Hi all,
Having viewed this forum for quite some time from the outer reaches, I've finally decided (no...find it necessary) to venture in as the forum's latest 'newbie', don't know nothin' much guy. But first let me say what a great group this is, not only knowleagable, but friendly and willing to help. (...well I'll find out I guess)
Now that the buttering up has been done, to my plight.
I have recently purchased an SW 8" reflector, mounted on an EQ5 stand and have only pointed it to the heavens about 6 times. My initial perception of this rig, is 'not too bad'. I think the mount is at its max with this OTA and the focusser could be a little smoother in operation inho. But I digress..
I have collimated the scope using a laser collimator, however when viewing Jupiter last night, and stars around 1st mag (15 - 20 deg. off zenith), I am seeing aberrations. Typical, orange along the left side and purple on the right.
The lenses that were used are Vixen LV 12mm, Meade 10, Super plossl 25 and shorty 2x Barlow.
The aberrations are more noticeable with the larger EP's, however the overall clarity of the targets, again imho, appears off. Buy that I mean not as crisp and sharp as I would have expected. Not pinpoint clarity.
I cannot fathom out what might be causing these ab's. Whether they are Chromatic or Spherical. The EP's, although not in the $300 range, should nonetheless give a reasonable quality view I would think.
Anyhow, does my scope need better collimating, is the SW 600 at it's max capability, is it my EP's? If someone can throw me a lifeline I'd be most appreciative. (I don't have a lot of hair and ill afford to lose more)
Thanks in advance, Phil
Last edited by strgazr; 13-11-2009 at 11:49 AM.
Reason: Clarify title
It is unlikely to be chromatic or spherical aberration. Most likely collimation. Depending on the type of laser and the method used. I would be inclined to investigate that further.
unless you spent a good 200 odd bucks on your laser then that is probably the 100% problem! the 8" dob spits out crystal clear views.
A quick test for your laser collimnator. put it in your 1 1/4 " adaptor and twirl it around and around, look at the laser dot on the primary mirror if it at all moves then your laser collimnator is out of alignment (look around the barrel there will be 3 hex head screws. start adjusting!
Thanks Andy,
Good to be aboard. I was hoping that might be the answer I received. I was inclined that way myself but sought a more experienced opinion before heading blindly down that track.
I have found that the adjusting screws for the secondary on the SW 600 are bit awkward, like not too precise, and do take a bit of tweaking.
For the record the collimator I'm using is a new GSO I bought from Andrews. You may like to comment if you wish Andy.
Reflectors do not have chromatic aberration.. Coma and sometimes spherical aberration, are common, though. This can be made symmetrical (but not eliminated) by collimation. However, eyepieces they do, especially if they are poorly designed and combined with cheaper Barlows. This manifests itself with coloured rims around bright targets (like you described)
Thanks Brendan,
That was also a reply I was hoping not to get, but thought i might. Oh well as they say. 'you do get what you pay for". Looks like it'll be off to the bank again (sigh).
Better cook dinner for my dearly beloved tonite...no a restaurant, that should win her over.
Star images with low power combined with lots of light , will look worse as they use a larger part of your pupil and they show up the inhomogenities in the cornea of your eye. I find it takes a 2 to 3 mm exit pupil before my eye aberrations aren't affecting the brighter star images.
You don't mention
a) whether your scope was fully acclimatised to give its sharpest possible images.. this can take up to a few hours.
B) whether you actually had steady enough skies to allow an 8" telescope to reach its theoretical perfection .
C) Are your star images at a defocus position showing say 10 rings looking symmetrical and even both sides of focus ( indicating good collimation and state of equilibration ) . This test needs to be done regardless of what your laser might tell you about the state of collimation. A star test is the `acid' test as it were.
Those issues need to be ticked before you can make any serious judgements about optical quality .
If you don't have the time to get to know your instrument in this way then an 80mm ED refractor is a good choice..little in the way of thermal issues and images will always look superficially sharp as the aperture does not have the resolving power to be affected much by local `seeing'.
a) whether your scope was fully acclimatised to give its sharpest possible images.. this can take up to a few hours. (Yes....4 -5 hours)
B) whether you actually had steady enough skies to allow an 8" telescope to reach its theoretical perfection . (I believe so, using my bino's as a measure)
C) Are your star images at a defocus position showing say 10 rings looking symmetrical and even both sides of focus (indicating good collimation and state of equilibration) (No!. As there was plenty of time given for equilibration, it would appear that it is a collimation problem)
Just checked out the collimation of on the GSO laser and it is also out. (Not a sharp dot, more like a line) Have tried to adjust the collimation using the 3 grub screws to no avail.
Appendix: Dismantled the collimator and removed the laser. (Basically the same as a key-ring laser acquired from toy shops) Placed a small piece of masking tape over the beam exit hole and pierced a hole about 1/2 mm dia making sure it was round and smooth. Removed one of the O-rings used to locate the laser inside the barrel (there were two). The single O-ring I replaced positioning it toward the front-end of the laser. The laser was now supported by one O-ring at the front and the 3 adjusting grub screws on the end of the collimator body. Collimating was a breeze and the output is now a pinpoint laser dot.
Thanks, but I realize that. However it does make for better pinpoint alignment on the primary centre point. Collimation of the laser is guaged by its ability to rotate around a central axis which indicates true laser alignment, in other words collimation.
Collimation is the process of aligning the optical axis of optical systems to the reference mechanical axes or surfaces of an instrument, or the adjustment of two or more optical axes with respect to each other.
Appendix: Dismantled the collimator and removed the laser. (Basically the same as a key-ring laser acquired from toy shops) Placed a small piece of masking tape over the beam exit hole and pierced a hole about 1/2 mm dia making sure it was round and smooth. Removed one of the O-rings used to locate the laser inside the barrel (there were two). The single O-ring I replaced positioning it toward the front-end of the laser. The laser was now supported by one O-ring at the front and the 3 adjusting grub screws on the end of the collimator body. Collimating was a breeze and the output is now a pinpoint laser dot.
Just saved myself some big bucks.
Phil. if you can publish even more detail + photos, I think there would be many interested in how to pull apart a GSO laser collimator and then improve its performance. I had one. The beam was a "sausage" but I didn't worry about that. But when I tried to collimate the laser using the grub screws it really didn't behave as I expected it to. Your comment on removing an O-ring was very interesting. Thanks Eric
Thanks Andy for that link mate, but sadly I had already been to it previously and again, sad to say, although having checked off the various procedures it didn't help. (Spit, kick!, damn).
I did another collimation last night on the OTA (after tweaking the collimator) and I think there was some improvement to the view in regard to clarity. There was still a small degree of lens aberration, but to be fair it really wasn't a good night for it on the Gold Coast with broken cloud and a residual smoke haze. I'll just have to wait a bit I guess as the forecast for the next couple of days doesn't sound too flash.
I did the same thing to a friends laser, firstly unscrew the gso case (as if you where changing the batteries), grab a hold of the laser pointer and pull it out there are 2 o rings holding the laser in. ditch one now while it is out unscrew the golden nose piece where the laser comes out, i used a piece of alfoil layed it over where the laser comes out then just screwed the laser cap back on used a thin nail that just fitted though the nose piece to pierce a hole in the foil, put the laser back into the gso case minus 1 o ring then screwed it all back together. use a v block mounted in a vice and a piece of paper for a target on a wall at least 4 meters away, then collminate. your laser will produce a nice round dot with a nice glow around it. this proves to work in the same manner as a barlow laser as the glow reflects a shadow back to the 45 deg cut away. its not bright but it does work!
Erick, I thought being an illustrator it would be quicker to throw a mod sheet together and explain using illustrations rather than photos. I've attached 2 file versions, one .pdf and the .jpg. Hope it does the job OK.
start adjusting!
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