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NMBob's FreeStar - for C6 only

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#1 CharlesC

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Posted 19 April 2017 - 11:00 PM

NMBob discovered Visual back will fit on secondary of Fastar compatible C6 in another thread and called it FreeStar.  This thread is to report results trying this F2 setup. 

 

I had several problems, many of my own doing, and location, that need to be sorted out.  I'm in a near white zone and conditions are not good tonight for seeing or transparency.  I get a lot of back scatter from the light dome in these conditions.  What did I find out?

 

o Usually I run x128 exposure on my LN300 camera, but with FreeStar I had to run x16.  It seems to be eight times brighter!

o I could not use a bright star for focus as making it smaller caused dim stars to disappear.  Finally used dim stars for focus.  Can't use batinov mask.

o Bright stars have a halo and you can see cables in halo.  This maybe due to streetlight getting in. Dew shield can not be installed to block streetlight.

o I could not get SharpCap to stack, so tried brief non-align stack.

o Need to work things out at a darker locatoin, learn how to focus, keep streetlight out, and get SharpCap to stack.

 

Bottom line, this will take more work to see how useful it can be.  I believe 224 camera would be more capable at this wide field of view.  Anyone do better than this?  Shouldn't take much effort.  confused1.gif

 

Here is poor first attempt on M51.  Using x8 exposure (.3sec) and stacked for 27seconds un-aligned.

M51_Stack_235.jpg

 

It was so bright, I tried turning off AGC and tried x256 exposure.  Not bright enough to stack.  Here is non-stacked single exposure.  You can see cables in bright star halo.  M51 is at top of frame.  I wonder if stars are one pixel and SharpCap minimum 2 pixel limit causes failure to recognize stars?

M51_5sec_NoAGC_noStack.jpg

 

M13 Using x8 exposure and stacked 5 seconds un-aligned.

M13_Stack_50b.jpg


Edited by CharlesC, 20 April 2017 - 12:22 AM.


#2 NMBob

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Posted 20 April 2017 - 12:26 AM

Yeah, at random times when I was moving the scope around I'd see some strange lights or reflections. Some of them like what you are getting. I don't think there was any streetlight or anything to cause them. I tried the .5X reducer too (from the FunStar thread). Couldn't make sense of what I saw. One corner of the display had about 1/4 of what looked like an out of focus star image, but it was bright red. The only thing bright red in the area was the power LED on the opposite side of the R2 camera. Running the focus didn't seem to change anything. Will play with it all more the next time.

 

I'll probably get one of the FunStar adapters for my C8. This is kinda fun! :)


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#3 CharlesC

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Posted 20 April 2017 - 12:32 AM

I wonder what causes the reflections on bright stars?  Maybe black felt in the baffle would help.  Maybe its something else entirely.  I was using a IR filter.  Maybe reflections were coming off that.


Edited by CharlesC, 20 April 2017 - 12:33 AM.


#4 NMBob

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Posted 20 April 2017 - 07:52 AM

I didn't have any filter, but then we may have had different reflections, too. If I hadn't had so much trouble getting the mount to track I would have checked things out more. I suspect it was the little knurled knob/clutch at the gearing end of the RA slow motion cable. It felt like it was tight/fully engaged, but I'm not so sure now. Or maybe I just didn't tighten it enough. Darned operators and their errors. :)

 

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#5 CharlesC

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Posted 20 April 2017 - 08:43 AM

Bob, post pics when you get the time.  I'm wondering if we have same problem and if there is a cure.



#6 GlennLeDrew

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Posted 20 April 2017 - 09:24 AM

If a bright star's reflection appears at about opposite the field center, this would suggest a Schmidt ghost. This results from a bright star spot on the sensor acting as a light source which reflects through the system and being autocollimated by the 'flat' Schmidt corrector, forming an out-of-focus image opposite the optical axis.

 

The sizeable halos more or less centered on a brighter star suggests the possibility of a reflection from the glass window over the sensor. Their radial offset which increases with increasing field angle would tend to support this.



#7 CharlesC

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Posted 20 April 2017 - 09:43 AM

Glenn, do you think a 1.25" polarized filter might fix it?

Perhaps a camera with anti-reflective coating on its window would be best.



#8 NMBob

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Posted 20 April 2017 - 10:39 AM

About the only pictures I had were the ones I posted. I mostly noticed the 'ghosts' running around when moving the scope around. Maybe they really were ghosts! I like Mr. Glenn's explanations. Much to look into next time!



#9 Don Rudny

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Posted 20 April 2017 - 12:36 PM

Had some clear skies last night and tried Bob's Freestar on my Evo 6 with Lodestar X2c.  I also have a Hyperstar, so we have a definitive comparison.  M8 came up, so I used it as the target.  All settings were the same.  5x45s.  I used a non linear contrast setting.

