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Viewing sessions with my Istar127mm achro .

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#51 stevew

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Posted 11 September 2013 - 10:49 PM

Steve and David , come on Guys , its a 127mm f8 Achromat what do you expect ?? .
This shot equates to about 90x thru the eyepiece .


Brian.

Bri, the image of Venus, and the Moon look great for color correction! Very impressive for an Achromat.
Congratulations on a fine refractor.
I hope you get many years of enjoyment out of it.

Steve

#52 beanerds

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Posted 12 September 2013 - 12:25 AM

:grin: Sony A200 DSLR straight in the focuser at the native 1000mm fl of the telescope , sized for CN's ( from 1.07mb to 128kb ) , zoomed in and cropped by windows media centre , that's it .
Brian.

Hi Brian,

I believe this image was taken afocally, and it looks to fairly low power given the whole moon is showing and it's small. Most likely you are seeing the objective is being stopped down by the exit pupil being much larger than the camera lens/diaphragm and so it's effectively making the objective a much longer F-ratio than normal, which of course improves all aberrations.

There has been a very detailed discussion of this lately on the ATM forum. The afocal images are only reliable under certain conditions (very high power to keep the exit pupil of the EP small), and a lot of cases of people posting images this has not been the case.

Clear skies,


Interesting observation, Gord. That leads me to ask if the image of Venus is prime focus or afocal?

David



#53 beanerds

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Posted 12 September 2013 - 12:30 AM

:) Thanks Steve , I like the way my Istar performs , well above the $360 I payed for it , I cant be happier with it .
Brian.


Steve and David , come on Guys , its a 127mm f8 Achromat what do you expect ?? .
This shot equates to about 90x thru the eyepiece .


Brian.

Bri, the image of Venus, and the Moon look great for color correction! Very impressive for an Achromat.
Congratulations on a fine refractor.
I hope you get many years of enjoyment out of it.

Steve



#54 Niklo

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Posted 12 September 2013 - 04:54 AM

Yeah, there is a little obvious CA, at least on my monitor, but , what is important , is if it makes you happy?

Hey, it is an achro,...achromats aren't magical......it is all about the physics...

My story;
I had an ISTAR 100/1200 one time, it was "alright".
Little over 1/4 wave, I paid 300 for the objective in cell with shipping.
I sold it for 200 shipped, what I figured it was actually worth.
I bought off the web site , which had an objective with a test report of 1/6th or close as an advertisement of the quality of the objective I was to receive.
Yes, I was disappointed that what I received was not what was advertised.
Was the objective I bought a "good" objective? Yes, I believe so.
Was it a "killer ' objective? No
Can one see the difference between a measured wavefront of 1/4 and 1/6th at the eyepiece? Yes.
Was I about to send it back and complain because I didn't get what I expected to receive?

No, because I don't have the time to play those kind of games. My curiosity has been satisfied.

I sold it with full disclosure of the interferometry report. I am sure the new owner is enjoying the glass and he got a "deal".

Another observer and I actually did a compare between the ISTAR 100/1200, the Carton 100/1300 and a Jaegers 103/1575. All in home-brew OTAs. Simple compare..Saturn. The Jaegars won.

Draw your own conclusions.

Thank you for your report. It's a pity that the objective wasn't 1/6 wavelength. You probably had bad luck :(
How was the CA comparing the Istar 100/1200 with the Carton 100/1300?
Saturn isn't a perfect object for comparing the CA but better than no comparison ;)
Clear skies,
Roland

#55 Jon Isaacs

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Posted 12 September 2013 - 05:26 AM

Brian, Good shot of Venus, hard to believe its only an achromat.
Does Venus show about the same color correction visually?

Steve


When I view Venus in a reflector (or color free refractor) against a dark sky, I normally see the blue and red-orange edges that are the result of the atmospheric refraction. I would expect to see that in this image as well but I am not seeing it.

Jon Isaacs

#56 star drop

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Posted 12 September 2013 - 11:17 AM

When I zoomed in on the Venus image it looks like the bottom of the purple fringe has a very slight reddish tint.

#57 Jon Isaacs

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Posted 12 September 2013 - 05:11 PM

When I zoomed in on the Venus image it looks like the bottom of the purple fringe has a very slight reddish tint.


When I zoomed in, I saw the purple fringe, didn't see any red, it might be there. What makes Venus extra challenging for an achromat is that there is blue-violet, orange-red thing going on underneath the existing chromatic aberration. So if you filter the purple, you filter the atmospheric refraction too.

Jon

#58 beanerds

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Posted 12 September 2013 - 07:42 PM

Thanks for your observations and comments guys I do appreaciate them , yes it has a tiny amount of CA and venus was getting fairly low down in the mirk ( 40*) but as many people and I said its very , very good for a 127mm f8 achromat .
I will try for a Jupiter shot over the weekend and post one or two up for your appraisal .

Thanks ,
Brian.

#59 beanerds

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Posted 15 September 2013 - 05:05 AM

Could not get Jupiter this weekend , morning clouds ,both mornings but I will try again in the morning (Monday) .
Here is the 'Acid" test I use on any telescope again in-case anyone has forgotten , full sun on a white roof .
Brian.

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