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careysub
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Surplus Shed Special on 6.1" Triplet F/1.25
      #5904026 - 06/05/13 02:04 PM

Surplus Shed has a special on those 6.1" F/1.25 wide field camera objectives.

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?


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rdandrea
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: careysub]
      #5904047 - 06/05/13 02:19 PM

Quote:

Especially anyone who has found an effective application for them




Burning ants on the sidewalk?


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: rdandrea]
      #5904375 - 06/05/13 05:17 PM

I may be mistaken, but I thought this item is merely the front part of a lens system. If so, it can't be expected to form a good image by itself...

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KevinS
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5905349 - 06/06/13 07:33 AM

Their price just dropped to $75 on this lens. I assume their customer base is not too interested in such a short FL. Looks like a very nice lens but I can't think of an easy way to make it useful for observing without stopping it way down.

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bremms
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5905375 - 06/06/13 07:47 AM

Yes it is part of a system. I believe there is a negative element behind the objective. I have a 5" cemented exotic glass doublet that needs a rear element as well. A $45 ant burner.

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Chuck Hards
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: bremms]
      #5905558 - 06/06/13 09:34 AM

I think Fred's remaining stock on these are "seconds", i.e., light scratches or staining present.

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.


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: Chuck Hards]
      #5941339 - 06/26/13 12:06 PM Attachment (64 downloads)

I finally made 'some' progress with this lens after almost two years - took this test shot
will wait for good weather for deep sky shot


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5941870 - 06/26/13 06:19 PM

Clouds are hardly a good test target. Why didn't you just aim the thing lower in order to capture some landscape details, which would certainly have provided a subject having vastly better definition and contrast.

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5941997 - 06/26/13 07:14 PM Attachment (64 downloads)

fair enough
test 1a


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5942006 - 06/26/13 07:23 PM

Is that at full aperture (f/1.25)? There seems to be more depth of focus than expected? And why the circular frame?

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5942016 - 06/26/13 07:31 PM Attachment (51 downloads)

one more with lodestar above was webcam

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5942027 - 06/26/13 07:38 PM

Is that at full aperture (f/1.25)?
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


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5942103 - 06/26/13 08:47 PM

Indeed, there would seem to be promise in this optic, based on these shots.

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.


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5942482 - 06/27/13 03:15 AM

Cool ill try that

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5942542 - 06/27/13 05:40 AM Attachment (45 downloads)

took this before work
look up lodestar by starlight express color camera


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5942865 - 06/27/13 10:38 AM

spec on camera
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).


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5943383 - 06/27/13 04:26 PM

Not the best image to work from; a tiny, somewhat de-focused target in a big field. But here goes, with an uncertainty of about 5%.

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
= 36.3mm

(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?


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5943562 - 06/27/13 06:14 PM

Is there a positive lens/group behind it, working like a focal reducer? - wow you are smart!

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


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5943723 - 06/27/13 08:00 PM Attachment (46 downloads)

I bought one of these from the recent sale. Sample image is below. This is from the full unstopped lens. There is no baffling at all in the system and no additional optics.

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...


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5943737 - 06/27/13 08:09 PM Attachment (49 downloads)

And here's the entire system. The focuser is basically a 6" helical focuser It feels plenty sensitive enough...but then again, experience is limited at this point.

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!


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5943743 - 06/27/13 08:12 PM

I should note that the exit pupil for this thing is significantly bigger than either my QSI520 or QSI583! Most people I've seen try and build around this thing used a barlow or some other negative lens to increase the focal length to GAIN photons! Try and pull in those unused guys hitting the camera housing so they hit the useful sensor instead

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5943851 - 06/27/13 09:38 PM

negative lens to increase the focal length to GAIN photons!

yes

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


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5943941 - 06/27/13 10:41 PM

btieman,
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.


