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Equipment Discussions >> ATM, Optics and DIY Forum

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btieman
scholastic sledgehammer


Reged: 07/24/08

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
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
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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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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
scholastic sledgehammer


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
Carpal Tunnel
*****

Reged: 09/06/06

Loc: NE USA
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
scholastic sledgehammer


Reged: 07/24/08

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
Post Laureate
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Reged: 05/03/10

Loc: The Great Basin
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
Carpal Tunnel
*****

Reged: 09/06/06

Loc: NE USA
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
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
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
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
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
Postmaster
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Reged: 01/18/07

Loc: bellevue ne.
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
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
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
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
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
Post Laureate
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Reged: 05/03/10

Loc: The Great Basin
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
scholastic sledgehammer


Reged: 07/24/08

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
Authoritative Ignoramus
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Reged: 06/25/08

Loc: MA
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
scholastic sledgehammer


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
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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
scholastic sledgehammer


Reged: 07/24/08

Re: Surplus Shed Special on 6.1" Triplet F/1.25 [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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