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Istar 8" F9 OTA Build

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#26 Jeff B

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Posted 13 January 2020 - 09:04 PM

Finally in blue.

Attached Thumbnails

  • 8 Istar, Chromacor U2,  Blue, Inside, full aperture.jpg
  • 8 inch Istar, Blue, Inside, full aperture.jpg
  • 8 Istar, Chromacor U2,  Blue, outside, full aperture.jpg
  • 8 inch Istar, Blue, Outside, full aperture.jpg

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#27 Jeff B

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Posted 13 January 2020 - 09:23 PM

As you can see, there is a nice improvement in the SA for all three colors, especially in blue.  However, the real news is that I could not repeatably measure much difference in focus between the three colors.  Before adding the Chromacor, I could repeatably measure the red and blue falling about 1/8" longer than green.  I suspect I will get even better results when I start tweaking the Chromacor's position in the light path.  So the things work.  Actually, I already knew that from much experimentation and star testing but it's nice to see it in DPAC too.

 

Next I'll stop the lens down to 7" and maybe 6.5"  and do the DPAC thing again.

 

Jeff


Edited by Jeff B, 14 January 2020 - 10:19 AM.


#28 Mark Harry

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Posted 14 January 2020 - 03:59 PM

I could repeatably measure the red and blue falling about 1/8" longer than green.
**********
 As it should be. Interesting last set of pics. I'd tend to shoot for a position where blue and green  have the best compromise, and let red be a bit loose, since your eye is likely less sensitive to it anyhow. Just my 2 cents.
I'd stand in line for a look-see.


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#29 plyscope

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Posted 14 January 2020 - 05:31 PM

"I'd stand in line for a look-see."

 

Me too, when are you coming to Australia Jeff? grin.gif


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#30 Jeff B

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Posted 14 January 2020 - 05:48 PM

I could repeatably measure the red and blue falling about 1/8" longer than green.
**********
 As it should be. Interesting last set of pics. I'd tend to shoot for a position where blue and green  have the best compromise, and let red be a bit loose, since your eye is likely less sensitive to it anyhow. Just my 2 cents.
I'd stand in line for a look-see.

I did some further tweaking to try to do exactly that Mark.   

 

First I pushed the Chromacor U2 forward an extra inch which helped even more but blue was still overcorrected, though green and red were looking very good.  Then I stopped the lens down to 7" and put in a 2" spacer giving a total spacing of the Chromacor around 6" from the focal plane.  With the Chromacor U2, blue was now well corrected but green and red actually turned a tad undercorrected.  Substituting my Chromacor U1 for the U2 made for excellent results in green and red with blue now about 1/6 to 1/7 wave over corrected with, again, no real differences in focus positions for the three colors.  This configuration seems to be an excellent balance.   I will stop the objective down to 7" or 6.5" (F10.2 to 11.1) when using the Chromacor with my bino-viewer in tow.   I will take additional DPAC pictures and also experiment with using the Baader 1.25x and 1.7X GPC's in the the viewer too.  I'll post these shots in the thread I started in the refractor forum, which is where we typically discuss the effects of using a Chromacor.

 

Jeff


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#31 Jeff B

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Posted 14 January 2020 - 05:48 PM

"I'd stand in line for a look-see."

 

Me too, when are you coming to Australia Jeff? grin.gif

Just send me a ticket Andy. lol.gif


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#32 MalVeauX

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Posted 15 January 2020 - 07:46 AM

This looks very interesting Jeff! I've been eye-balling this objective for a while for use in an ultra-narrow-band system and have been curious about its performance at single wavelength.

 

Did you build the tube or did you source it? My main issue is figuring out how to mount the objective cell in either a truss system or a tube.

 

Very best,



#33 rwiederrich

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Posted 16 January 2020 - 01:57 PM

Fascinating Jeff.  I'm still confused about the acceptability of the inner zones that appear depressed...and the imperfect bands I find questionable too.

 

I'm probably letting my mirror making experience get in the way of what objective glass ronchigram qualifiers look like.