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btieman
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Reged: 07/24/08

Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5943978 - 06/27/13 11:08 PM

Glen, no space between pixels. 11.84mm width is what QSI lists as well. The imaged width is not quite 7 1/4"--probably closer to 7" but I'm not going to quibble over 1.135 vs 1.25 given how casual the data was acquired.

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...


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5944341 - 06/28/13 07:57 AM

Btieman are you any other lens or just prime
Focus. ?

I did this awhile. Back lots of sa


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5944357 - 06/28/13 08:07 AM

Quote:

Btieman are you any other lens or just prime
Focus. ?

I did this awhile. Back lots of sa




Prime focus.

What was the nature of your sa? I'm only using about 1/3 of the exit pupil so the effects may be minimal...


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Chuck Hards
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5944544 - 06/28/13 10:30 AM

Your results are similar to mine. I posted a terrestrial shot through the lens last year and thought it showed promise (can't seem to find the thread now). It was also hard for me to achieve a precise focus due to the "rigged" nature of the test setup.

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: Chuck Hards]
      #5945997 - 06/29/13 05:23 AM Attachment (43 downloads)

OK no more pictures of cars, trees, vents , air conditioners
here is a star shot - with 72mm corrective lens set
its my best so far - 15 sec - there still is a fair amount of coma - but star test is the best


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5945999 - 06/29/13 05:26 AM Attachment (41 downloads)

just to show what a pain in the buttskie it been with this lens here is shot with 90mm corrective lens set

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5946001 - 06/29/13 05:29 AM

above shot was 25 sec - you can see north American nebula
but it looks like Scottie put us in warp drive!


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mikey cee
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5946278 - 06/29/13 10:24 AM

Wow! Looks like it's back to ant burner city!!

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Mike I. Jones
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: mikey cee]
      #5946724 - 06/29/13 02:57 PM

Actually, it could be a low-f/# long-range objective lens for a night vision system. Any military designations or codes on it? If so, its color correction is likely optimized for Class B filtering, from 0.64-0.9 microns. It might just make a dandy H-alpha nebula lens, if you can get a 3-5nm passband H-alpha filter in the path ahead of the camera.

On the other hand, night vision systems didn't have to be super-corrected. Resolution of 20 lp/mm would be a superbly sharp lens for a fiber faceplate photomultiplier objective. But, them stars do look pretty sharp!

Mike


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: Mike I. Jones]
      #5946874 - 06/29/13 04:43 PM

that was my idea use it for h-alpha only
the first star shot ain't perfect bad not that either


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Chuck Hards
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5946915 - 06/29/13 05:12 PM

I wonder if it was originally used with a curved film-holder, somewhat like a Schmidt camera?

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btieman
scholastic sledgehammer


Reged: 07/24/08

Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: Chuck Hards]
      #5947004 - 06/29/13 06:22 PM

highfnum (which is a kinda humorous moniker playing with this lens!) what is the chip size in your star image with the high coma? And what is the focal length of the 90mm secondary? Is it the 90mm that's supposed to go with this primary? Something else? If it has a positive focal length, it's probably making coma worse?? If it's a negative lens, it's not looking good

Still, my hope is photometry and not pretty pictures. Coma is more difficult to deal with, but not impossible and I've written my own photometry software so adding a way to deal with different apertures for different stars is doable.


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: Chuck Hards]
      #5947009 - 06/29/13 06:23 PM Attachment (22 downloads)

Quote:

I wonder if it was originally used with a curved film-holder, somewhat like a Schmidt camera?




I can't find it again now, but there was another thread here on CN from several years ago where someone had the entire lens set. As I recall, it was the 155mm by 195mm focal length primary, a 90mm by -78mm secondary, then two 30mm lenses for transfer optics--I think 75mm and -75mm focal lengths or thereabouts. Between the transfer optics was a beamsplitter to a secondary port...wish I could find the thread again!

Mine's now buttoned up a bit better and mounted on my son's CGEM. Looks a bit ridiculous up there Not looking good to get it under stars tonight, looking more and more like rain. But hopefully soon!


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kw6562
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5947320 - 06/29/13 11:21 PM

This is the "big" thread on that lens - 6" triplet --Keith

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btieman
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Reged: 07/24/08

Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947476 - 06/30/13 02:51 AM Attachment (30 downloads)

Quote:

OK no more pictures of cars, trees, vents , air conditioners




Agreed! I outlasted the clouds and got the setup pictures above out under the stars for a few hours...

Below is my rendition of the North American Nebula using this lens.

This is full aperture, no-additional optics. Using The Sky, this calculates to something like 3.46 degrees across the width of the image. Certainly some coma, but does it seem as bad as highfnum got in his shot?? Maybe...

There are still other optical issues to sort out though. The rectangular blob with halo in the left is puzzling me. It's not astigmatism as it changes orientation with the camera and thus seems independent from the lens. I thought for a bit that it was the long adapter plate that sticks out in front of the lens in the picture of it mounted on the scope above. Problem is it moves with the camera and its bi-symmetric whereas the plate should only be on one side. The tube is flocked with felt paper, and I think the camera is mounted pretty tightly to the housing but the PVC blank the camera is mounted to is not flocked and there are some metal bits holding the camera in place so maybe it's reflections off that off the primary again? Hard to say, but baffling may be next on my todo list.

Image is a single 30s exposure on the North American Nebula.


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5947486 - 06/30/13 03:07 AM

Btieman,
All stars above a certain brightness have that vertically oriented 'bow tie' flaring. I wonder if it has anything to do with the very fast light cone's interaction with the pixels???


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5947504 - 06/30/13 03:37 AM

Quote:

Btieman,
All stars above a certain brightness have that vertically oriented 'bow tie' flaring. I wonder if it has anything to do with the very fast light cone's interaction with the pixels???




It could be I guess? I don't see anything similar when I saturate bright stars at f/7 with my 11" on the same camera. I thought the butterfly effect and square stars were due to microlenses. No microlenses on this camera and front side illuminated...should be no surface structure on this CCD to delineate pixels...just a hunk of Si.

I find it suspicious that the effect has a clear circular boundary which you can see around the brightest star. And the star is off center in the circle. I know the cutout the camera mounts to is canted (cut with a hacksaw by hand) so the chip isn't square to the lens so a reflection off the lens back surface makes some sense...except doesn't explain the wings. There is some metal holding the CCD in place, but it's got four fold symmetry--not two. The metal mounting is 45 degrees to the chip but I can't see where the light/dark pattern would arise.

Ack...getting too tired! I think I'll flock the back and think about some baffling then give it another go some other night!


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5947506 - 06/30/13 03:39 AM

Kieth, thanks for linking the "big" thread! That is the one I was thinking about that discusses the full lens system this primary came from.

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5947710 - 06/30/13 09:09 AM

btieman - that (your star shot) also show halo from not all color in focus
I had a similar results - GlenDrew expressed valid critic of a similar going raw shot.


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5947719 - 06/30/13 09:13 AM Attachment (49 downloads)

well I tried again
1) I added red filter to control color
2) went black and white
3) 2- 35 second shots
4) full and cropped
5) its best so far - but lots of coma
6) btieman yes im using positive lens - I ordered a negative to see if I can correct


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947721 - 06/30/13 09:14 AM Attachment (36 downloads)

sweet section

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947724 - 06/30/13 09:15 AM Attachment (33 downloads)

just one more set for the hell of it

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947726 - 06/30/13 09:17 AM Attachment (27 downloads)

cropped - note - I stretched first crop shot

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947733 - 06/30/13 09:23 AM Attachment (36 downloads)

as a reference I too this shot with computer lens and same camera - 200 sec so now you know area

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947844 - 06/30/13 10:49 AM

by the way makes a great low power monocular
human eye adapts to curve
saw nB visually


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Chuck Hards
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947861 - 06/30/13 11:08 AM

At what size exit pupil?

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btieman
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Reged: 07/24/08

Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947868 - 06/30/13 11:13 AM

Quote:

btieman - that (your star shot) also show halo from not all color in focus
I had a similar results - GlenDrew expressed valid critic of a similar going raw shot.




That may be. There's clearly something making the stars rectangular and adding the wings. I doubt that's color and until it's figured out, I'll withhold judgement on how color corrected these lenses are.

In the end, I don't much care about color as I intend to always use it filtered. Depending on how the final detector mounting goes, I can get a filter up right close to the CCD where it won't vignette.


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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5947933 - 06/30/13 12:12 PM

Quote:

6) btieman yes im using positive lens - I ordered a negative to see if I can correct




Yikes! If I'm looking at things right, that's Debeb in the lower right? If so, that's an 8+ degree FOV!! If you're cramming all that onto the lodestar which is an ~5mm chip, you're much lower than f/1.25!

By comparison, my image is on a chip that's ~12mm wide and is only a 3.5 degree FOV...

I don't think a negative lens will "correct" the image in any significant way, but it might focus in on the central region more where coma is less and essential hide the problem...at the expense of FOV.

Here's a link with an article where someone made a scope out of this lens:

http://www.atmob.org/newsletters/201302.pdf

The article is near the bottom and the gentleman explains where he used a 2x barlow to make the lens effectively f/2.5. No mention of coma, however...and he was using it for visual use!


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5948729 - 06/30/13 09:30 PM

Note that narrow-band dielectric filters are limited to how fast a light cone they can handle. For example, a 7nm filter should not be placed in a focused beam faster than f/2.8, or f/2.5 perhaps.

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btieman
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5948815 - 06/30/13 10:30 PM

Quote:

Note that narrow-band dielectric filters are limited to how fast a light cone they can handle. For example, a 7nm filter should not be placed in a focused beam faster than f/2.8, or f/2.5 perhaps.




Glass photometry filters...no worries . Not interested in pretty pictures so don't have much use for anything near 7nm band pass. The dust moats might be too tight that close to the sensor to deal with, though...

I think the only risk to narrow band pass interference filters in a steep light cone is the band pass is no longer guaranteed since the steep angle pushes the light path to be too long for the destructive interference to be effective, right? Please educate me if this is wrong


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5948992 - 07/01/13 01:52 AM

Correct; the too-long light path through the dielectric layers alters the bandpass. From the central parts of a fast lens, the light transits the filter sufficiently near perpendicular. But ever closer to the edge of the entrance pupil, those rays become progressively too shallow at the entrance angle.

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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5955456 - 07/05/13 12:39 AM Attachment (35 downloads)

took some advice from others a little more improvements

a) added an astrotech coma corrector
b) went Ha 7nm - that help a lot as someone said
c) found out I have a small tilt which hurts in a fast system.

so here is latest - cropped a little lodestar 60 seconds no dark frame no stack


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highfnum
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5955471 - 07/05/13 01:01 AM Attachment (29 downloads)

80 second with dark frame and sw curve
well this is it - I need to do some physical changes to scope


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DonnyM
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: highfnum]
      #5975093 - 07/16/13 09:57 PM Attachment (27 downloads)

I have the full version of this lens. I bought it from Surplus shed back when everyone was talking about them. Price was $550 I think. Not 100% sure. I made a mount for it and took a few pictures with it. I let some of the bashing get the best of me so I quit playing with it after I sold my mount. It is a nice short Refractor as far as I am concerned. I have it up for sale but If it don't sell and I get a mount I will be back to wide field shots with it.
http://www.flickr.com/photos/66682720@N05/sets/72157631713363490/

Above is a link to some of the images I shot with it and the astrometry to go with it so you can see field of view. Some were done with a Sony A-300 10mp and a couple with a Canon t4i 18mp.
The lens can zoom in and out quite a bit. The amount of light coming out the back end is huge. More than enough to fill a large format film camera. Which is what was mounted to it. Mine came with a electronic shutter. The Iris opening is just over 1.5".
The attached image has reduced quality to fit in the forum rules. It was taken with my Sony A300 at ISO 800. 2min. The image is not cropped.

