More Aperture Masks

The other day, I went out to the shed in the rain to cut out a couple of aperture masks for my small 50mm guide scope.  I want to try to use this guide scope as an imaging scope for imaging the full disk of the sun.  I have previously worked out a potential configuration to do the same using my 72mm ED refractor but I want to compare the two and then continue to use the one that produces the best images.  

I need these aperture masks because I need the guide scope's focal ratio to be in the f5 to f6.5 range.  Without these aperture masks, my guide scope is at f3.2 which is too fast for this particular use.  I'm using my Quark Chromosphere solar filter so I need the focal ratio to be between f4 and f8.  I'm thinking that closer to f4 might be better for capturing prominences while closer to f8 will be better for capturing details on the 'surface' of the chromosphere.  This surface isn't really a hard surface though...  it is a surface of plasma gas in the form of hydrogen and helium.  

Unfortunately, although I'm happy about my woodworking skills, these two wood aperture masks didn't fit onto the scope all that well.  While trying to figure out how to size these aperture masks to fit perfectly, I actually came up with a better idea.  

I found that I had an adapter that fits onto the front end of this 50mm guide scope.  This adapter had m48 threads which makes it very useful.  In the photo below, I screwed on a 1.25 inch nosepiece which makes for an aperture of around 25mm (24mm if I screw on a UV/IR Cut filter).  25mm was the size of my smaller wood aperture mask (aperture mask on left in the above photo) so that is the perfect size.  24mm will give me an f7 focal ratio.  I also have step down rings to make this opening a 39mm opening (34mm if I screw on a UV/IR Cut filter) which is very close to the size of the wood aperture mask on the right in the above photo.  34mm will give me an f5 focal ratio.  I think this will work out better than using the wood aperture masks.  

I screwed a UV/IR Cut filter onto the front end of this new aperture mask just to see how that would work.  I'll probably just use this filter on the front end of the Quark filter rather than the front end of this scope.  

Now I'm not sure what to do with these two wood aperture masks.  For now, I guess I'll just put them in the drawer with my other aperture masks.

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A couple of days have passed and my step down rings arrived.  I purchased a couple of step down rings to get the aperture of this little guide scope down to 38mm (photo below).   

I was hoping to get down to 33mm which would provide a focal ratio of f5 but I can't find any adapter rings smaller than 39mm.  An aperture of 38mm, however, puts me at f4.3 which is within the operating range of my Quark Chromosphere solar filter so that will have to do.  I suspect I'll be using the 25mm aperture (photo above) more often.  The 25mm aperture will put me at f6.5 which should be good for providing a bit more detail to the surface of the sun.  Time will tell, but the larger f4.3 aperture might be better for imaging prominences.  

Anyway, here is a photo of the 38mm aperture...  

Now I just need a clear, sunny day so I can test this.  Actually, I could use a full week of clear skies and good health so I can do a bunch of astronomy things on my to-do list.

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EDIT:  4 July 2023 - I found that when I add a UV/IR Cut filter to the 39mm thread, that constricts the clear opening further.  With the filter, my aperture opening is now 34mm which gives me an f5 focal ratio.  This is great and almost identical to my original plan.

When I add a UV/IR Cut filter to the 1.25" nosepiece (shown in one of the earlier photos) that is used as a front end snout, that gives me a clear opening of 24mm which gives me an f7 focal ratio.  I like this focal ratio too!  I had an m42 filter holder on hand so I decided to use that instead of the 1.25 inch nosepiece.  This can be seen in the two photos below.

When I started off with the idea of finding the correct size adapters, I had forgotten that when I add a filter (or adapter), the filter or adapter itself will constrict the clear opening a bit more.  That is often not desirable but, in this case, the filter(s) do limit the clear opening which puts me at a better focal ratio(s) for use with a Quark Chromosphere solar filter.  Having clear apertures of 34mm and 24mm is right where I wanted to be in my original plan of making aperture masks out of wood.  

The photo below shows the whole assembly.  I'm not exactly sure of the spacing between the scope and the Quark so what is shown here is just a rough guess on the correct spacing.  It was overcast and thundering when I shot the photo so I could not test this assembly to get the spacing exactly right.   I'll have to do that on another day...  on a rare sunny day!

Below, you can clearly see the 1.25 inch UV/IR Cut filter on the front end of this telescope assembly.  These filters always appear magenta in light.  Since this is a 1.25 inch filter, the clear opening is 24mm for a focal ratio of f7.

I believe that these adapter rings will work far better than my original plan of making wood aperture rings.  The apertures needed for this small scope are so small that finding the right size adapter rings was pretty easy.  The downside is that I had to purchase some additional adapter rings.  It seems that no matter what I attempt to do in astronomy, I need to purchase some additional adapter or accessory!