 

Where are you going to use this scope at?  In your OB next to your 11", or at home out on one of your mounts there?

 

I'm impressed that you are *still* making refractors.    I have a 5" on the bench that I just can't seam to get to any more...….

 

 

Rob



#34 Jeff B

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Posted 16 January 2020 - 04:47 PM

I'm glad to hear from you Rob.  I hope you are well.

 

Remember, the photos I'm showing are in double-pass so everything looks twice as bad as it really is, including the "bowl" so the curving and bit of waviness in the lines is really not the bad except in blue.  But that's just it, a mirror with a bit of SA will give a similar level of SA for all colors.    

 

Yeah, this will more than likely be my last large refractor ATM project.   I'm doing it mostly for the challenge of making a large, slower, refractor OTA as light as I can.  I did one once before but was not happy with the lens, even stopped down.  This is another go at it but with a better lens and I now have an excellent DPAC set up to have good looks at what's going on with a lens and where on the lens that going on is....going on.  And it really lets me see the effects of my various Chromacors.  DPAC is a fabulous tool.

 

Jeff



#35 Jeff B

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Posted 16 January 2020 - 09:39 PM

Comparisons at full and stopped down to 7".  So the lens is modestly more "civilized" in SA when stopped down to 7".  Just hover the cursor over the image for the caption.

 

 

Attached Thumbnails

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


#36 Jeff B

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Posted 16 January 2020 - 10:02 PM

Here is what happens with the worst offender, blue, when the lens is stopped down to 7", and I position my Chromacor UI about 6.5" from the focal plane.  

 

The Istar is a notably more civil now.  As a reference, I've also included a DPAC shot of my TEC 140ED, in blue, outside of focus.  Again, hover the cursor over the images to read their captions.

 

Also, in this configuration, and, more importantly, without touching the focuser, when I focused to a null in green then carefully and precisely substituted the red and blue LEDs and their matching color filters (same brand) for those in green, I saw nulls for red and blue as well, which basically means the focus position for each color fall on top of each other.   Without the Chomacor UI but still stopped to 7", red and blue focused measurably forward of green, just like an achromat should.

 

Barring any significant astigmatism or coma, I suspect that this lens will produce rather sharp, low color error on-axis images at high power when stopped to 7" and used with the Chromacor UI properly placed in the light path.

 

Jeff

Attached Thumbnails

  • 8 inch Istar, Blue, Outside, full aperture.jpg
  • 8 Istar, Blue, 7 Stop, Outside.jpg
  • 8 Istar, Blue, 7 Stop, UI, Outside.jpg
  • 140ED Blue Outside of Focus.jpg


#37 Jeff B

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Posted 16 January 2020 - 10:13 PM

This looks very interesting Jeff! I've been eye-balling this objective for a while for use in an ultra-narrow-band system and have been curious about its performance at single wavelength.

 

Did you build the tube or did you source it? My main issue is figuring out how to mount the objective cell in either a truss system or a tube.

 

Very best,

Thanks!

 

I designed and built the tube myself from Hastings 9.0" O.D, .050" wall tubing. 

 

https://www.hastings...laneous-tubing/

 

I had the "counter cell" for the objective cell made locally but it was designed my me as was the focuser backplate that attaches the focuser to the tube (also machined by D&D) : 

 

https://demachine.com/

 

You can see my fancy drawings above on page 1.   I designed and made the baffle system myself though.

 

Jeff 


Edited by Jeff B, 16 January 2020 - 10:17 PM.

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#38 MalVeauX

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Posted 19 January 2020 - 09:53 AM

Thanks!

 

I designed and built the tube myself from Hastings 9.0" O.D, .050" wall tubing. 

 

https://www.hastings...laneous-tubing/

 

I had the "counter cell" for the objective cell made locally but it was designed my me as was the focuser backplate that attaches the focuser to the tube (also machined by D&D) : 

 

https://demachine.com/

 

You can see my fancy drawings above on page 1.   I designed and made the baffle system myself though.

 

Jeff 

Thanks Jeff! waytogo.gif

 

Very best,




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