Donny


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Chuck Hards
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: DonnyM]
      #5975544 - 07/17/13 08:11 AM

I sold mine a couple of weeks ago to a local who just HAD to have one. Got about what I paid for it, less shipping. Maybe he'll take it further than I did; I included the housing I made for test shots. Just too weird and heavy a lens for what you get out of it. Good luck to those who soldier-on.

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btieman
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Reged: 07/24/08

Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: DonnyM]
      #5981416 - 07/20/13 01:03 PM

Quote:

I let some of the bashing get the best of me so I quit playing with it after I sold my mount. It is a nice short Refractor as far as I am concerned. I have it up for sale but If it don't sell and I get a mount I will be back to wide field shots with it.




Donny,

I never let bashing from forums such as this get me down. My experience is that rarely are the bashers as knowledgeable as they think they are and information transfer through forums is such that even when they are, there is often much confusion over what you're specifically trying to accomplish. Plus, it is my experience that truly knowledgable people do not bash. If they believe you are wrong, they will say so, but they will back it up with reasons and explanations in a constructive manner. And they're generally open to the possibility that they do not fully understand the situation.

You're frame looks very good for my potential use case Of course, I only have the primary. The secondary would have been really nice to get ahold of My sensors are a bit smaller than the a300 though so maybe it's not so bad. Either way, I don't care much about some coma. As long as the individual stars are still resolvable, I don't much care their shape.

Is there a posting for the lens you have up for sale somewhere? I'm curious to see what it looks like.


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DonnyM
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: btieman]
      #5982140 - 07/20/13 10:33 PM Attachment (17 downloads)

http://www.cloudynights.com/classifieds/showproduct.php?product=83262&sor...
There is a little coma but that could be cropped out or maybe a field flatener.
Some where saying its darker than say a 6" refractor. That I can't say. I never got a chance to do much with it. If I had a mount that would hold it and track I would be out all summer with it.

Donny


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mikey cee
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: DonnyM]
      #5982316 - 07/21/13 01:28 AM

Like they say "if you like to fiddle, tinker or dick around with optics that's fine and dandy"....nothing wrong with that. But in the end there is no "free lunch". Mike

Edited by mikey cee (07/22/13 07:16 PM)


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jshinal
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: DonnyM]
      #5988098 - 07/24/13 04:42 PM

Can this lens possibly be used with a big ED barlow for purely visual use ? I'm fairly shameless; if it means turning it into an f/7 system to get rid of the coma, I'm OK with that.

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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: jshinal]
      #5988107 - 07/24/13 04:48 PM

Highly doubtful! The light cone is very much steeper than any Barlow is designed to handle. It would be a mess, unless perhaps only the central ~1/4 of the lens aperture is used; but then that's awfully inefficient. A cheap 70mm-100mm f/5-7 achromat will blow this thing away.

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jshinal
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: GlennLeDrew]
      #5988141 - 07/24/13 05:07 PM

Thanks, Glenn. If I am visualizing this properly, the difficulty is both the quite wide aperture needed for a negative lens, PLUS the typical thickness of a cemented Barlow element means the different red/blue rays cross the negative element's boundaries at very different points.

A suitable negative lens for this would need to be thin, is that correct ? Has anyone identified something suitable yet ? I've read the two threads, but didn't notice any recommendations.


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GlennLeDrew
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Re: Surplus Shed Special on 6.1" Triplet F/1.25 new [Re: jshinal]
      #5988365 - 07/24/13 07:46 PM

It's not that a Barlow would need a wide aperture, or need to be thin. It would require steep curves, and more likely additional elements so as to provide the required degree of correction. Reshaping the wavefront on such a steeply converging light cone requires significant optical work.

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