Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

NMBob's FreeStar - for C6 only

  • Please log in to reply
34 replies to this topic

#26 GlennLeDrew

GlennLeDrew

    Hubble

  • *****
  • Posts: 15849
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 21 April 2017 - 04:06 PM

Charles,

Adding a Moon filter would be pretty much exactly like shortening the exposure. For instance, a 13% transmission Moon (neutral density) filter reduces transmission to 1/8. And so the equivalent would be to make the exposure 1/8 as long in duration.



#27 CharlesC

CharlesC

    Gemini

  • ****-
  • topic starter
  • Posts: 3489
  • Joined: 04 Jun 2008

Posted 21 April 2017 - 04:16 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

 

 

IMO, when we focused on bright star, we were actually focusing on the reflection which defocused all dim stars.  I maybe wrong, but I think a good experiment would be to look at a bright star field like M16 with a moon filter or some dark colored filter to dim incoming light.  If all the halos go away, I'm right, if halos remain I'm wrong.

 

Your link shows spherical mirrors inherently have spherical aberration, but later mentions the corrector takes care of that.  No doubt there is some spherical aberration at point of Freestar, but I don't think we are experiencing it yet as reflections overwhelm everything else.



#28 CharlesC

CharlesC

    Gemini

  • ****-
  • topic starter
  • Posts: 3489
  • Joined: 04 Jun 2008

Posted 21 April 2017 - 04:20 PM

Charles,

Adding a Moon filter would be pretty much exactly like shortening the exposure. For instance, a 13% transmission Moon (neutral density) filter reduces transmission to 1/8. And so the equivalent would be to make the exposure 1/8 as long in duration.

If what we are seeing is spherical aberration a moon filter shouldn't have any effect.  If it does, problem is a reflection, IMO.

 

I believe older Mallincam's didn't have a window on sensor.  Would be interesting if someone tried a windowless Mallincam camera on Freestar.  If reflection is the problem, it would not be seen in this type of camera.



#29 GlennLeDrew

GlennLeDrew

    Hubble

  • *****
  • Posts: 15849
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 21 April 2017 - 05:21 PM

If the same camera used with Hyperstar does not show the same halos as when FunStar/FreeStar is used, then we can certainly rule out camera window reflections as the cause of these halos. Hyperstar optics can do nothing to suppress such reflections. This would leave only SA as the culprit.


  • Don Rudny likes this

#30 GlennLeDrew

GlennLeDrew

    Hubble

  • *****
  • Posts: 15849
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 21 April 2017 - 05:40 PM

Additionally to the ClipOnStar corrector scheme...

 

An additional clip-on corrector to 'beef up' the SA reduction of the existing Schmidt corrector need not have coatings applied. The ~8% light loss is minimal. The potential for reflections induced by uncoated surfaces is rendered moot simply by tilting the corrector in angle equal to or larger than either the FoV width or 1/2 the FoV width (I forget ATM, and would have to work it out.) The extremely low optical power of such a corrector, in conjunction with its thinness, permits tilts of several degrees before lateral chromatic aberration would begin to rear its head, and most certainly so for such small image scale as here.



#31 CharlesC

CharlesC

    Gemini

  • ****-
  • topic starter
  • Posts: 3489
  • Joined: 04 Jun 2008

Posted 21 April 2017 - 05:41 PM

Experiments will get to the bottom of this.


Edited by CharlesC, 21 April 2017 - 05:42 PM.

  • NMBob likes this

#32 CharlesC

CharlesC

    Gemini

  • ****-
  • topic starter
  • Posts: 3489
  • Joined: 04 Jun 2008

Posted 23 April 2017 - 10:14 PM

Tried some dark filters, and they had no effect.


  • NMBob likes this

#33 GlennLeDrew

GlennLeDrew

    Hubble

  • *****
  • Posts: 15849
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 24 April 2017 - 09:14 AM

Tried some dark filters, and they had no effect.

An ND filter simply dims everything. If you then compensate by increasing the exposure to get back the image intensity desired, you're kinda back where you started, but having to wait longer for the image to update. ;) And the dimmer image automatically means greater camera noise. It's a lose-lose.



#34 Nicole Sharp

Nicole Sharp

    Surveyor 1

  • -----
  • Posts: 1595
  • Joined: 12 Jun 2018
  • Loc: Cumberland, Maryland, USA

Posted 12 July 2018 - 04:21 AM

I am confused by this.  Is a "FreeStar" simply taking the Celestron Visual Back from a C6 out of the back and putting it in the front instead?  Then attach a camera to the Visual Back to take photos at f/2 without needing to buy a Revolution FunStar or Starizona HyperStar?



#35 mclewis1

mclewis1

    Thread Killer

  • *****
  • Posts: 19228
  • Joined: 25 Feb 2006
  • Loc: New Brunswick, Canada

Posted 12 July 2018 - 08:52 AM

Yes, that's correct.


  • Nicole Sharp likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics