We know how even with APS-C sensor you need a 2″ focuser to avoid significant vignetting. However 2″ filters are significantly more expensive (which can translate to quite some money for some nice AP filters that were recommended to me), and not as many options available (even fewer are available as clip-on and usually even more expensive), so for the sake of not overstretching my budget I looked at whether I could do something about it.
So, to delineate the problem, here we have a nice flat frame from my 200p-DS’s 2″ T-mount:
There is a slight vignetting at the edges, which makes me feel a bit better about not shelling out for a full frame. But it is very little. However, if I would like to use a 1.25″ filter, I would have to use a 1.25″ t-mount to thread the filter on, which results to this flat frame:
As you can see, the vignetting is intense. Even though flat frames will help the final image if we want to keep most of the frame there is no way to make up for all that light that did not reach our sensor.
Knowing the diagonal of my APC-S sensor is about 1.05″ made it clear to me that there should be a way to improve things and the problem is the long 1.25″ tube. So I looked into my box with adapters to figure out a way of installing my 1.25″ filter close to the sensor, more or less trying what clip-on filters do. And the simplest way I could see was based on a very simple 1.25″ t-mount I had (Search on ebay for one of the cheapest – around £10 – that has a filter thread along its entire length, also on Amazon.co.uk), by cutting the insert side at its base, as shown here:
I used a simple hacksaw, which of course is not the best way and I will need to file it down a bit (and perhaps paint the silver) mainly for aesthetic reasons, but it seemed to allow me to thread the filter on either side of the T-to-1.25 “ring” I created. In fact, given the distance to the mirror of a Canon, I can screw the filter in the cut-off side of this ring, and screw this ring on the inside of a Canon T-Ring. I screw it only halfway, as the other side screws to a 2″ T-mount and tightening locks them both in place. Here is the result:
Note that if your camera does not have the flange length to do this (it should – you might just have to screw a bit more to get some clearance from the mirror) you can always thread the filter in the other side like this (and this is also how you should install filters that don’t work in either direction):
But did we manage to get any benefit? Let’s see:
That’s some nice improvement, it probably looks closer to the full 2″ flat frame than the regular 1.25″.