First light for Pentax XW-30

OneOfOne (Trevor) · #1 22-08-2006, 08:15 AM

I didn't get a lot of time to view last night, we were clouded over by just after 9:00 and I didn't finally get set up until nearly 8:00. Anyway, I swung the scope around to Eta Car first (I was actually testing my new UHC filter) and put in the 10. The whole area basically filled the view, the neighbours spotlight made viewing pretty hard though. I noted the sharpness, what detail I could see etc. and then swapped over to the 30. The first impression was how bright the background sky became compared to the 10, to be expected though as the sky was quite bright anyway (I could only see three stars in Crux). The whole of Eta Car was contained to a small area in the centre of view. Another thing I found is the amount I had to move the focusser out, pretty much to the limit. I may have to look at a spacer of some sort to try to make it a little more parfocal with the 1 1/4's. The second biggest surprise was the sharpness of the image. I was expecting, with a magnification of 33x and FOV of about 2 degrees, to get some noticable dropping off in sharpness toward the edge...but it was as sharp near the edge as the 10! Now that is impressive.

Next I tried the Butterfly Cluster, this is big enough to completely fill the FOV of the 10. Put in the 30, the cluster was tack sharp in the middle. Unfortunately, there weren't enough stars to fill the whole view. I moved the cluster to one edge of the view and then over to the next one, watching the focus as it traversed from side to side. Even with the cluster touching the edge of the field, I could not discern any appreciable falloff in quality. If I was to focus on the cluster when it was on the edge, I don't know if I would bother to change it compared to the centre....if I didn't know it was the 30, I could convince myself I was looking through the 10 with the quality I could see.

In summary:

Such a low magnification will brighten the sky a LOT. Investing in a 2" UHC filter maybe a worthwhile investment.

The out travel may require a spacer to allow you to swap to a 1 1/4" without having to wind it in and out so much.

Although, I didn't get a chance to view much, my first impression of the sharpness and focus across the field is "impressive".

The eyepiece is heavy, so if you have a dob, you may need a counterweight of some sort when you put it in.

I need to find a cluster that spans up to 100 or so arc minutes so I can fill the view with stars all at once to be able to better judge the overall "quality" of the field.

Unfotunately, Melbourne is supposed to be clouded over until Saturday...fingers crossed!

janoskiss (Steve H) · #2 22-08-2006, 08:56 AM

Thanks for the report Trevor! For benefit of those not familiar with Celeron's model numbers, Trevor's got an f/5 8" Newtonian.

How do you like the 14mm XW in that scope? I found the same thing Mike did at f/5 and f/4: too much field curvature, unsharp near edge of field. OTOH, the 10mm and under are perfect. Yours is the first report I've read about one of the 2" XWs. Sounds like a winner!

Be sure to bring the XW-30 to the Ballarat camp!

OneOfOne (Trevor) · #3 22-08-2006, 01:13 PM

I have noticed that near the edge of the 14 there is a bit of "something", either "out of focus (field curvature)" or "coma (a coma corrector may help to answer this)" or something else. It is interesting that both the 10 and 7 appear not to suffer from this problem, as far as I have seen so far. The 30 appeared to be more like a "10". I was expecting the 30 to have "more" of the same problem as the 14, picturing a sort of graph in my head of focal length vs degree of "field curvature" if you like.

Yes, I will definately be bringing the 30 with me to Ballarat for others to give their more experienced opinion. Maybe I should charge "rent" on it!

janoskiss (Steve H) · #4 22-08-2006, 01:37 PM

Quote:

Originally Posted by OneOfOne

I have noticed that near the edge of the 14 there is a bit of "something", either "out of focus (field curvature)" or "coma (a coma corrector may help to answer this)" or something else. It is interesting that both the 10 and 7 appear not to suffer from this problem, as far as I have seen so far. The 30 appeared to be more like a "10". I was expecting the 30 to have "more" of the same problem as the 14, picturing a sort of graph in my head of focal length vs degree of "field curvature" if you like.

It's FC. Here are the graphs you might be thinking of:
http://www.pentax.co.jp/japan/tech/xo-xw/64.html. As you can see the XW-14 and 20 are not really any worse than the XW-5,7,10, but the field curves the opposite way. The XW-10 and under curve the same way as the field of the parabolic mirror of a Newt, the 14 and 20mm curve the other way, hence give an unsharp view near the edge. The focal surface of the XW-30 also appears to curve the wrong way but the curvature is considerably less than for XW-14 & 20.

Quote:

Yes, I will definately be bringing the 30 with me to Ballarat for others to give their more experienced opinion. Maybe I should charge "rent" on it!

I bet you could too!! How about $1 for the first minute, $0.50 for every additional minute?

OneOfOne (Trevor) · #5 23-08-2006, 07:57 AM

Quote:

Originally Posted by janoskiss

Here are the graphs you might be thinking of:
http://www.pentax.co.jp/japan/tech/xo-xw/64.html.

Interesting graphs, it shows why the 10 is considered to be so good, with the 7 not far behind. Yes, it sort of implies why the 30 is not too bad, and better than the 14. It goes the wrong way, but not by as much as the 14 or 20. Pity I can't read Japanese.

ausastronomer (John Bambury) · #6 23-08-2006, 08:55 AM

You can extract further information from those graphs. The divergence between the 2 lines shows the degree of astigmatism in the eyepiece. In all cases it is very minimal and you would never detect it with your own eye, but the graph shows the XW 30 to be effectively free of astigmatism completely.

CS-John B

Don Pensack · #7 01-09-2006, 06:42 AM

Yes, though the positive field curvature should add to the positive field curvature of most scopes, which could theoretically make field curvature visible, in practice the amount is so small the 30mm is a fine eyepiece in any scope. It does not correct coma in a short f/ratio, but it might be the first time you've seen what pure coma looks like (without astigmatism, field curvature, or rectilinear distortion thrown in).
Visually, in daytime use, the 30mm is significantly better than the 31 Nagler or the 30mm Meade UWA. Pentax's background in spotting scopes shows through, here.
At night, on stars, the 30mm, though superb, doesn't have the field of view of those others. Frankly, though, it's less expensive than the other two, and would make a *Best Buy* at that focal length if the wider field wasn't wanted or needed.
I chose the 35 Panoptic as my lowest-power eyepiece, but, if I had to do it over again, the 30mm XW might edge it out. I really don't view that many objects that need a 1.22 degree field of view. 1.15 degrees would be just fine.
I've now looked through all the XWs except the 40mm, and this one, along with the 10mm, are my favorites in the series.