Surplus Shed Special on 6.1" Triplet F/1.25
Posted 05 June 2013 - 01:04 PM
There was a lengthy thread on this, but such lengthy threads get tedious to peruse to dig out and piece together the conclusions.
It might be helpful as a buyers guide considering taking Surplus Shed up on this offer for someone to summarize their experience with these.
IIRC, the upshot was that these are fast all right, a bit too fast for visual work - the very steep light cone makes constructing a visual system that exploits it apparent potential impractical.
If anyone with actual experience could weigh in here that would be good. Especially anyone who has found an effective application for them.
Could a secondary lens be used to lengthen the effective FL for easier use?
Posted 05 June 2013 - 01:19 PM
Especially anyone who has found an effective application for them
Burning ants on the sidewalk?
Posted 05 June 2013 - 04:17 PM
Posted 06 June 2013 - 06:33 AM
Posted 06 June 2013 - 06:47 AM
Posted 06 June 2013 - 08:34 AM
The oft-mentioned negative element is sub-diameter, but not required for imaging. I took some terrestrial test images with the triplet alone and it showed promise. It might actually have use for wide-field "patrol" imaging. But it's such a weird lens that such a project is way down on my list of things to investigate and build.
Get yourself a 6" PCX lens and make the world's largest ortho eyepiece.
Posted 26 June 2013 - 11:06 AM
will wait for good weather for deep sky shot
Posted 26 June 2013 - 05:19 PM
Posted 26 June 2013 - 06:23 PM
Posted 26 June 2013 - 06:38 PM
I don't know (there is other lens(s) in train -yet once I get deep sky shot and timing
ill ask for your help or if fails I will go away on this
I had many dead ends before
but so far this is the best
I got a lens train on back based on what was said on this thread plus talking to Fred at SS
plus some of my cobbling of stuff
compared to other shots that were pure mush
this is the best yet
Posted 26 June 2013 - 07:47 PM
The addition of an additional lens (or lenses) must certainly alter the focal length/ratio. Given the depth of field seen here, it must certainly be rather slower than f/1.25.
If you know the sensor size, you can image a yard stick placed at a measured distance from the camera and thereby directly determine the focal length.
Posted 27 June 2013 - 04:40 AM
look up lodestar by starlight express color camera
Posted 27 June 2013 - 09:38 AM
CCD type: ICX429AK Sony Exview interline CCD with low dark current and vertical anti-blooming.
■CCD Full resolution Pixel data: Pixel size: 8.6uM x 8.3uM, Image format: 752 x 580 pixels
■CCD Image area: 6.4mm (Horizontal) x 4.75mm (Vertical).
Posted 27 June 2013 - 03:26 PM
The 30 arcminute Moon occupies 1/20 the field width, which tells us the FOV is ~10 degrees.
The CCD width is 6.4mm, and so
FL = CCD / TAN(FOV)
= 6.4 / TAN(10)
= 6.4 / 0.1763
(Technically, we should first calculate the half angle, the double it, but for a 10 degree field angle the error will be smaller than the measurement error of the target dimension.)
If this is indeed that 6.1" lens at work here, a 36mm f.l. is remarkably small! Is there a positive lens/group behind it, working like a focal reducer? Did you supply the correct chip dimensions?
I've been assuming a 6.1" aperture, but now wonder if that figure referrs to the focal length, either as-is, or nominal for the manufactured (and complete) unit. What is the clear aperture?
Posted 27 June 2013 - 05:14 PM
there is pair of lens(s) that shrinks image down like condenser
like focal reducer
you think it would make things worse right - like when I put a strong reducer in my dob you get more coma
but with this lens it acts like a corrector
those ccd numbers are from spec
Posted 27 June 2013 - 07:00 PM
Camera is a QSI520: 1600x1200 pixels at 7.4um/pixel. Not exactly a rules, but the image is a vent grate. The full width of the grate is just a hair larger than the field of view. The grate is 7 1/4" with so Use 7" for easier math The grate is almost exactly 9' from the front of the lens. My math computed that this is indeed at f/1.25 but somebody please double check it
The biggest problem I'm having with further developing this thing is the images are just too darn bright! This image was taken in a darkened room and is still only a 0.15s exposure. I've got it pointed out my kitchen window at some houses up the hill...it's cloudy out and I still can't expose short enough to not saturate! The other night I had to wait until about 30 minutes after sunset to finally get some distance focus images. I was even able to focus at infinity and get some stars, but transparency was really poor so other than feeling pretty comfortable I was in the range of infinity focus, I didn't get much else.
The images I got of the houses that are about 1km away did show some promise though! Focused well enough to count shingles on the roofs anyway...
Posted 27 June 2013 - 07:09 PM
Again, no baffeling yet...there's some black construction paper wrapping the inside of the tube, but that's it. Also, the camera is just pressed flush against the sawed off hole left where the nut on the blank was removed. I'm sure it leaks light like a sieve!
I'm hoping to clean up the camera mount and secure the lens and everything well enough to start thinking about putting it on my CGEM so I can try and take some real sky shots! I'm hopeful it will reach a tight focus, but my plans are more to try some photometry with it so I can live with a soft focus--will probably even want one on purpose!
Posted 27 June 2013 - 07:12 PM
Posted 27 June 2013 - 08:38 PM
I just did some more checking not all good news either - Glen
the lens(s) im using are trimming cone about 1/2" round edge
so its a 5.1 inch -- need some negative to stretch
that may explain improvement in image since the edge is the worst - this lens is daunting 2 steps forward 1 step back
Posted 27 June 2013 - 09:41 PM
Do I take your CCD's pixel dimensions as as they are for calculating the chip's imaging area, or is there some separation between pixels to be added?
If I simply take 7.4 microns/pixel times 1,600 pixels, your sensor's active width is 11.84mm.
A 7.25" target at 9 feet is an angle of 3.84 degrees. 3.84 degrees across an 11.84mm chip implies a focal length of 176mm.
If the clear aperture is 6.1" (155mm), the f/ratio is f/1.135. This result is affected by the nearness of the target, as well as the unknown distance from the target to the lens's nodal point. Better to use a distant target for a more accurate measurement; beyond about 50 times the focal length, it can be considered as practically at infinity for this purpose.
I note a softness in your photo of the grate which *suggests* significant spherical aberration. A target containing finer details (of high contrast) would allow better assessment of this.
Posted 27 June 2013 - 10:08 PM
By ruler, the clear aperture is 155mm and the chip is placed at very near 195mm from the back edge of the glass when near focus.
I wouldn't read too much into the softness from that image. I'm sure I was leaking light around where the camera meets the PVC since I did nothing but set the PVC on the CCD housing. No internal baffling of any kind to stop leaked photons from bouncing around. Also, with the camera sitting flat on its back it wasn't getting enough air flow to cool effectively so I didn't make much of an attempt at getting a perfect focus. And all that while trying to image white on white!
Now that I've gotten the basics worked out, I need to put a little more time into mounting things properly and baffling at least enough to keep the light from reflecting off the housing right near the sensor. I didn't save any, but the few images I did get of the houses up the hill were pretty good with no obvious aberrations, but again, the biggest issue working against me right now is too darn much light!!
So far I've dropped less than $100 in the entire "scope" and it's showing great promise for my intended use. My last big concern is that light pollution in my current location will prove to be too much of a limiting factor to make use of it
I'll post back when I get past the playing around stage and can manage to take more rigorous images...
Posted 28 June 2013 - 06:57 AM
I did this awhile. Back lots of sa