Off axis confusion

[Okeydokey]

I noticed that almost all off axis images on google related to Dobs/Macs and what not!  I guess this is to avoid the front mirror... makes sense.

But my refractors dew shield has a lens type cover which is D. 145mm for my D. 120mm Helios.  On this cover there is a D. 55mm raised section with another lens type cover in the centre, looks about the same size proportionally as those on off axis filters.

Now when I look through the 20mm eye piece the view doesn't change whether the dew shield's smaller cover is on or off.  My point is and the reason I ask is that I am fabricating a solar (Baader filter holder) for the scope and was wondering if there is an obvious reason why I don't make it at D. 55mm as opposed to D. 145mm?

Because with a 155mm aperture I get one filter plus one spare, for my £24.00,  whilst at 55mm I get one filter plus eleven spares.  I suspect I will need to change the EP at some point to increase the image size but have gone a bit numb trying to work out what is happening

Many thanks for the sanity check!

[ArmyAirForce]

I'm sure I read somewhere that due to seeing, a white light solar filter won't see any more detail larger than 4 inches ( 100mm ), so there's not much point going bigger than that. I would go for the biggest size you can up to that size, but leaving you with enough for spare filters. A 4 inch hole would be better than a 2 inch one, so find a compromise size.

On my SW200PDS, I've gone for about 95mm which is the biggest size that fits between the secondary supports.

[Okeydokey]

Many thanks, a response loaded with info:  I just found out a beer can is about 55mm D.  May still have to order quite a few for product development purposes, though I might not get away with it now.

[ArmyAirForce]

This is mine, with rotating covers to protect the film when not in use. There is also a filter for the finder, and a glare shield around it, which is useful for lining up on individual sun spots and positioning for imaging. It is held onto the front of the scope with Velcro.

[StarryEyed]

Reducing the apature but using a smaller solar filter (stopping down) also increases the focal length and with some optics can improve the quality of the view. You might try two or three different sizes to see if there is a benefit. I'd they is none you will still have two or three filters.

[Okeydokey]

Many thanks everyone, I read some where that an f10 would be ideal for solar viewing, clearly with my scope that would equate to stopping down to 100 from my current 120mm, I think the coin has dropped!  I will revisit plan A accordingly.  I had a play with Stellarium and used an aperture of 55mm with an 18mm EP and the difference becomes obvious.