abutu
sage
Reged: 03/09/07
Loc: Gijón (Asturias) - SPAIN
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Re: Startest of 4" apo
[Re: leexeen]
#2528187 - 07/19/08 11:26 AM
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Is the APO finally a TSA102 or a NP101?
My TSA has identical ring patterns inside/ouside focus. But my FS60 has the similiar problem as yours. It is a little undercorrected. But the FS60 is in every way a FANTASTIC scope. Remember that the star test is very very sensitive even to the slightest errors. So no need to be too worried with a imperfect ring pattern.
Me too. My TSA102 has also identical patterns at both sides of focus point. However, in my previous scope, a TV85, I had the same result that the author comments, the ring patterns inside and outside the focus were quite different. I don´t think the seeing is a factor to explain these differences. I think that a doublet can not offer the same pattern to both side of focus.
Regards,
Alex
Edited by abutu (07/19/08 11:28 AM)
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deSitter
Still in Old School
Reged: 12/09/04
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Re: Startest of 4" apo
[Re: peleuba]
#2528436 - 07/19/08 02:02 PM
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An ugly infocus diffraction pattern? Meaning what?
Not perfectly round, and too many rings. A perfect Airy disk will actually look shaded, like Jupiter, dim on the sides. If your scope shows that, and 1 ring and a hint of a second on say a pretty dim naked eye star, then it can't be anything but good, and it does not matter at all what the out-of-focus tea leaves are revealing.
-drl
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lukman
super member
Reged: 06/27/04
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Re: Startest of 4" apo
[Re: Daniel Mounsey]
#2529046 - 07/19/08 08:45 PM Attachment (31 downloads)
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Daniel, Ridwan, Eddgie, Leexeen, Drl, Paul, Chris, Alex…Thank you very much… I did not expect that many responses. There is a lot of good information in this thread and I need to read it a few times to understand it.
Today I went to the local astro shop and bought a Green filter (#56). I was lucky and got the chance to test my setup with a webcam. It was clear just for an hour or two before the clouds roared in and began raining. I did not have time to do a good star test. I did capture 30 seconds avi in-focus and another 30 seconds avi out-focus with a green filter. RegiStax has been used to capture the following photos. Please do keep in mind that these photos don’t tell the whole story at the moment. I need to do a proper star test yet with the webcam in combination with a green filter. I would appreciate any advice how to capture the in-focus and out-focus image with a webcam.
The imaging train that I used:
Refractor--2x Barlow--green filter--webcam
The first image is in-focus.
Edited by lukman (07/19/08 08:56 PM)
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lukman
super member
Reged: 06/27/04
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Re: Startest of 4" apo
[Re: lukman]
#2529054 - 07/19/08 08:51 PM Attachment (31 downloads)
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This is the out-focus image...
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leexeen
scholastic sledgehammer
Reged: 08/08/07
Loc: Beijing, China
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Re: Startest of 4" apo
[Re: lukman]
#2529106 - 07/19/08 09:28 PM
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Lukman, the outside-focus image seemed racked outward more than the inside-focus image is racked inward. And also it is better to limit the rings to 3. From your pic it is a lil hard to judge.
Edited by leexeen (07/20/08 12:29 AM)
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lukman
super member
Reged: 06/27/04
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Re: Startest of 4" apo
[Re: leexeen]
#2529127 - 07/19/08 09:44 PM
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Leexeen, rain and clouds spoiled the test. This was a quick test. Next time I will try to capture 3 rings and also pay attention to the image size. Though it’s very difficult with a webcam, the image is just too bright with only three rings visible. Maybe I need to adjust brightness of the webcam.
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peleuba
professor emeritus
Reged: 12/01/04
Loc: Southern PA
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Re: Startest of 4" apo
[Re: abutu]
#2529334 - 07/20/08 12:12 AM
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However, in my previous scope, a TV85, I had the same result that the author comments, the ring patterns inside and outside the focus were quite different. I don´t think the seeing is a factor to explain these differences. I think that a doublet can not offer the same pattern to both side of focus.
The ring pattern in extra-focal image was less "contrasty" primarily due to spherochromatic aberration (sphercochromatism). This is one aberration in the TV85 that is not corrected to the level of the modern triplets.
Using green filters #56 and #58 will isolate the wave length of light that our eye is most sensitive to and that which most lenses are nulled. On the TV85, I have seen where the Fresnel pattern cleans up nicely and is nearly identical on both sides of focus when star testing using these filters alone and then while stacked.
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THESKY
scholastic sledgehammer
Reged: 02/16/06
Loc: SOCAL
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Re: Startest of 4" apo
[Re: peleuba]
#2530706 - 07/20/08 07:39 PM
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Hey Lukman, I am glad that you have a chance to capture the star test. If you persist with the star test and keep the testing a life you will learn a lot. I think that it is a very useful skill you can to acquire as an amateur Astronomer.
Another test you want to consider is called Roddier Test. They have a group discussion on Yahoo group. Basically it is a software that analyze a star test image. I have not try the software myslef but it is my next thing on my to do list. Have fun.
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THESKY
scholastic sledgehammer
Reged: 02/16/06
Loc: SOCAL
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Re: Startest of 4" apo
[Re: peleuba]
#2530707 - 07/20/08 07:39 PM
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Hey Lukman, I am glad that you have a chance to capture the star test. If you persist with the star test and keep the testing a life you will learn a lot. I think that it is a very useful skill you can to acquire as an amateur Astronomer.
Another test you want to consider is called Roddier Test. They have a group discussion on Yahoo group. Basically it is a software that analyze a star test image. I have not try the software myslef but it is my next thing on my to do list. Have fun.
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lukman
super member
Reged: 06/27/04
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Re: Startest of 4" apo
[Re: THESKY]
#2530892 - 07/20/08 09:28 PM
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Thank you R_Angkasa. I just did a quick research on the the Roddier Test. The program seems to be in French. I found the following link:
http://www.cityastronomy.com/roddier-6inchachro.htm
The only small problem that I have with the program at the moment is stacking images. It seems if you stack images you may loose some contrast and some details me be smeared. For sure would love to hear your opinion when you have tried the Roddier Test.
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