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Old 23-05-2010, 03:06 PM
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Paracorr Focusing Tests

I have performed some Focusing tests with the Paracorr and my 200mm / F6 Newtonian, which I think will be of interest to anyone using or purchasing a Paracorr.
The Televue recommended spacing between the Paracorr and the imaging chip is 55+/-4mm; however I have found that a tolerance of +/-4mm does not appear to ensure that good focus is achieved.
Here are the test shots that I have taken (refer to the text underneath for the test descriptions):
http://upload.pbase.com/david_fitz_h...11410/original
Note that the optimal focus distance between the CCD chip and Paracorr will also most likely vary with the focal ratio and aperture.
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Old 23-05-2010, 04:17 PM
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Interesting test David. Do you think the improved focus at larger spacing is a function of the scope being f6?

I've got my paracorr at 55mm spacing with the qhy8 on my f5 Newt. I focus on a star using a Bahtinov mask then slew to the object I'm after.

Here's how the centre and corners of my image are turning out.

Peter.
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Old 23-05-2010, 04:51 PM
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Quote:
Originally Posted by peter_4059 View Post
Interesting test David. Do you think the improved focus at larger spacing is a function of the scope being f6?

I've got my paracorr at 55mm spacing with the qhy8 on my f5 Newt. I focus on a star using a Bahtinov mask then slew to the object I'm after.

Here's how the centre and corners of my image are turning out.

Peter.
Yes, I definitely believe that the optimal focus position depends on the f-ratio and perhaps the aperture. Televue have not published any data on this that I can find, but ASA have published data on their 3" Keller corrector for Newtonians which shows this. Of course they may be different optical systems, but it seems likely that there will also be some relationship for the Paracorr. In the Keller design, the longer the f-ratio the larger the optimal spacing; also for a given f-ratio, the larger the aperture the larger the spacing.

Looking at your images it is a little hard to tell, as the top left / bottom right stars look elongated, but the others look closer to round. I suspect that a larger spacing will be optimal for you based on my experience, but you probably first need to find out why the focus is not symmetrical in the field. Maybe collimation or image tilt - I think someone suggested before that you take some test shots and rotate the camera to see how that affects the image.
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Old 23-05-2010, 04:57 PM
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David,

Based on CCD inspector it looks like the colimation is out (7.2") on my scope causing the slight elongation in top left and bottom right. The tilt results reported tilt in X -0.1" and tilt in Y -0.1". Total tilt 4% @123 deg.

I'm expection a spacer set to arrive soon so I'll be able to try a few different spacings.

Peter
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Old 24-05-2010, 12:02 PM
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Hello Peter,
yes I think you are right about it being collimation. If it was just tilt I would expect the elongation at opposite corners to be at right angles to each other. Using a Paracorr with the CCD beyond focus results in sagittal elongation (ie. in line with the centre of field), and having the CCD inside focus results in tangential elongation (at right angles to a radial line going through the field centre). So, given that the centre of field is in focus, if tilt was causing the distortion one corner would be inside focus and the opposite corner outside focus.
I'm a little skeptical of CCDInspector's conclusions though - I did a rough
collimation a while back and it told me I was within 1" collimation error, which I found hard to believe. Even your 7" doesn't sound like enough to cause the focusing error that you have? Also, I'm not sure how it can unambiguously determine tilt, given that different optical systems distort in different ways in / out of focus? Maybe someone else can shed some light?
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Old 24-05-2010, 05:26 PM
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David,

I don't think I could have tilt between the paracorr and the QHY8 as it is all threaded connections (unless Mr Televue has a QC problem). There could be tilt between the paracorr and the focuser draw tube. In terms of colimation I'll have to check the focuser and the mirrors.

Not sure about CCD inspector and how it works as I don't have any experience using it.

Peter
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Old 26-05-2010, 11:25 PM
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David,

Thanks a lot for posting this - very interesting and a very good end result.
Are you using the Paracorr designed for STL PSB-1100? If so, how hard is it to adjust the distance?

James
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Old 27-05-2010, 12:13 PM
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Quote:
Originally Posted by Moon View Post
Are you using the Paracorr designed for STL PSB-1100? If so, how hard is it to adjust the distance?

James
Hello James,
No, I'm not using the STL PSB-1100 Paracorr. I'm using the Universal Paracorr with Tele Vue's 2.4" Imaging System (IS) camera adapters / spacer rings. Televue states on their website that the STL PSB-1100 is suitable for systems ranging from f3.5 to f5. The Paracorr optics are suitable for systems from f3.5 up to f8. My current system is f6, and as I also used it with a DSLR for a short period I bought the Universal Paracorr. I initially used the adapters recommended by Televue (ie. a 0.375" + 0.25" + STL adapter) which gave a distance to CCD chip of 54.9mm (taking into account the optical backfocus of 37.3mm for the STL-11000). I have found that this is not good enough however, and that using 0.5" + 0.25" + STL adapter (giving a distance to CCD of 58.1mm) gives greatly improved results.

From these tests, I am skeptical that the STL PSB-1100 would give optimal results over the whole f3.5 to f5 range, although I am not able to test this currently.
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Old 27-05-2010, 04:33 PM
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James, just further on this - I assume then that you have an STL PSB-1100? If you want to test a different spacing, you could do what I did as an initial test before I got the extra adapters / spacers. I cut out some large "washers" from some hard plastic sheet so that they fit over the threaded section of the adapter (that screws into the STL-11000) and are thus seated on the flange at the base of the threads. I bought the plastic sheet from an art & crafts store in town (Eckersley's) that was 0.6mm thick, and made three of them for a maximum added space of 1.8mm. Since your scope is f5, it is more likely that you will want added distance to the chip rather than less (my 200mm/f6 needed an extra 3mm over the 55mm optimal distance that Televue quotes).
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Old 27-05-2010, 04:40 PM
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David,
No I don't have one - I'm just thinking about putting together a 12.5 custom newt rig similar to yours sometime down the track. Perhaps I should wait and see how you go!
But I will try out the spacing technique on my existing RC and see what happens.
James
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