Jump to content


Photo

Effect at image plane of "too small" Newt tube?

  • Please log in to reply
13 replies to this topic

#1 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1158
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 03 December 2012 - 03:00 PM

I've been studying Albert Highe's "Portable Newtonian Telescopes". The diagrams in the book and in the accompanying spreadsheets show that an "underdimensioned" tube (or upper cage rings) has much less vignetting (if that is the right word) than I expected. Even for the case where the aperture at the top of the tube equals the diameter of the mirror, the off-axis illumination drops off very slowly, much more slowly than an undersized diagonal or a too-long (or too-skinny) focuser tube.

My main question is: in a truss or strut Newtonian where tube currents are a lesser issue, does having the front aperture smaller than customary affect the image at the focal plane in other ways besides the apparently small loss in off-axis illumination?

#2 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 8241
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 03 December 2012 - 03:47 PM

does having the front aperture smaller than customary affect the image at the focal plane



it brings it closer to the secondary. this enables the use of a smaller secondary. smaller co = better contrast.

#3 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44724
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 04 December 2012 - 06:19 AM

does having the front aperture smaller than customary affect the image at the focal plane



it brings it closer to the secondary. this enables the use of a smaller secondary. smaller co = better contrast.


If one is using a tight upper ring, then the off-axis illumination will be vignetted by the upper cage. One can also use a smaller secondary with larger upper ring and accept a similar amount of vignetting.

But in a scope large enough that trusses and struts are desirable, I am thinking 15 inches, contrast on the planets is almost entirely a function of seeing and thermal management.

Jon

#4 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1158
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 04 December 2012 - 08:43 AM

If one is using a tight upper ring, then the off-axis illumination will be vignetted by the upper cage. One can also use a smaller secondary with larger upper ring and accept a similar amount of vignetting.


What surprised me is that the amount of vignetting isn't similar (according to Highe). Vignetting from the front ring causes a gentle linear drop-off in illumination across the field. For example, I just changed his example spreadsheet from 19" "tube" for a 18" scope to a 18" tube. Vignetting starts immediately at the center of the field, but at radius 1", illumination is still about 94%! The drop-off is linear.

But in a scope large enough that trusses and struts are desirable, I am thinking 15 inches, contrast on the planets is almost entirely a function of seeing and thermal management.

Jon

You may well be right, but if Highe is correct, and there are no other drawbacks (like some induced aberration), then it should be possible to have your cake and eat it too. I assumed the advice to have a tube 1.5-2" larger than the primary applied to truss scopes as well as tube scopes, but now I see no reason to go beyond 1/2", or at most 1", larger diameter at the upper OTA.

So again my original question, perhaps clarified: are there any image degradations (like aberrations) caused by having vignetting at the top of the OTA, other than light fall-off?

I guess mechanical misalignment due to construction or assembly should be considered, so that the vignetting may be asymmetrical.

#5 eyepiecedropper

eyepiecedropper

    Vostok 1

  • -----
  • Posts: 108
  • Joined: 31 Dec 2011
  • Loc: aschaffenburg, germany

Posted 04 December 2012 - 11:55 AM

hi, i don´t think so.
centering the optical axis/collimation could be more criticial to avoid an uneven field illumination or to much vignetting. but visually only in theory.

some thoughts:
orion.uk intentionally has very small tube diam. which is the restricting part of the 100% illuminated fov here. they say everything´s a compromise. with my 8"f8 solid tube orion.uk i have severe tube currents btw.

intes-micro with their mak-newts have a very very low 100% illuminated fov of about 3mm and less and they are the best of all scopes i´ve used visually.

#6 KerryR

KerryR

    Gemini

  • *****
  • Posts: 3068
  • Joined: 05 Dec 2007
  • Loc: SW Michigan

Posted 04 December 2012 - 12:40 PM

Gary Seronik has an interesting article on his website about field illumination. That article suggests that it's not necessary to have a large full-field illumination, unless observing variable stars. If memory serves, the article suggests that as long as edge of field illumination doesn't drop below %50, and the %100 rays are not vignetted, the full field size is of little concern for most visual observers; i.e. it can be, essentially, zero diameter. The article is specific to secondary mirror size selection, but the take home message would, I think, apply to tube diameter/vignetting as well.

The article might be worth a look in understanding the impact of vignetting, even though this is secondary specific...
Sizing Up the Newtonian Secondary

As others have pointed out, thermal management is probably a larger concern, this could also apply to open tube newts at the mirror box opening, depending on design.

#7 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1158
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 04 December 2012 - 12:54 PM

The Highe book and spreadsheets calculates the vignetting from the OTA, the diagonal, and the focuser tube, and puts them on a nice chart. The vignetting is additive, BTW.