 

One thing to point out is that if I try to get a better focus on the bright stars, the smaller dim stars go out of focus.  I'm hoping Glenn sees this and can explain what's going on.  Some on SGL have said that this won't work without some corrective lens because of spherical aberrations.  I thought that would only be a problem out toward the edges.

 

In any case the results are not very good.  First one is Freestar, then Hyperstar.

 

M8.Lagoon.Nebula_2017.4.20_00.35.58.jpeg

 

M8.Lagoon.Nebula_2017.4.20_00.15.16.jpeg


Edited by Don Rudny, 20 April 2017 - 12:37 PM.

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#10 nytecam

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Posted 20 April 2017 - 01:45 PM

Don - via SA the brightest stars 'bloat' to fill and saturate the SA 'disk' and lesser stars less so !  Comparison to the H-star is stark.

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#11 OC Telescope

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Posted 20 April 2017 - 01:55 PM

Had some clear skies last night and tried Bob's Freestar on my Evo 6 with Lodestar X2c.  I also have a Hyperstar, so we have a definitive comparison.  M8 came up, so I used it as the target.  All settings were the same.  5x45s.  I used a non linear contrast setting.

 

One thing to point out is that if I try to get a better focus on the bright stars, the smaller dim stars go out of focus.  I'm hoping Glenn sees this and can explain what's going on.  Some on SGL have said that this won't work without some corrective lens because of spherical aberrations.  I thought that would only be a problem out toward the edges.

 

In any case the results are not very good.  First one is Freestar, then Hyperstar.

 

attachicon.gifM8.Lagoon.Nebula_2017.4.20_00.35.58.jpeg

 

attachicon.gifM8.Lagoon.Nebula_2017.4.20_00.15.16.jpeg

That's a great comparison of photos,

 

I would ask if that's the right question though?  Is the right question between the FunStar image and the image through the 6SE at F10 or 6.3 or .5x?  And is it really the "image" or the view (eaa).  

 

What I was amazed with is the view when using and the ability to shift focus and exposure to actually "See what you've been missing".

 

Mike



#12 Astrojedi

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Posted 20 April 2017 - 02:52 PM

Charles, Don,

 

The 'halo' around bright stars in your images does not seem to be a halo. It seems bright stars are not focusing properly i.e. they are extremely comatic as in both your images as I can see the shadow of the cables as well.

 

This could be due to a combination of spherical aberrations and a lack of collimation (due to camera sensor's tilt relative to the plane of the image). At F2 collimation needs are extremely critical and even a small tilt can cause significant coma. In the freestar and funstar just the act of tightening the locking screw I would imagine is sufficient to completely destroy the optical alignment of the 2 planes including along the optical axis.

 

It is also possible that when the light cone is so steep different wavelengths are not able to come to focus at the same point without some help. Maybe using an UV/IR cut filter would help.

 

Goes to show how complex optically a SCT system is relative to say a refractor or a Newtonian.

 

Hiten


Edited by Astrojedi, 20 April 2017 - 02:53 PM.

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#13 Don Rudny

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Posted 20 April 2017 - 03:24 PM

Just researching spherical aberration, I really think the problem is the spherical design of the SCT primary without correction.  Here is a link to Starizona's explanation of spherical aberrations.  My results appear to be consistent with this explanation.  The lack of  proper focus with a spherical mirror affects the field equally, not just at the edges.  There's just no way to bring everything  to focus.

 

https://starizona.co..._spherical.aspx

 

Hiten, I thought of the collimation issue as well.  I did use a Baader twist lock holder that should minimize the tilt.  There is no collimation adjustment used on the Hyperstar capture.   Errol suggested in the other thread that I try an NB Ha filter to reduce the IR affect on bloat, so I'll do that.  But, I just don't think it will solve all the problems.  It may help though.



#14 CharlesC

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Posted 20 April 2017 - 03:49 PM

Don, I don't think its bloat, but internal reflection Glenn suggested. Below a certain threshold of brightness stars are perfect.  I had the same problem, even using IR filter.  Only bright stars have the problem, you can see cables dangling in the reflection.  The image is eight times brighter than normal.  Maybe dimming light with a LPC filter would kill reflection.  Heck, maybe a moon filter would kill reflection, but certainly not a good solution, but interesting test.


Edited by CharlesC, 20 April 2017 - 03:57 PM.

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#15 alphatripleplus

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Posted 20 April 2017 - 04:59 PM

Yeah, Charles does a better job in explaining my hunch than I did in the other thread - if Charles is right that there is a brightness threshold below which the stars are okay, then a narrowband filter might help this non-linear behaviour. Of course, I am just guessing. smile.gif



#16 GlennLeDrew

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Posted 20 April 2017 - 11:30 PM

The circular, hard-edged halos more or less centered on bright stars I now see as spherical aberration (SA). (I initially suspected reflection via the window near the sensor; no longer.)) For any one place in the FoV, the SA affects *every* image point equally, be it star or nebula, bright or dim. The dimmer stars show a much reduced aberration simply because the dimmer outer halo is too faint to register. But that halo is there.

 

If not for the Schmidt corrector, the SA would be monumentally grotesque!

 

SA here is not like the 'bloat' caused by differential focus by wavelength, or longitudinal chromatic aberration. A cut-off filter will avail nothing beside dimming things. The Schmidt corrector is of such low net optical power (essentially zero), and possessing of a very gentle 4th order curve, that chromatic aberration is vanishingly small.


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#17 CharlesC

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Posted 21 April 2017 - 11:49 AM

I don't have a clue about optics, so here is idea that maybe completely useless, and certainly "outside the box".

 

When referring to a washer Bob mentioned here the inside diameter of visual back is 1.9".  You can get a round 43mm (1.7") prescription lens from Zenni for $9.95.  One could put an o-ring in lens groove and shove it into visual back. Use extenders for camera spacing?  Zenni gives you the lens for free when you buy $9.95 frames like this with free shipping. Heck, save the empty frames for Halloween and go as John Lennon.

 

Lens prescriptions specify; sphere, cylinder, axis.  Pretty sure sphere is one to adjust while cylinder and axis should be zero.  Does anyone have an idea about appropriate lens prescription?  This would be fun and cheap to tinker with.

 

I see Edmunds sells spherical aberration lens for $175-$300 here. . Aberration values go from +1.00 to -1.00. Is that sphere?  Could order Zenni glasses with left of -0.25 and right of +0.25 and see which improves it, then order another pair further in direction that shows improvement.  Only $5 per lens that way.

 

Like using a chainsaw for brain surgery. jawdrop.gif


Edited by CharlesC, 21 April 2017 - 12:36 PM.


#18 GlennLeDrew

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Posted 21 April 2017 - 12:44 PM

Those Edmund spherical aberration plates are optimized for collimated, or parallel light, and are intended to be placed near a pupil (e.g., the entrance pupil, which here would effectively be at the Schmidt corrector.) In the highly convergent f/2 beam they would probably perform rather poorly.

 

But guess what? The SCT already has a spherical aberration plate--it's the Schmidt corrector. What one needs to to is to figure a replacement that likely has a stronger 4th order curve. The existing SA indicates at least several waves of error. Even a somewhat ham handed optician can cook up a replacement that will surpass this current state of affairs. Even a 1-2 lambda error would be a *huge* improvement, which significantly cuts down on time to finesse and thus reduces cost. Then the larger issue will likely be one or both of coma and field curvature.

 

If I had my own facility I'd love to tackle such a project, starting preferably with a corrector taken from an otherwise broken scope.


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#19 WhitezonePrisoner

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Posted 21 April 2017 - 01:13 PM

Heh Glenn, maybe you don't need a replacement to the existing Schmidt corrector - how about just adding an additional full aperture  "clip-on corrector" with the appropriate degree of SA correction in front of the existing Schmidt corrector (with a cental hole for the camera, which still screws into the Funstar or FreeStar in the middle of the original corrector). You optic guys can figure out how much additional correction the clip-on corrector needs....plus you get to use your C6 normally by just removing the clip-on corrector, remove the camera, and put back  the secondary. Sounds easy....

 

Man, I guess I just need to work with some optics guy,  patent the thing, build it and away we go lol.gif

 

Edit: Can we call it the ClipOnStar?


Edited by WhitezonePrisoner, 21 April 2017 - 01:24 PM.


#20 Censustaker

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Posted 21 April 2017 - 01:44 PM

Freestar needs a COSTAR!


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#21 mclewis1

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Posted 21 April 2017 - 01:56 PM

Gee, I wonder how long it will take to rack up the costs equivalent to the $700+ for the HyperStar. fingertap.gif

 

Remember the HyperStar also has collimation capabilities.


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#22 WhitezonePrisoner

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Posted 21 April 2017 - 02:25 PM

I dunno about cost....I think the collimation issue can be solved with something off-the shelf like this tilt adapter for 50 Euros from TS in Europe https://www.teleskop...astrophoto.html.

 

I'd be surprised if a corrector plate is gonna cost anything like $600 or $700, so I think we could be way cheaper than Hyperstar. Let's see if the optics guys say it can be done as a clipon corrector to the front of the scope.


Edited by WhitezonePrisoner, 21 April 2017 - 02:27 PM.


#23 CharlesC

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Posted 21 April 2017 - 03:40 PM

I found an article on CN that explains the bright star "bloat" we are seeing. 

I think Glenn was right in his first assessment.

Look at figure 13 in below article in below link on "stray reflections".  Identical to what we are seeing.

 

IMO, problem is filter or glass cover of CCD.  The reflection happens within the glass due to extreme brightness of F2 image.  A camera without CCD glass cover wouldn't get this "stray reflection".  Perhaps anti-reflective glass cover would fix it.  I think, as a test, trying a moon filter should eliminate all stray reflections. 

 

https://www.cloudyni...mize-perf-r3013

"Stray reflections. On it’s own, the HyperStar is free of stray reflections; however, if you add any filters to the system, it is possible to introduce significant stray reflections around bright stars. Figure 13 shows how these stray reflections are formed, why they are so large, and what they look like. Of course, this is the exact same problem that will occur with any other fast optical system when the filters are used in a converging beam. The size of stray reflections can be reduced by using very thin filters and by placing them as close as possible to the focal plane.  However, placing the filters close to the sensor will increase the brightness of the stray reflections. Another strategy is to move the filters so far from the focal plane that the intensity of the stray reflections becomes negligible. The problem with that strategy is that it requires very large filters. In the HyperStar adapter, the filter position is fixed so you get what you get. In my experience, it is probably best to use the HyperStar system without any filters."


Edited by CharlesC, 21 April 2017 - 03:53 PM.

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#24 GlennLeDrew

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Posted 21 April 2017 - 03:59 PM

The ClipOnStar notion has merit. Perhaps even a sheet of (selected) float glass could be used. Just cut 'n core it, spin polish with one or both of a petal-shaped and a small, sub-diameter lap, concentrating on the 0.707 radius to the required depth in fringes, plus or minus a couple, and in pretty short order a useable element could be turned out.

 

If the side to remain flat has notable fringe 'wiggles' and variation in depth of more than a couple of fringes, a short spell (of perhaps 10-20 minutes) of flat polishing should improve that considerably.

 

The profile of the aspheric surface could be determined by testing; no need to calculate at all. A compact Ronchi tester based around a video camera and lens would be inserted where the imaging camera goes. The lens of course must have a field of view large enough to take in the f/2 beam, but that's not so wide.

 

A suitably distant point source of bright light would be the target; about 100 feet should be not too close for this not-demanding system. Or another telescope could send collimated light from a pinhole located at its focus.

 

If an ATM were to churn out some number of these things, fabricating the glass element alone might(?) be economically viable at about $200 each. Cell and coatings would be extra.

 

Of course, a feasibility sample would be wise to make, to assess outer field performance re field curvature and coma.


Edited by GlennLeDrew, 21 April 2017 - 04:01 PM.

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#25 Don Rudny

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Posted 21 April 2017 - 04:04 PM

I found an article on CN that explains the bright star "bloat" we are seeing. 

I think Glenn was right in his first assessment.

Look at figure 13 in below article in below link on "stray reflections".  Identical to what we are seeing.

 

IMO, problem is filter or glass cover of CCD.  The reflection happens within the glass due to extreme brightness of F2 image.  A camera without CCD glass cover wouldn't get this "stray reflection".  Perhaps anti-reflective glass cover would fix it.  I think, as a test, trying a moon filter should eliminate all stray reflections. 

 

https://www.cloudyni...mize-perf-r3013

"Stray reflections. On it’s own, the HyperStar is free of stray reflections; however, if you add any filters to the system, it is possible to introduce significant stray reflections around bright stars. Figure 13 shows how these stray reflections are formed, why they are so large, and what they look like. Of course, this is the exact same problem that will occur with any other fast optical system when the filters are used in a converging beam. The size of stray reflections can be reduced by using very thin filters and by placing them as close as possible to the focal plane.  However, placing the filters close to the sensor will increase the brightness of the stray reflections. Another strategy is to move the filters so far from the focal plane that the intensity of the stray reflections becomes negligible. The problem with that strategy is that it requires very large filters. In the HyperStar adapter, the filter position is fixed so you get what you get. In my experience, it is probably best to use the HyperStar system without any filters."

I don't think so, Charles.  Why wouldn't get the same thing with Hyperstar?  I used no filters, and the Lodestar has a glass cover, but it's right up against the sensor.  I have never gotten any reflections off of it.  The Starizona article I posted explains what's going on.  Without more correction, the SA causes much of the light to be unfocused.  It's so unfocused that it starts focusing the central obstruction and cable.  If I try to focus the bright stars, the dim ones go out of focus.  The brighter stars show up more because the unfocused light is dimmer.  Smaller stars have the SA, too, but it's too dim to show up.  Even the nebula has a softened look to it because some of the light is unfocused.

 

Don




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