a shorter tool for structural reasons and to speed up the poses. Thanks . Good day

[Giulio TiberinI]

[Author]

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