A Pentax lens based imaging rig
by Mark Striebeck of
1+ year ago I started to work on a Pentax lens based imaging system. I saw some wide angle shots that were made with 30 - 60mm lenses that looked amazing. And because of the resolution, these images can be taken without guiding and just a good enough pointing model. Almost "point and shoot" style.
Why Pentax lenses?
Pentax 67 lenses have three properties that make them suitable for astro-imaging:
These lenses have a backfocus of 84.95mm(!!!) - enough to put focuser and filter wheel between it and the camera
These lenses have an imaging circle of almost 8cm(!!!) - more then what most CCD cameras have.
Being made by Pentax, the optics of these lenses is remarkable. Definitively good enough for astro-imaging.
I purchased a Pentax SMC 67 45mm F4 lens - it's very short and will create a LARGE image circle of more then 700 arc minutes!!!
First, I had to get an adapter. I was planning to use a PDF focuser from FLI and the CFW2-7 filter wheel from FLI plus my MLx694 camera:
MLx694: 21.0 mm
CFW2-7: 20.9 mm
PDF: 29.84 mm
That means that the adapter has to be 84.95mm - 71.74mm = 13.21mm thick - from the Pentax bayonet to the PDF focuser thread. Unfortunately, preciseparts can't make the adapter shorter than 24.00mm. I actually ordered the 24mm adapter, but couldn't achieve focus with it.
I checked some local companies who make adapters like these. But most didn't even get back to me - they probably only make adapters in MUCH larger numbers.
Next, I ordered a Pentax bayonet adapter with a different other side and tried to make one myself - but my skills on the lathe weren't nearly good enough to make one that is rigid and exact enough .
A different approach would be to use a Robofocus focuser to actually turn the focuser of the lens itself. This would remove almost 30mm from the system and be plenty enough (see here for an example). But that sounded tricky as I would have to mount the focuser next to the lens stable enough that it can turn the lens. This didn't pass my test for a simple, robust system...
Finally, Richard borrowed me his adapter that he made for himself for the same purpose a few years back.
Problem #1 solved.
Now, assembled, this is the whole imaging train:
My first attempt was to 3D-print an adapter to put this on top of a losmandy dovetail:
This worked, but it moved the whole thing pretty high up which made it less stable and also required A LOT of counterweight to balance it.
Luckily I realized that the holes in the FLI camera are exactly the same distance as the holes on the dovetail!!! That made it much easier to mount it!
FIRST LIGHT - WEIRD HALOS
Finally, I could take out my camera and try it. This was the result:
Not too bad! But when you zoom in:
There are some weird halos around the stars. This turned out to be a major pain in the ...
First, I tried to use a UV cut filter. These lenses are very sensitive to UV light and it could be that this is the cause (though unlikely as this is a narrowband image, i.e. UV should already be filtered out).
Next, I thought that this might be stray light and put a lens cap on - same result.
I tried different f-settings - no improvement.
Finally, I took my setup to Richards ranch and we worked on it. He also couldn't find anything wrong. But he checked the setup of the lens. And in a diagram, he noticed that the last element of the lens is almost extruding from the lens and is curved. He suspected that what I see are reflections from the lens.
FIRST LIGHT 55MM LENS
To check this, I purchased a PENTAX 67 SMC P 55mm F4 F/4 lens. I'm glad that I didn't go the route of using a Robofocus focuser, otherwise, I'd have to fuzz around with the setup as the focuser wheel of this lens is in a different position as the 45mm lens ...
Looking good! Zooming in:
Yei! No halos. Some fuzziness around the bright stars left - which is probably a result of the UV-sensitivity (I should get a UV cut filter for this lens).
I want to use this rig with my MyT mount, so that I can build a pointing model and use that to track without a guidescope.
Sounds easy, but plate solving at this scale is a challenge!
Using the entire image usually doesn't work as there are WAY too many triangulations (it's kind of fun to watch this in TheSkyX which shows which frames are considered - it's jumping around like crazy!!!)
Pinpoint doesn't work at this scale at all to begin with.
Through a lot of trial and error, I determined that these are the best settings:
Exposure: 8 sec
Image Crop: 50%
One challenge remains if I image from out backyard: light pollution! Because of the relative small aperture, the stars don't stand out from the light-polluted background and are often missed. Nothing I can do about that .
Another challenge is that often earthly objects (trees, our house...) cover some of the image and create "artificial stars". Though the resulting images are sometimes really cool:
This was (and still is) really tricky.
Because of the short focal length, the critical focus zone is 78 microns(!!!)
This means three challenges:
Focusing has to be very exact
The focuser can only be a little bit away from the focal point - otherwise the stars are so out of focus that they aren't recognized anymore.
I have to refocus a lot during the night.
And of course, focusing routines also suffer from the same problem as plate solving: our light polluted backyard - which makes stars often not stand out enough ...
I am using the following settings (in SGPro):
Number of data points: 9
Step size: 70 (these two settings are the most sensitive ones: too small and I have to move the focuser too close to the focus point to start with plus it doesn't move out far enough to create a good v-curve, too large and the focuser moves too much out and SGPro doesn't recognize any stars)
Minimum Star Diameter: 6
Exposure time: 20 sec for LRGB, 45 sec for narrowband filters
Taking flats is something I have not figured out at all:
Sky flats always have gradients (because of the large image circle). Even if I put a white t-shirt on it to diffuse the light - there are still gradients.
Flats with a flat panel create weird patterns (probably because of the proximity of the panel to the lens).
With all this figured out just in time for OSP, I took my first two images: the North America Nebula and the Double Cluster/Heart Nebula/Soul Nebula together. I took the LRGB data for both images at OSP and augmented it with narrowband data taken from our backyard.
I have a lightweight, wide-field imaging rig!!!
But because of all the remaining instabilities in focusing, plate-solving, flats... it's pretty far from the point-and-shoot vision that I had :-( I'll keep trying, but maybe instead I should reconsider getting that awesome RH 200 Veloce from Officina Stellare