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View Full Version here: : Parcorr first light....


OneOfOne
10-11-2006, 08:04 AM
Well last night, Thursday, was the first clear night for yonks! The day was even clear enough to get the PST out a few times at work to show off the transit.

I decided it was a perfect time to get the scope out and give the Paracorr a play for an hour or so, I bought it 2 weeks ago. The first thing you notice with this is that it is a pretty heavy piece of glass and ali! The weight is comparable to one of my Pentax's. It is 2" and is used in a similar way to a Barlow, most of the length slides inside the focuser so you only need to pickup 30 odd mm of in focus to get it working. I didn't have any problems bringing any of my Pentax's to focus. It does create some problems doing a comparison as you have to keep an image in your head while you take out the eyepiece, put in the Paracorr, eyepiece back and adjust focus. The Paracorr also adds 15% to the magnification which could affect the results slightly, although I expect making this small a difference should not be the only reason why stars may "pop into view". All tests were done with my Pentax XWs. Next time, I will try out the Panoptic24, Stratus and the Plossls I don't use any more.

First target 47Tuc...I put the 7 in on its own. I could see good resolution of the surrounding stars. As far as I was concerned, the image was pretty damn good. Put in the Paracorr. Well, unfortunately, although the view without it was "pretty damn good" I now found myself struggling for a description to apply to the view with the Paracorr in. Although I would not say it was an amazing difference, it was quite noticeable. Stars seemed to be just that little bit "tighter", the dots appeared smaller and brighter. I felt that some stars in the centre were now resolved there weren't before! The edges seemed to have more stars?

Anyway, that may have just been my expectations. I moved onto M7. This cluster is fairly large and so I changed to the 10. The view was nice and I took note of some stars near the edge of the field and some that were right on the limit of vision. Drop in the Paracorr and stars that were visible with averted vision were visible directly now. Again, the stars just appeared to be slightly smaller and..well...neater? At this point I began to think that maybe it wasn't just my expectation after all, or not wishing to think I wasted my money...there was a difference, and the difference was good.

I tried a few more objects and found much the same, there just appeared to be a few more stars. This may be caused by the slightly higher magnification, but I don't know if going from 100x to 115x would produce this degree of effect. Next time I may try the XW-7 on it's own against the Stratus 8 with the Paracorr. This should give very similar magnification, and I would "expect" that the Pentax should produce a slightly better image.

Anyway, I decided to give NGC253 a go. I have never been able to see this in my backyard before, the yard last night was bright enough I could read the writing on the eyepieces without need of a light. Swung it around to 253 with the 7. I looked carefully, and could see a couple of stars but nothing else. I could glimpse "something" there with averted vision, but when I looked, it was blank. Put in the Paracorr and there it was! Not exactly obvious, but if I looked, I could see a definite smudge, no detail to speak of but it was there! Took the Paracorr out and tried again, it was gone.

Last test was Neptune and Uranus. The improvement here was quite marginal, but then again, how much detail do you expect to see on either at the best of times. I would like to say the moons jumped into view...but that would be an lie.

The Paracorr has a adjustment that moves the distance between the eyepiece and the correcting optics, TeleVue have a table of settings for their eyepieces. I guess I would have to spend some time trying different settings for each eyepiece to find the optimal setting, but I tried the two extremes with the 7 and could not find a large difference between them, but maybe the correct position was in the middle...duh! The image was still excellent without taking this adjustment into account.

In summary? It makes a discernible difference, to me at least. I checked stars right to the very edge of the field and they looked quite sharp. I moved some from one edge to the other with no significant change in image sharpness. Is it worth the cost? You don't get a lot of change out of $600 so if you have $50 Plossls you may not be able to justify the price. Next time I will try some of my Celestron Plossls to see what difference it makes just to get an idea of an answer to this question. If you have premium eyepieces (Pentax, TeleVue etc) you may not think it will make a lot of difference, but my observation is that you will notice a difference. If you would like to get an image that is significantly better, you won't get it with a Paracorr, but I think you would have to sell your Newt and get an RC or something like that to get this sort of improvement. If you have a Dob, you will probably have a bit of a balance issue, but this is nothing new anyway if you have premium eyepieces anyway!

If you would like to take one for a test drive, I will be at Ballarat and I will have this beast with me. Ask nicely, and you may just get the chance!

Satchmo
10-11-2006, 08:16 AM
Thanks for the review!

You'll get by far the biggest improvement across the field with your low power eypieces at the edge of which which is where the strongest coma is.

I note that most of your testing was with a 7mm eyepiece on an F5 scope. You've noted a perceptable improvement in star sharpness a extra stars visible, which is what was predicted, given that Coma gets worse from centre to edge in a linear function, In good seeing all eyepieces views should experience an improvement, no matter how big or small the field.

Mark

Don Pensack
10-11-2006, 08:36 AM
To find the optimum setting for a non-TeleVue eyepiece, start at the outermost setting, focus the center and carefully evaluate the shape of the stars at the edge of the field. They won't be perfect.
Then, move the Paracorr tunable top in one setting, focus the center and compare the edge again.
Repeat.
There will be one setting that gives the best results (the tiniest stars at the edge). Write this down for each eyepiece.

Then, use a small label maker to put a number on the Paracorr at each of its 5 settings (e.g.1-2-3-4-5), and put a corresponding small sticker on each eyepiece that corresponds to the correct setting for that eyepiece. That way, in the field, you'll never have to remember which is the correct setting--you'll just match the numbers up.

When I change eyepieces, I install the eyepiece first, then loosen the tunable top and move it to the right setting and retighten the tunable top screw. Takes 5 seconds, and works like a charm.

I had the same remarks about the Paracorr, and especially noticed more stars in the field. I think this is due to a lot of stars at the limit now becoming visible because their star images aren't smeared out to invisibility.
I've used it for a couple years, and never noticed any deleterious effects, only positive. All the way to my 5mm eyepiece (365X).
I have an f/5 scope, but tried it recently in an f/5.5 scope and also noticed the improvement in star images, just not as great. The literature shows some positive effects in very wide fields all the way to f/8. I believe that now.
On the other side, a friend's f/3.66 dob is usable with one and is pretty bad without it.