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

MCT Secondary Baffle Mod

  • Please log in to reply
192 replies to this topic

#176 hardwarezone

hardwarezone

    Messenger

  • -----
  • Posts: 459
  • Joined: 26 Apr 2013
  • Loc: Singapore

Posted 07 September 2014 - 10:10 AM

Thanks for the good information, Don

 

 

The vignetting in the daytime photos are caused by improper alignment of the eyepiece to camera.

I had serious blackout issues on the camera preview screen and tried to align the camera higher and lower it with the geared center column on the tripod.



#177 Chassetter

Chassetter

    Viking 1

  • *****
  • Posts: 575
  • Joined: 08 Jul 2009
  • Loc: New Brunswick, Canada

Posted 22 October 2014 - 07:36 PM

I would like to flock my nexstar 4se bafle with the sandpaper method as I can see the baffle interior off axis through the corrector when i do the  led light test.

Does anyone know it's dimensions or can I get away with the last 4 inches or so?



#178 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • topic starter
  • Posts: 12050
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 22 October 2014 - 07:52 PM

No, I do not know the dimensions, you might have to measure them.

 

I removed the meniscus and measured the opening at the end of the baffle, then measured the visual back opening and the length as accurately as possible. The cut it too large to fit, test fitted it, and trimmed it as needed until it fit snuggly with the rolled ends pressing against each other inside the baffle for a tight fit.

 

A wooden dowel or something similar can be used to help press and seat the flocking inside the tube without the use of any adhesive. It took a few tries to get it just right. Others may have done it differently, but this works.


  • Chassetter likes this

#179 ChristianG

ChristianG

    Apollo

  • -----
  • Posts: 1439
  • Joined: 18 Oct 2012
  • Loc: Ottawa, Canada

Posted 28 October 2014 - 09:16 PM

No, I do not know the dimensions, you might have to measure them.

 

I removed the meniscus and measured the opening at the end of the baffle, then measured the visual back opening and the length as accurately as possible. The cut it too large to fit, test fitted it, and trimmed it as needed until it fit snuggly with the rolled ends pressing against each other inside the baffle for a tight fit.

 

A wooden dowel or something similar can be used to help press and seat the flocking inside the tube without the use of any adhesive. It took a few tries to get it just right. Others may have done it differently, but this works.

 

Hi. This is also how I did my C90 and 127 Mak, although I used a telescopic antenna to measure the length of the baffle tube from the back, I did not remove the corrector cell for that. It's a trial-and-error process, but cost is very low. I find that fine needle-nose pliers are useful for pulling the sandpaper back out for trimming with an x-acto knife and metal ruler. The wood dowel is indeed a necessity to get a good fit. Have fun!

 

--Christian


Edited by ChristianG, 28 October 2014 - 09:49 PM.

  • Asbytec and Chassetter like this

#180 Chassetter

Chassetter

    Viking 1

  • *****
  • Posts: 575
  • Joined: 08 Jul 2009
  • Loc: New Brunswick, Canada

Posted 29 October 2014 - 05:25 PM

Thanks guys, maybe this weekend!



#181 schang

schang

    Surveyor 1

  • *****
  • Posts: 1654
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 16 November 2014 - 06:00 PM

I would like to flock my nexstar 4se bafle with the sandpaper method as I can see the baffle interior off axis through the corrector when i do the  led light test.

Does anyone know it's dimensions or can I get away with the last 4 inches or so?

I just got around on this for my C90 Mak...You can take the visual back out (or not) to do the measurement.  The diameter of the baffle can be measured with a ruler, and the circumference can be calculated.  The length can be measured with a long straight wire, with one end bent 90 degrees.  Stick it into the baffle till it hooks on the end of the baffle, then make a mark at the other end.  This will be you length for the sandpaper.  You can cut a test paper(I used the Manila folder) of the length and width, and rolled it into the baffle tube to see how it fit, before I cut the sandpaper.  

 

This is an informative thread...I saw the diffraction loop like feature in my C90 Mak when there is a bright light on the edge of the FOV.  I did not pay much attention to it due to the fact that I seldom view things with bright lights on the edge.  However, recently I began to deal with this looping issue, when I found out that no matter how long my light shield is, I still saw it.  So I started with adding blackened sandpaper strip into the primary baffle tube.  Voila, the diffraction loops are gone; and I do not need a light shield at all....But I am still going to use the light shield to prevent the corrector glass from dewing up.



#182 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • topic starter
  • Posts: 12050
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 20 November 2014 - 06:59 PM

However, recently I began to deal with this looping issue, when I found out that no matter how long my light shield is, I still saw it.  So I started with adding blackened sandpaper strip into the primary baffle tube.  Voila, the diffraction loops are gone; and I do not need a light shield at all...

 

 

That's interesting, thanks. I'm still unsure, or cannot remember the cause, what causes that bright loop. One might expect it to be an internal reflection traveling up and down the optical train. But if flocking the baffle cured it, that's interesting. I would think any reflections inside the baffle would be very diffuse while an optical reflection would be better defined like this off axis loop folks are seeing. Other than that, have you noticed any difference in the FOV darkness or reduced scatter?



#183 ChristianG

ChristianG

    Apollo

  • -----
  • Posts: 1439
  • Joined: 18 Oct 2012
  • Loc: Ottawa, Canada

Posted 24 November 2014 - 11:21 PM

 

However, recently I began to deal with this looping issue, when I found out that no matter how long my light shield is, I still saw it.  So I started with adding blackened sandpaper strip into the primary baffle tube.  Voila, the diffraction loops are gone; and I do not need a light shield at all...

 

 

That's interesting, thanks. I'm still unsure, or cannot remember the cause, what causes that bright loop. One might expect it to be an internal reflection traveling up and down the optical train. But if flocking the baffle cured it, that's interesting. I would think any reflections inside the baffle would be very diffuse while an optical reflection would be better defined like this off axis loop folks are seeing. Other than that, have you noticed any difference in the FOV darkness or reduced scatter?

 

 

Hi guys.

 

First, technically this 'loop' or 'diamond ring' effect is just one light reflection from the inside of the stock primary baffle tube, which is quite shiny so reflections are not diffuse at all and in fact quite well defined! By the way, It is not a diffraction artifact, this would involve the wave nature of light. And yes, flocking the inside of the tube fixes the problem.

 

Flocking the inside of the primary baffle tube has by far the most effect on contrast. If however one observes from areas with street lights etc. (for instance), a light shield of the proper length may help, but the effect will be subtle. It is easy to measure the minimum length of a light shield to stop light from going between baffles. It's called the 'yard stick test'. One finds 10" for a C90/Apex 90, and 12" for a 127 Mak. No need to make it longer or shorter.

 

In my experience, flocking the inside of the primary baffle tube, and to a smaller extent the inside of the main tube plus the use of a proper light shield, has a huge effect on the instrument's contrast. For almost no cost, performance in that department is on par with a good Questar with Broad Band coatings. Not bad for a 200$ telescope! Hope this helps.

 

--Christian


  • Asbytec likes this

#184 schang

schang

    Surveyor 1

  • *****
  • Posts: 1654
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 24 November 2014 - 11:48 PM

 

However, recently I began to deal with this looping issue, when I found out that no matter how long my light shield is, I still saw it.  So I started with adding blackened sandpaper strip into the primary baffle tube.  Voila, the diffraction loops are gone; and I do not need a light shield at all...

 

 

That's interesting, thanks. I'm still unsure, or cannot remember the cause, what causes that bright loop. One might expect it to be an internal reflection traveling up and down the optical train. But if flocking the baffle cured it, that's interesting. I would think any reflections inside the baffle would be very diffuse while an optical reflection would be better defined like this off axis loop folks are seeing. Other than that, have you noticed any difference in the FOV darkness or reduced scatter?

 

Norme: Sorry for the late reply...Like Christian stated, that the "loop" from the bright light on the edge is quite defined, and appears to change its size depending on the distance from the very edge.  Its impact is quite dramatic to the extent that it overwhelms the image near the loop. It does not appear to affect the image darkness or reduce scatter, but seems to be superimposed on the image, so contrast is pretty much affected by this loop.  This was observed during the night on terrestrial object with street light near by.  On celestial objects, the loop is seen with the moon (Venus or other very bright stars) near or a little outside the edge of the field.  but it disappears when the object is inside, but away from the edge of the field. The flocking of the primary baffle tube eliminates the loop problem, though I still keep my dew (light) shield on for obvious reason.   


  • Asbytec likes this

#185 Quixylvre

Quixylvre

    Explorer 1

  • -----
  • Posts: 76
  • Joined: 23 Sep 2013
  • Loc: Grand Rapids, MI

Posted 10 March 2015 - 08:57 PM

Apologies for thread necromancy, but I need some help. How, pray-tell, does one measure and calculate the correct curve-radii and angular span for the flocking material to apply to the secondary-baffle?



#186 GlennLeDrew

GlennLeDrew

    Hubble

  • *****
  • Posts: 15452
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 11 March 2015 - 05:13 AM

Do this graphically, from which a template results. But one calculation is required--formula below.

Get these 3 measurements:
- widest diameter
- narrowest diameter
- distance separating the two diameters, as measured along the baffle's side.

Draw the side view of the resulting trapezoid. Continue the sloping sides to their convergent point. Now you have a cone, with two known *side* heights, as measured along the *side* of the cone (we're not concerned with the *base* height, as measured along the cone's axis.)

On a suitably big sheet, and using some form of compass, draw two concentric circles of radii equal to the two *side* heights previously obtained. The flocking material to be used lies within the annulus defined by the two circles.

To determine the fraction of the circle our conical section occupies, we will use these parameters:

- maximum baffle diameter, d
- longest cone *side* height (defining the largest radius), r
- pi, or 3.1415926

The arc on the annulus, in degrees, equals:

(d * pi / r) * (180 / pi)


Note: I just worked this out from first principles, and hope I haven't mucked something up. ;)

Edited by GlennLeDrew, 11 March 2015 - 05:15 AM.


#187 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • topic starter
  • Posts: 12050
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 11 March 2015 - 06:13 AM

Thanks, Glenn, for tending to this thread past and present. 

 

I did just that, really, but my measurements were a bit sloppy. So, I cut it too large, test fitted it, then trimmed it up as needed. 

 

No glue required, it fits snug enough to stay put.


Edited by Asbytec, 11 March 2015 - 06:13 AM.


#188 Quixylvre

Quixylvre

    Explorer 1

  • -----
  • Posts: 76
  • Joined: 23 Sep 2013
  • Loc: Grand Rapids, MI

Posted 11 March 2015 - 01:59 PM

Question for anyone who's done this with the newer model (introduced in 2010) Celestron C90, the one that's made by Synta. Anyone who's done more than just flock the primary baffle, have you done what PowellAstro did here?

post-91356-14074205190323.jpg

If so, did it appreciably improve the scope's steadiness when adjusting focus?



#189 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 35770
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 11 March 2015 - 02:31 PM

When the pressure plate is tightened down on the mirror, the mirror does not move relative to the central baffle from that point on.

The tape would aid in proper centering of the mirror on the baffle, but it would have absolutely no effect on slop in the focuser.

The slop in the focuser is almost entirely due to the clearance between the outer baffle, to which the mirror is attached, and the inner baffle attached to the back cell of the scope.

There is thick grease in between these two baffles, and the outer baffle slides up and down the inner baffle as you focus.

So, what can be done to lessen the slop that causes image shift?

 

1) Thicker grease.  if the grease does not flow away from the pressure points as one baffle is tipped relative to the other, this will reduce image shift.  Such a super thick grease might not work well at sub-zero temperatures, though.

2) tighter tolerances between the inner baffle O.D. and the outer baffle I.D.  Ball bearings running in a track would have done it, but would have increased the production costs.

a smaller gap would have been hard to grease and could result in metal-on-metal scraping.

3) a strong spring behind the primary mirror around the baffle that presses the mirror forward.  This might lessen image shift, but it would also mean the focuser would be harder to move in one direction than the other.

4) Having the pressure to change focus come from the center instead of the side (e.g. helical focuser, center-mounted).. Some SCTs did have this.

5) A fixed mirror with a rear-mounted focuser.  This was deemed unacceptable because it would significantly restrict focusability.  Current SCTs focus over a very wide range from near to infinity with eyepieces, cameras, reducers, different sizes of star diagonals inserted or removed, off-axis guiders, etc.  A rear-mount focuser only would limit the usage of the scopes.

 

There is one simple thing every user could do that would help.  Every time the scope is used, run the focuser from one end to the other about a half dozen times to get the grease well-distributed.  It will reduce image shift.  Then, once a year, remove the focuser and use a pencil eraser through the hole in the back to turn the primary mirror around in a circle several times.  This will distribute the grease uniformly over the interface between the two baffles.  Then, grease the focuser threads and reinstall the focuser.

 

In the C90, heavy grease of suitability over the -50 to +150 temperature range goes a long way toward making focusing smooth.  If the focus is central (i.e. helical), that's all you need.  There's no harm in centering the mirror, however.



#190 Quixylvre

Quixylvre

    Explorer 1

  • -----
  • Posts: 76
  • Joined: 23 Sep 2013
  • Loc: Grand Rapids, MI

Posted 11 March 2015 - 02:57 PM

Good to know, thank you for the explanation. Was hoping to hear from PowellAstro or someone else who's DONE that on the C90 how much (if any) that extra layer of tape to take-up slop betwixt the mirror itself and the slider holding it improved the scope's stability.

 

P. S. What about having the moving-mirror focuser for COARSE focus adjustments COMBINED with a more traditional draw-tube focuser for FINE-focus adjustment?


Edited by Quixylvre, 11 March 2015 - 02:58 PM.


#191 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 35770
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 11 March 2015 - 06:03 PM

Good to know, thank you for the explanation. Was hoping to hear from PowellAstro or someone else who's DONE that on the C90 how much (if any) that extra layer of tape to take-up slop betwixt the mirror itself and the slider holding it improved the scope's stability.

 

P. S. What about having the moving-mirror focuser for COARSE focus adjustments COMBINED with a more traditional draw-tube focuser for FINE-focus adjustment?

The problem is back-focus distance.  You don't want to add any length to what sticks out the back of the scope because that moves the primary mirror forward and lengthens the focal length of the scope and reduces the true field of view in every eyepiece.  You want the back end to be as short as possible--preferably no longer than the accessories that come in the box.



#192 Zachrey

Zachrey

    Sputnik

  • *****
  • Posts: 30
  • Joined: 08 Dec 2016
  • Loc: Taos, NM

Posted 04 February 2018 - 03:14 PM

I'm not sure if this stuff is still available but it could be useful for people with open truss dobs like the Meade Lightbridge, ES, Orion Skyquest, etc.

 

https://www.showtex....urs-shakespeare

 

or 

 

https://www.showtex....moliere-tds.pdf

 

 

You have to ask for "ultra black".

 

Apparently this stuff is used in NASA educational programs:

 

https://www.showtex....oonrise-mission



#193 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 35770
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 04 February 2018 - 09:15 PM

https://www.youtube....h?v=FPFZbqbjbP0

http://www.astronomy...um.com/c90.html

related:

https://www.astromar...?article_id=594




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.







Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics