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

Chromacor Effects in DPAC

  • Please log in to reply
8 replies to this topic

#1 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 14 January 2020 - 10:26 AM

During my DPAC testing of an Istar 8" F9 objective lens for an OTA build I'm doing, I installed a Chromacor I, model U2 in the light path ~ 4" from the focal plane.  This is sub-optimal for this particular lens but the DPAC images do show that the thing really does work.

 

The images and discussion can be found at this thread here:

 

https://www.cloudyni...8-f9-ota-build/

 

Beginning at I believe, posting number 24.

 

Cool stuff.  

 

Jeff

 


  • eros312 likes this

#2 francesco italy

francesco italy

    Vostok 1

  • *****
  • Posts: 150
  • Joined: 07 Apr 2008

Posted 15 January 2020 - 03:12 AM

Thank you Jeff for your work, very interesting, I am trying a Chromacor N II on my 250 f11 Istar R30 "Prometeo" , waiting for a N I unit, can't wait to have news from you.

 

aa2.jpg


  • Jeff B likes this

#3 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 16 January 2020 - 05:09 PM

So here is an interesting update.

 

I stopped the lens down to 7" giving an F ratio of ~10.2.  I did this as I want to use my Chromacors with my bino-viewers without an OCS/GPC/Barlow type of element in the optical path for high power use on solar system and double star objects.  My preferred configuration to do that is my viewer in conjunction with either my Baader BBHS silver or Zeiss prism diagonal using the Baader quick change system.  This gives the shortest light path possible while still giving me room to properly place a Chromacor in the light path and not have its inlet clip the aperture.   

 

Just stopping the 8" F9 aperture to 7" F10 improved its performance, especially in blue where it is rather over-corrected (actually, the objective is "polychromatically" over-corrected, meaning it is also a bit over corrected in green and red as well).  Here are the DPAC photos stopped to 7".

 

Hover the cursor over the photo to read the caption.

Attached Thumbnails

  • 8 Istar, Green, 7 Stop, Inside.jpg
  • 8 Istar, Green, 7 Stop, Outside.jpg
  • 8 Istar, Red, 7 Stop, Inside.jpg
  • 8 Istar, Red, 7 Stop, Outside.jpg
  • 8 Istar, Blue, 7 Stop, Inside.jpg
  • 8 Istar, Blue, 7 Stop, Outside.jpg

Edited by Jeff B, 16 January 2020 - 05:10 PM.


#4 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 16 January 2020 - 05:40 PM

Now compared with the full aperture, outside of focus but still with no Chromacor.

 

As you can see the lens is a bit more "civilized" in terms of spherical aberration when stopped to 7" and you can see that as well in the at-focus green image too, with the bowl a bit less emphasized over the reduced aperture.

Attached Thumbnails

  • 8 inch Istar, Green, outside, full aperture.jpg
  • 8 Istar, Green, 7 Stop, Outside.jpg
  • 8 inch Istar, Blue, Outside, full aperture.jpg
  • 8 Istar, Blue, 7 Stop, Outside.jpg
  • 8 Inch Istar, Red, Outside, Full Aperture.jpg
  • 8 Istar, Red, 7 Stop, Outside.jpg
  • 8 inch Istar, Green, At Focus, full aperture.jpg
  • 8 Istar, Green, 7 Stop, Focus.jpg

Edited by Jeff B, 16 January 2020 - 05:44 PM.


#5 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 16 January 2020 - 06:00 PM

And here is what happens when I installed my Chromacor UI about 6.5" forward of the focal plane.  In this position, the Chromacor adds ~ 1/6 to 1/8 wave of undercorrection, in an attempt to compensation for the native overcorrection of the objective.   

 

And, indeed, that's exactly what it does in a modest but nice way.  Now with the bino-viewer/diagonal system installed, the Chromacor UI will sit about an inch or so more forward and apply even better SA correction.

 

 

Again, hover the cursor over the images to read the captions.

 

 

Attached Thumbnails

  • 8 Istar, Green, 7 Stop, Outside.jpg
  • 8 Istar, Green, 7 Stop, UI, Outside.jpg
  • 8 Istar, Red, 7 Stop, Outside.jpg
  • 8 Istar, Red, 7 Stop, UI, Outside.jpg
  • 8 Istar, Blue, 7 Stop, Outside.jpg
  • 8 Istar, Blue, 7 Stop, UI, Outside.jpg
  • 8 Istar, Green, 7 Stop, Focus.jpg
  • 8 Istar, Green, 7 Stop, UI, Focus.jpg


#6 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 16 January 2020 - 06:33 PM

Here are comparisons, first in blue, from full aperture/no Chromacor to 7"aperture/with Chromacor U1 and then at focus in green between the full aperture sans Chromacor UI and stopped to 7" with the Chromacor UI.

 

The lens is indeed, decidedly more civil in SA performance (meaning straighter lines) stopped to 7" + the Chromacor than at the full 8" aperture without the Chromacor.

 

What's not shown though is that with the 7" stop and Chromacor in place, when I focused to the green null you see in the picture and then carefully and precisely changed out the green LED for the red and blue ones along with their matching color filters (all the same brand), I was still seeing nulls in red and blue.  This means all three colors were focusing pretty much on top of each other.   Without the Chromacor in place, the red and blue focused measurably further out than green by maybe 5 to 6/64" or so.

 

So here you have non subjective proof that the Chromacors really do work when properly implemented.  They not only reduce the longitudinal CA (due to focus differences from red to blue) but also the spherochromatic contribution to CA (the changes in spherical aberration from red to blue).  

 

I have a feeling that, once I get the scope out under the stars and a good view of the Moon, and barring any significant astigmatism and coma, the views at 7" aperture with the Chromacor U1, though dimmer, will be considerably sharper and with significantly less "CA" that at full aperture without the Chromacor.

 

Another great use for DPAC!

 

Jeff

Attached Thumbnails

  • 8 inch Istar, Blue, Outside, full aperture.jpg
  • 8 Istar, Blue, 7 Stop, UI, Outside.jpg
  • 8 inch Istar, Green, At Focus, full aperture.jpg
  • 8 Istar, Green, 7 Stop, UI, Focus.jpg

Edited by Jeff B, 16 January 2020 - 06:40 PM.


#7 ValeryD

ValeryD

    Vendor (Aries)

  • ****-
  • Vendors
  • Posts: 1,942
  • Joined: 26 Nov 2005
  • Loc: Kherson, Ukraine.

Posted 17 January 2020 - 03:09 AM

Hi Jeff,

 

The Chromacor saga continues. :)  Nice to know.

 

It looks like this achromat was made for H-a  solar works.  This makes it nully corrected at 656nm and slightly overcorrected in green and very severe overcorrected in blue.

 

UI should correct green and especially blue while barely touch red.

 

I'd better try to save the aperture maxially (just stop down the turned edge). 

 

BTW  do you still own and operate the big cannon  11" D&G ?

 

 

Valery



#8 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 17 January 2020 - 11:55 AM

Hi Valery!

 

I'm glad you are watching.  Yes, the Chromacor story continues but now with some actual testing.

 

I had imagined that H-a might be a good use for the scope but the retainer ring on the front of the cell makes no mention of it, even though Istar did indeed make such H-a lenses.

 

"UI should correct green and especially blue while barely touch red".  And that is precisely what is happening as you can see in the DPAC pictures wink.gif .

 

I tried a 7.5" stop and it completely covers that regional turned edge but that does not bother me for low power visual use where I want maximum aperture.  The 7" stop seems to be a really good aperture for higher power, especially with the bino-viewer, Chromacor UI and the Baader diagonals (with their shorter light paths).  I still need to do a star test and I'm waiting for a clear (and WARMER) night as well as letting some strained rib muscles to heal (takes forever).

 

In the mean time, I'm going to use the scope indoors to test each one of the Chromacors I have to make sure their labels are correct.  For example, the UI I've been using is marked as an OI but the model U2  is indeed U2.

 

I still have access to and use the 11", F12.3, D&E naval gun.    An exceptional achromat and works very well at full aperture with the bino-viewers and the Chromacor O1, even though the lens is rather neutral in green correction (have not checked red and blue).  However, around here due to poor seeing it is rare to use the full aperture for high power solar system viewing so I made 9.5", 8.5" and 7" aperture stops.  The 8.5" stop (~F16) in combination with the viewers and the U1 with 4" of extra spacers on the diagonal nose is a very, very good combination (!) and often used.

 

Jeff


  • peleuba likes this

#9 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6,299
  • Joined: 30 Dec 2006

Posted 19 January 2020 - 10:49 AM

I had an interesting chat with Valery.  I wanted to be sure that what I am seeing in DPAC is an accurate representation of what I may seeing on real objects.   Indeed, there is the possibility that as DPAC doubles the errors it may double the effects of the Chromacor too, and mitigate alignment issues.  

 

So, like all bench testing, I will verify it under the stars.  

 

But it's so cold right now.  coldday.gif cloudy.gif coldday.gif




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