I wouldn't think of a very tight diameter on a tube Newtonian, since I don't know how to deal with the tube currents, and I have seen the effect they can have. But on a truss OTA, where most of the optical path is open (or inside the mirror box and the possible shroud quite large), then the smaller upper cage diameter seems to make sense.

I think I finally understand why Dave Kriege puts a 13" upper ring ID on his 12.5" scopes.

#8 operascope

operascope

    Mariner 2

  • *****
  • Posts: 253
  • Joined: 03 Sep 2008
  • Loc: Canada

Posted 04 December 2012 - 01:49 PM

Check out this web page for a clear explanation. Of course, this is for truss scopes, where tube currents aren't a problem.
http://www.bbastrode...rUpperCage.html

#9 Nils Olof Carlin

Nils Olof Carlin

    Surveyor 1

  • *****
  • Posts: 1624
  • Joined: 26 Jul 2004

Posted 04 December 2012 - 03:09 PM

What surprised me is that the amount of vignetting isn't similar (according to Highe).



Making a drawing should make this quite obvious - the secondary is much smaller and more affected than the upper tube opening. Even worse is vignetting by an overly tall and narrow focuser drawtube - possible but not common I hope.
So, if making the tube opening minimal saves significant space, go ahead, but at least see to it that the optical axis is centered, by offsetting the secondary properly (down the tube as always, but also away from the focuser, as often omitted if the tube is wide).

Nils Olof

#10 cjc

cjc

    Ranger 4

  • -----
  • Posts: 358
  • Joined: 15 Oct 2010
  • Loc: Derbyshire, England

Posted 04 December 2012 - 06:36 PM

I also have had problems with tube currents on an Orion UK 8" with the older closed back 3-point cell. I have installed a 40mm fan, but am about to upgrade it to 60mm, to give closer to the throughput recommended by Bryan Greer.

The end ring is only 202mm internal diameter for a mirror that is 199mm, but about 196mm once TDE is taken into account. The secondary looks to have a bigger vignetting effect, but in practice I have not found it visually to bean issue.

#11 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1158
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 04 December 2012 - 09:23 PM

What surprised me is that the amount of vignetting isn't similar (according to Highe).


Making a drawing should make this quite obvious - the secondary is much smaller and more affected than the upper tube opening. Even worse is vignetting by an overly tall and narrow focuser drawtube - possible but not common I hope.
So, if making the tube opening minimal saves significant space, go ahead, but at least see to it that the optical axis is centered, by offsetting the secondary properly (down the tube as always, but also away from the focuser, as often omitted if the tube is wide).

Nils Olof

I am greatly reassured I am on safe ground, hearing from one who has put long productive thought into these things. I will indeed do my best to address optical centering.

The drawtube is not my problem, but Highe shows it can be, dependent on the ratio of drawtube width to length. And indeed the effect is severe: you might practically count on the beginning of drawtube vignetting as being the edge of the visible field, since the falloff in illumination is so severe.

Tack sa mycket!
Lee (who once had a personnummer and is hoping for a small Swedish pension:)

#12 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1158
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 04 December 2012 - 09:27 PM

Check out this web page for a clear explanation. Of course, this is for truss scopes, where tube currents aren't a problem.
http://www.bbastrode...rUpperCage.html

Bartels' observations are corroborated and quantified in Highe's book. Good link - I have followed Mel's writings but missed this one.

#13 Nils Olof Carlin

Nils Olof Carlin

    Surveyor 1

  • *****
  • Posts: 1624
  • Joined: 26 Jul 2004

Posted 05 December 2012 - 06:54 AM

I will indeed do my best to address optical centering.


You can (fairly) easily check your unvignetted field of view - if you like the learning experience.:
put a piece of clear plastic (OH foil, cling film?) over the focuser opening (not far from the focal plane). Look from some distance, and move your eye until you see the outlines of the secondary (smaller) and primary (larger) just touch - mark the line of sight on the film (with a sharpie or similar), and repeat at several orientations.
Now you have the circle outlined (or ellipse?) of the field unvignetted by the secondary. You can do the same by marking where the tube opening just becomes visible (if at all!) at the edge of the primary (it is on the opposite side of the former). Now you see if the vignetting is symmetric enough, or if you need to adjust the position of the secondary (to/from the focuser in the latter case, up/down the tube in the former - unless sideways of course).
(The image was taken in the opposite direction - down the tube, showing the focuser opening with markings on the film).

Nils Olof

Attached Files



#14 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4456
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 05 December 2012 - 07:06 AM

Here's a puzzle. What about the vignetting behavior when using a Paracorr? The field lens of the Paracorr is far in front of the focal plane and doesn't the Paracorr serve also as a kind of relay lens? The only way I could estimate vignetting with the Paracorr in place was to look at out of focus star images.

JimC






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics