I have a SkyWatcher newtonian 200mm and I have been trying to use my Celestron Omni 2x barlow; however, when I look at planets (Saturn for instance) the image at the eyepiece is hard to focus. In fact, it stays fuzzy at the best focus I can achieve. I think it does better on nebulae and deeper sky stuff, but I am unsure if this really is the case...too many clouds lately to test this on a range of objects. Oh, yeah, I'm not trying to go too high in power - I've been using my 25mm, 12mm and 10mm EPs...
Saturn is relatively low in the skies (only getting to around 46 deg max at 11:43pm today) so if you are viewing any planet at less than this altitude, poor seeing conditions can make it difficult to get a pleasing image. I've noticed that views of the Moon and planets at anything below 40 deg altitude tend to be quite disappointing.
Sometimes, you may get a fleeting moment of good seeing when detail on the planet "snaps" into focus and you have a "wow" moment. Webcams can capture these "wow" moments and stack several together, producing detailed and sharp images of the planets.
If you are looking at deep sky stuff higher up (say 50 deg and more) then you are viewing through a thinner slice of our atmosphere and so may be getting more pleasing views?
Another possible cause could be poor collimation? If the OTA is badly out of collimation, planetary and lunar images that should show fine detail will tend to look soft and unsatisfying.
Do a star test on a mag 3 or 4 star, at a high magnification (say x250 or more), at least 60 deg altitude and have a look at the slightly defocused images, inside and outside focus and check if you have a nice set of concentric diffraction rings. This will require reasonably steady seeing; otherwise you’ll just see a confused blob.
Well, still not happy with the focus. I have improved it by moving the barlow out of the focusser and working from there, but it is still not a crisp image.
Have checked the collimation - it is good, apart from the fact that I can't seem to adjust the primary mirror. The secondary is adjustable and I have done so when I brought the scope home, but the primary doesn't seem to have locking screws and adjusting screws, just one set of screws at 120 degrees separation. Even so, the mirrors seem to be well-placed and centred.
I still have to do the star check and see if I get the concentric rings.
Hi john,
I'm also not entirely happy with my barlow. I have the same sort of problem. My scope was collimated on Saturday night at Kulnura and seeing was pretty good. Satrun at 12:30 showed a terrible view throught my barlow. I had two images it was so out of focus. With my straight up 10mm it was crisp anf magnificent. So I'm interested in ideas.
As to your primary adjustment it sound like you're having the same problem i had. My scope did not appear to have locking screws until I realised that there is a cover of the back and the three screws you are looking at is probably to remove that cover and reveal the actual adjusting and locking screws. Took me ages to get the nerve up to undo those. The manual said nothing about a cover
Yes - that is my problem exactly. Using my (relatively expensive) 12mm EP I can get a very sharp and clear image, but once I introduce the barlow it goes fuzzy. What I have done is to put my EP in the barlow and insert the barlow into the focusser without tightening the screws on the focusser. What I do then is to adjust the focus by moving the barlow in and out until I get a reasonably focussed image, then lock the screws in place. From there I use the focusser to refine. But, having done this I still don't get a sharp image...but my "perception" is that it is better being displaced a little in the focusser.
On the primary mirror issue, yes again that sounds correct. My scope is a SkyWatcher and (again) yes, the book made no mention of removing a cover. I will have a closer look at what you suggested, but my feeling is that the collimation on the scope is pretty good. The secondary mirror was out, so I fixed that up, but it seems to be positioned in the middle of the primary, so collimation doesn't appear to be a problem...I hope!
I'm going to contact the shop I purchased the barlow through for some suggestions; if I get any I will let you know.
IMHO, you need to tackle this by settling one issue at a time. First you must collimate your 'scope, even if you think it's already collimated (which if it's not been done since you bought it, I seriously doubt) by performing a star test. Collimation is part of any Newtonian telescope's regular maintenance anyway.
Second, (once good collimation is established) your local seeing conditions might be causing you problems when using the barlow with your 12mm ep (at f6 : 1200mm f/l / (12mm/2) = 200x). Is there any possibility of checking your scope with a high power / short f/l eyepiece without the barlow in the optics train? Borrowing one for example?
Thirdly, try your barlow in someone else's Newtonian 'scope.
Hi John,
Mine is the skywatcher 150mm (f=1000mm), pictured below. It does have a cover.
The collimation of mine was way off. But it was corrected on the weekend by someone who knows how to do it. With good seeing and checking the collimation against a star (all looked great with my 10mm EP) The Barlow still showed satrun out of focus. I didn't think much of it as the dew was getting heavy and EP's foggin, etc so i just put it down to that.
Dindsy: Well, I have removed the cover from the back of the scope, discovered the locking and adjustment screws, and had a fiddle. Last night I did the star test and, yes, the scope is a little out on the primary mirror. I would adjust it properly tonight, but the sky is not cooperating...so I have made minor tweeks using a ruler at the front of the scope to try and get the mirror image equidistant. Of course, this won't correlate with doing the star-style job, but at least I feel confident enough to collimate the primary mirror when the sky clears.
Chunkylad: As said above, I will be working on the collimation. For a newbie, fiddling around with my new $1000 toy is a little un-nerving to say the least, but I have managed to fiddle with more expensive pieces of my amateur radio gear, so I guess it's just a confidence and experience thing! Ack the comment about borrowing a more powerful short f/l EP. I will have to get in contact with some local yokals here in Brisbane and do that.
