32 replies to this topic

[photo444]

There have been searches for cheaper yet performing FR for EAA purposes.  I have been trying various combinations of COTS lenses from vendors like SurplusShed and posted here at CN. (http://www.cloudynig...aa#entry6521341 and http://www.cloudynig...table reducer)  

 

So far the result have not been successful until I tried a special lens designed for microscope industry.  This lens is an achromatic doublet positive meniscus lens which seems perfect for this purpose. I found one such lens made by Baush and Lomb for their stresoscope to increase magnification x2.  I quickly made an PVC assembly to put in a 2" draw tube just like a 2" eyepiece.  Its focal length is 4" so the distance between the center of the lens and a CCD plane becomes 2" for reduction of x0.5 which is conveniently manageable.  

 

Below is pictures taken with this reducer showing negligible coma and NO VIGNETTING!  In order to minimize star elongation, the scope was guided (was not perfect due to short focal length of the guide scope, 75mm.)  A CLS filter and LN300 PAL camera was used. Both were taken at 2.5 sec exposure and stacked 128 frames.  The scope was 6"F5 Newtonian and the reduction was 0.45.

 

The market price of the x2 Baush and Lomb lens is more than $120 and it is scarce in the market.  A cheap and perfect replacement for this, as we all are looking for, was found on Ebay which allow even easier and far cheaper DIY project than the BnL lens.  All you need is a lens

 

http://www.ebay.com/...=item27fa150e06

 

and an adapter 2" to 1.25" like 

 

http://www.ebay.com/...=STRK:MEBIDX:IT.

 

With a slight modification you can have an excellent reducer!  I will cover this part in the next post.

 

Clear skies!

 

Paul

 

 

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[photo444]

For your reference, the Bausch and Lomb 2x lens attachment (31-26-19) looks like this.

 

http://www.science-i...Microscopes.pdf

 

You can compare other commercial higher priced reducers as shown in 

 

http://www.cloudynig...rs#entry6554989

 

Paul

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[Dwight J]

Very good find there Paul.  

[photo444]

Thank you Dwight.  I will test tonight and see how the cheap Chinese lens behave.

 

Paul

[photo444]

I have to show the result of the 2x lens from the NewOptical Factory in China.  It was disappointing comapare to the results I obtained with the Bausch & Lomb 2x lens.  As you see below it shows a strong coma with no vignetting.  Reduction was x0.4 rather strong.  I will try at x0.5 or higher to see how coma behaves.

 

I may stick with B&L for now.  I will show DIY aspect of the reducer.

 

This lens comes and goes on Ebay since it is an old stock.

 

Paul

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Edited by photo444, 08 August 2015 - 07:39 PM.

[Dwight J]

Quite a difference in terms of coma, wow!   The extra cost for the B&L seems to be worth it.   I had a Meade Chinese F3.3 FR and it had the same amount of coma - too much and playing with spacing didn't help.  The Japanese version has excellent lenses and no coma visible in the field of my Mallincam Extreme (1/2" chip) even using extension to reach F 2.5 on my C11.  The problem is that neither of these are available unless you get lucky on the used market as I did.  The quality ones are very expensive and I think you found a well priced alternative Paul.  I haven't seen much of the AVS reducer yet but what I did see looked good although I believe it is designed for the 1/3" chip so a larger chipped camera may show some coma and vignetting but this is only a guess.  

     I recall reading in M. Covington's astrophotography book years ago that FR's perform better when placed well up into the baffle tube on SCT's but all designs now are not like that.  I wonder....

[Howie1]

Or there's this way to make a diy FR ... https://www.youtube....h?v=9l66Sch9PWY

[photo444]

I just saw a listing of the lens on Ebay at $30 which is darn cheap.  Anybody who are interested may check it out.  I will pass since I have two.

 

http://www.ebay.com/...=item463f461b52

 

Paul

[photo444]

Howie1,

 

Thank you for bringing that tutorial. That is excellent DIY project. It even works with 1/2" ccd chip!

 

Paul

Edited by photo444, 10 August 2015 - 08:51 PM.

[mclewis1]

I just saw a listing of the lens on Ebay at $30 which is darn cheap.  Anybody who are interested may check it out.  I will pass since I have two.

http://www.ebay.com/...=item463f461b52  Paul

It's a nice idea but you're not going to save much money. 

 

$30 + shipping + the housing + your time 

 

vs.

 

$45 for the excellent Antares 2" .5x reducer (free shipping in the US from AgenaAstro)

[photo444]

Mark,

 

Yes, Antares 2" .5x reducer is cheaper and quicker.  Do you have any example of pictures taken with the reducer?  Some have reported coma and bent field. I wonder how this reducer perform for EAA with 1/3" CCD.

 

Still fun of DIYing can't be thrown away for some people.

 

Paul

[mclewis1]

Paul,

 

Don't get me wrong I think DIY projects are valuable, more than just the dollar value of the parts.

 

I have also heard a few negative comments about the Antares recently ... but I don't remember if there was also complete certainty in the correct setup (spacing) of the reducers in question. 

 

Here's an ugly quick image taken last year with my TMB refractor and an Antares 2" .5x reducer (older model, the one without the filter threads). It's NGC6992 (part of the veil), 30s with a Lumicon Deep Sky filter and an Xtreme video camera. I was playing with the APC settings to bring out the nebula so the stars really suffered. My mount was also having tracking issues (it was very imbalanced) so if you look closely the fainter stars are elongated in the vertical direction.

 

The point of the image despite it's many many faults is that the reducer hasn't introduced any appreciable coma or other optical aberrations on the 1/2" sensor. I believe (from memory) that the spacing with the reducer is just under 70mm.

 

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[photo444]

Mark,

 

Thank you for taking the trouble looking for the picture of the case.  There is no doubt that this Antares 2" 0.5x reducer is performing well even to 1/2".  I may get one to try out, too.

 

Paul

[Relativist]

Coma, isn't that a feature where outlying stars point to the object at the center so we know where to look?

Very nice DIY project. I'm still hoping for a less expensive coma correcting FR. I have yet to find a reducer that is a good fit for a fast newt. The DIY reducer you've constructed might just work, even though I'm at f/4.

[photo444]

I had PVC work done and the sky last night was clear for a short time so I was able to take a picture below in between passing clouds.  The distance between the lens and the camera was reduced to 25 mm and the reduction was 0.57x. Coma was greatly reduced as seen in the picture, though there are some hints of coma at the 4 corners.

 

It was with my 6"F5 Newtonian yielding F2.85.  With F4 optics (Feff=2.28) as the case of Relativist I would expect the coma will be a little bit more.

 

The camera was LN300PLA, exposure was 2.5 sec, and 16 frames were used for stacking.  

 

Paul 

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[photo444]

I used two PVC parts from Lowes at a total cost of $2.50.  After joining together with dabs of superglue it was machined on a lathe: 1) outside diameter to 2.00" to fit in a 2" draw tube, 2) inside was bored to 1.25" to accept 1.25" parts, and 3) cut to a short length, as seen in the pictures. The face of 2" side was bored to 47 mm diameter and 4 mm deep, and this bore will mate with the machined end of the lens housing which was turned to 47 mm as well to have a tight fit,

 

Since the reducer eats up the back focal length, the assembly may be inside of a 2" draw tube.  In this case the 8-32 hold screw can be replaced with a set screw.

 

For those who prefer Aluminum rather than PVC, a 2"-1.25" adapter can easily machined in similar manner.

 

As you see in this thread B&L 2x attachment lens (31-26-19) works better.

 

I have another brand (Nikon) which I will try later see how it works.  It has a larger aperture than above mentioned lenses but can be turned to 2" housing.

 

Paul

 

 

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[GlennLeDrew]

A meniscus form on the reducer would seem to behave like a field flattener. With the concave surface facing the sensor, this 'bends' the field so as to 'push' the otherwise up-turned outer field down nearer to flatness. A common achromatic doublet designed for an infinite conjugate necessarily induces positive field curvature whose focal surface is concave toward the objective. A meniscus reducer such as this would appear to be inducing a more or less compensating negative field curvature.

 

The actual degree of resulting field flatness will depend to some extent on the telescope's own field curvauture, which in turn depends on its focal length. For given objective type, field curvature scales as the focal length; a shorter f.l. exhibits stronger field curvature due to the shorter radius of curvature. And the ratio of reduction performed by the reducer (which depends on reducer-to-focus distance) will likely have an impact as well. And so the degree of good focus across the field will likely differ at least somewhat from system to system...

[photo444]

Glenn,

 

That is why I was looking for positive meniscus lens for the purpose. Unless it is designed for the purpose it is not possible to correct 100%. Here the goal is a little more than first order remedy.  

 

My case and Curtis' are similar because both are simple fast parabolic mirrors, one is F5 and the other F4.  It may also be true that 12"F4 is easier to correct 6"F4 because of inherent curvature of the focal plane, as you have elaborated. 

 

Paul

[photo444]

I now added one more lens to the fleet, it is similar 2x microscope attachment lens made by Nikon for their stereoscope.

 

I also have bought a nice item for DIY, 2" to C-mount adapter from ScopeStuff (http://www.scopestuff.com/ss_c2bf.htm) (#C2BF - C-Mount to 2" Eyepiece Barrel Adapter with Filter Threads).  It provides rather short tube length than other similar adapters and has 1.25" filter thread inside of the adapter.

 

All of my reducer lenses are machined to be fitted into the 2" filter thread, so I can easily interchange. Total tube length can now be adjustable by adding or subtracting number of 5 mm extensions tube similar to this (http://www.ebay.com/...=item51a0b37397).

 

Below table is a result of two lenses NIKON and NOF (NewOptical Factory in China) in effort to find real focal length of the lenses and the effective tube length of the set up.  I used PL300 PAL for the test.

 

It is worth to point out that each lenses has its own B0 (minimum tube length).  The focal length came out to be 115 mm for both lens.  I will confirm with Bausch & Lomb lens when the adapter is ready.

 

I am in a process to estimate how much distortion (mainly coma and vignetting) each combination gives.  I already have taken good number of NGC7635 shots with different combination and tabulating them (weather has to cooperate!). I haven't seen any vignetting with 1/3" ccd (LN300 PAL) yet!  

 

Paul

 

 

 

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[photo444]

Here are two shots made with two different lenses without any 5 mm extension.  The first is NIKON and the next NOF CN lens on 6"F5 Newt.  

 

Reduction factors calibrated with the same object w/o reducer are 0.574 and 0.557, respectively.  NOF generated more coma.  It even showed more coma than NIKON lens w/ one 5 mm spacer (R=0.535). A lot more work is ahead to fill all the matrix elements.

 

Paul

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[Relativist]

A couple of suggestions, one is lets try open clusters. Second if we can put a crosshair with a few grid lines, then we can tell how far out from the center the coma (or whatever optical abberation is really occurring) is noticeable.

[photo444]

Curtis,

 

I am working now to generate equi-distortion concentric circles on a star field taken with APS-C sensor with/without reducer.  As you have pointed out in our PM the distortion already exist in F5 image plane is substantial.   Since the reducer shrinks the field which is already distorted by the primary mirror, one has to take caution not to confuse it as the distortion caused by the reducer.

 

I took part of Milky Way near Cygnus for the purpose to fill the entire field with stars.  Is there any star cluster which is big enough to fill the APS-C sensor at F5?  Another difficulty is controlling collimation to keep the distortion circle concentric. The bottom line is the quality of optics which is necessary to attack this study. 

 

A way of quantitative definition of distortion is needed, for example the size of elongation can be measured in terms of number of pixels.  Since this is for EAA not high resolution AP a qualitative argument still is OK IMHO.

 

I also have ordered Antares 2" reducer to compare. I will have it by next week.

 

This project seems getting bigger and bigger but loaded with a lot of fun.

 

Paul

[photo444]

I have worked out with APS-C frames of gamma-Cyg and NGC6910, w/ and w/o reducer.  The reduction was 0.65 which is rather weak.  The pictures were re-sized to fit the CN format in which some details were lost.

 

The first one is a frame w/o reducer.  Joining two opposite distorted stars gave the center point where distortion is minimum. Then two concentric circles were drawn to show equi-distortion radius.  White rectangles are the size of 1/3 and 1/2 inch ccd sensors. Blue boxes are equivalent coverage of 1/3 and 1/2 sensors recorded with the reducer. Please ignore orange boxes.

 

The second picture is a frame taken with the reducer (0.65x).  Equi-distortion circles and boxes representing 1/3 an 1/2 inch ccd sensors are shown.

 

The third is a frame taken with LN300 PAL 1/3" CCD, showing NO distortion to the size of the pixel.

 

A few conclusions may be drawn from above:

1. No sizable distortion is visible for 1/3" sensor at the prime focus of a 6"F5 Newt when no reducer is used.

2. Slight coma can be seen for 1/2" sensor even when no reducer is used.

3. It is reasonable to say for 1/3" sensor that any distortion seen with a reducer is responsible to the reducer. 

 

I will try to compare frames of an identical object at R=0.5x for the three lenses.

 

 

 

 

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[GlennLeDrew]

Good work, Paul! Just the kind of detail-oriented investigating I tend to delve into.

 

I take it that the use of "distortion" here is really referring to aberration. It's important to observe the distinction, as distortion refers to image scale variation with field angle, commonly called pincushion or barrel, depending on the sense in which straight lines in the image bend. I kept picturing image scale variation whenever reading "distortion", until the post preceding mine here.

[photo444]

Glen,

 

Thank you. The term I used distortion is all of aberrations.  I need comments like yours in order to analyze logical and systematic ways.  At this time I was focusing on coma-tic aberration which is easier to deal with. Determining pincushion from star field is rather tedious process to me, but I may have to soon. Hope there is some straight lines with the stars somewhere.

 

I may make a note that the first APS-C frame taken without reducer already is lacking rotational symmetry, it may mean that the primary mirror surface is not perfect parabola and/or the secondary is not perfectly flat.  

 

Paul