From: Roger Samworth
Sent: 10 November 2016 20:18
Subject: Barlow for Mikrokular
Having now purchased a MikrOkular full HD, primarily for solar imaging, the question now arises as to how to make best use of the full HD.(1920 X 1080). The primary reason that I got it was for the extra pixels, and to get a reasonably high resolution of the whole solar disc (as well as detailed images of individual features) but the aspect ratio (16 X 9) is hardly ideal for a circular object!
The obvious solution is to image 2 hemispheres and then stitch them together. To make best use of the available pixels, the solar disc size needs to be something approaching the 1920 pixel axis. Using the MikrOkular on the Lunt LS35THa gives a small-ish image, but NOT the whole solar disc. Adding a X2 barlow makes the disc too big – more than the 1920 pixels. What is needed is a X1.5-ish barlow. Another consideration is that the Lunt, like the PST, has quite a small focussing range, so the barlow also needs to accomodate this.
There didn’t se em anything obviously available, so I decided to make one.
A barlow is simply a negative lens spaced away from the eyepiece (or chip, in this case). The further away from the eyepiece it is, the more magnification you get. So, to get the X1.5 it needs to be quite close. With imaging devices such as the MikrOkular, or PD or cameras like that, the chip is quite small and near to the optical axis of the system, so you can get away with a simple lens as off-axis aberrations don’t set in. Additionally since solar imaging is monochromatic, chromatic aberrations aren’t an issue.
The MikrOkular fits inside a 1.25″ O/D adaptor so we could make a barlow and tube that closely fits and slides inside this adaptor, and fasten a simple negative lens to this.
A piece of plastic cistern overflow pipe is nearly the right size, but a bit small. So I cut a piece and slitted it lengthways. I then cut a second piece, slitted and cut out a section from it. This then slides inside the first piece to spring it out to the correct diameter. In the resultant gap, I glued a bent piece of plastic cut from a flower-pot to act as a spring to hold the finished barlow in place. I then got got hold of the negative eye-lens from a pair of Celestron freeby cardboard binoculars that I was given at some exhibition, and glued this to the end of the tube. You’ll get the idea from the attached pictures. Cost = £0.00
So, does it work?
I have attached an image I did from the window-sill this morning of the full solar disc. This is a composite of 4 images, 2 for the disc, and 2 for the prominences. Since the sun is a bit feature-less at the moment I couldn’t get them to stitch automatically, so I did it manually, using Canon’s Photo-stitch utility.