2011-05-22

Testing continues

22.05.2011

Testing the imaging quality proved difficult to say the least. With nine points to adjust and some structural flexing focusing and collimation is almost impossible. Overall the system seems a bit soft and low contrast, but at least on H-alpha narrowband filters it gave a fairly good resolution on the focused area. Still, the high intensity regions bleed over darker ones reducing the contrast a bit too much. Additional baffling migh help keep the stray light away, but the glow around bright areas does indicate that high contrast isn't a priority for these lenses.

2011-05-21

Initial focus testing


Initial focus testing
Originally uploaded by Mickut
Via Flickr:
I wish I had a DSLR with live view, it made focusing and testing a lot easier. At short focal lengths the image is soft and low contrast, so unfortunately the optics seem useless for a fats, widefield astrograph. I still have evaluate at a significantly longer FL to see is it any good as a planetary scope.

Short and stout


Short and stout
Originally uploaded by Mickut
Via Flickr:
The telefocus optics keep the overall length very short compared to focal length. This rig is a major pain to collimate, but luckily its main purpose in life is to evaluate general usefulness of the lenses and the distances between flanges for a suitable focal length and sufficient back focus distance.

Assembled for testing


Assembled for testing
Originally uploaded by Mickut

Secondary secured to the board

Via Flickr:
Camera mount in the background

Clamps for main lens


Clamps for main lens
Originally uploaded by Mickut

2011-05-19

Lots of clearance


Lots of clearance
Originally uploaded by Mickut
I didn't break anything, although it looked a bit dangerous at times. I had a small measurement error, so all holes needed to be enlarged a tad.

Drilling


Drilling
Originally uploaded by Mickut
Making a hole for the main lens. The small central hole was used to align all boards for threaded rod holes and as a guide hole for the cutter tool. Everything should be fairly well centered for getting the measurements right for the actual OTA.

Raw material for test rig


Raw material for test rig
Originally uploaded by Mickut
Via Flickr:
Thick, 27mm plywood for the main lens, thinner 10mm ones for negative lens and camera mount. Four holes will be drilled into each, three for threaded rods and a big one at the center for optics.

2011-05-18

Astrograph project starts


Fast positive triplet
Originally uploaded by Mickut


Negative doublet
Originally uploaded by Mickut
My mystery optics order from Surplus Shed of a very fast air spaced triplet and negative doublet arrived today. The weight is a bit of a concern, the triplet lens (D=155mm, f/1.23) weighs a whopping 2.5 kg and creating a stable focusable system for these two is going to be challenging. The smaller, but not lesser, negative doublet weighs "only" 414 grams, and is unmounted, edges painted black with elements glued together. I took a chance on these, as the big lens for half off, and at a total price well below 200€ shipping, taxes and customs included wasn't too bad if these can be combined to even a reasonably good fl=465mm f/3 astrograph. Since I'd use mainly narrow band filters with it, I'm not too concerned about color aberrations, mainly about the image plane flatness for the fair size CCD sensor in SXVR-H18.


The next step is to create a temporary rig for determining correct spacing between elements in order to reach the desired focal lenght, the thin lens equations say it's in the order of 150-160 mm, but this is now the real world. In theory short test exposures should be doable with it as well before creating the actual tube assembly.

The structure for the OTA served a challenge, as the back focal distance from the negative element to image plane at infinity focus is fairly short, in the order of 80 millimeters at desired focal length. This leaves out the option of building a rigid tube and attaching a commercial focuser at the back, but I realized I might just be able to construct a Huge Crayford focuser and put the optics into the inner tube and attach the camera to the outer tube, eating only 15-20 millimeters of the focus travel. As a result the final tube would end up around 20cm diameter and 20cm long looking more like a snare drum than a telescope, but who cares.