I was just able to view through an eyepiece 18mm + 2x Barlow) and was able to see the small flares off one side, so it looks like it's not the coma corrector.
Okay. Well that eliminates one piece of the puzzle.
Here's the view through my Cheshire eyepiece. My focusing tube is protruding a bit. I think it might be still out a bit but I'm not messing with it further as it produces good images at the moment.
Thanks for the input Steve, I had thought of this before. With my current tools, no laser or anything, it seemed ok to me. I used a steel ruler along the inside of the focuser and measured the distance from the front of the OTA. all seemed pretty right to me, but I know there are better ways to work it out but thats the best I could do.
I am interested to know why others with the BT-200 8" f/4 scopes have not reported a similar problem. Even if they all look okay, are they really and I am sure many wouldn't be perfectly collimated so would show some form of problem.
Going by what Ray has mentioned, it does look like the Secondary is not offset, I used the method of shining a light down the focuser (had to make an adapter for the LED torch to sit neatly into the focuser tube) onto the secondary and bounding it off the primary onto a piece of grease proof paper, taped to the front of the OTA... the result is below.
yep, looks like that is the problem - no offset and your input light is glancing by one side of the OTA edge, producing a lopsided diffraction pattern.
You can't get the light column centered without putting in a secondary offset.
yep, looks like that is the problem - no offset and your input light is glancing by one side of the OTA edge, producing a lopsided diffraction pattern.
You can't get the light column centered without putting in a secondary offset.
Ray, I see the results differently:
1- Yes, I agree that the secondary mirror is not mounted with away-from-focuser offset. This is obvious based on the fact the spider vanes reflection is centered against the secondary mirror silhouette
2- Because the secondary mirror was not mounted with away-from-focuser offset, it is expected that optical axis of the primary mirror to be tilted towards the focuser. This is obvious in the photo of post # 10. The flash photo reaffirmed what I can see in post # 10 photo.
3- The tilting of the optical axis towards the focuser will not cause any lopsided diffraction pattern as you have suggested
I do not see any issues in the photos uploaded by the OP. I do see the spider vanes clips in the light path but I doubt this is the reason. The OP stated that one side of the inner wall of this focuser was visible via the secondary->primary->secondary reflection. I examined that photo but it was unclear to me if this was the case.
Ray, you had to apply the away-from-focuser offset because you stated that your OTA opening is too tight which means tilting the primary mirror towards the focuser will cause the OTA edge get in the light path. I do not see it in the OP's case.
My setup is similar to the OP's though I use my scope for only visual.
Hi guys, I really appreciate the input, thanks Ray and Jason, lots to think about. Kevin, thanks for the picture, from that I was able to match the view through my cheshire. Unfortunately it looks like we are going to have at least a weeks worth of cloud and rain so looks like I wont be able to test it for some time. Will keep you posted on how it goes.
Well I am happy to say that, I have almost got it I think. Images appear much sharper, but need to have some clear, steady skies to test properly. Here is a shot of Sirius and some small windows, zoomed in. There are tiny tails on the Stars, this could be either the collimation, which just needs a tweak, or the wind buffeting the scope during the image run, or both Focus was a touch out too I think, going by the double diffraction spike when zoomed all the way in.
Canon 1100D, 8" f4, MPCC Mk1, 5 x 1sec ISO800. The image is sharper than I have ever seen, and it's almost like a new scope, which could be the reason for the weather for the next 2 weeks
I forgot to take some Out of focus star images, but to me it looks good so far. Collimation is slightly out with an off centre secondary shadow, but it is the clearest I have ever seen the Star image. I also took a Quick snap of the moon this morning and it too is the sharpest I have seen it. Oh focus also snaps in, I am almost happy...
I forgot to add (oops) That I got a long way after Kevin (Cometcatcher) sent me a picture of his secondary holder with ruler on the adjustment screws so I could get the right spacing. Thanks you very much to all who tried helping. Ray, Kevin , Jason, Trevor, Steve and Ken. Your help was really appreciated.
I still have a little way to go before I will be 100% happy, but am good for now.
Well I just copied the distance that the screw heads were away from the secondary holder. Which is now strange, because mine shows that I have a gap of about 10mm between the secondary holder and spider... But it lines up so I guess thats the main thing. Maybe there are some slight differences with similar BT-200 scopes?
These are the images I followed, I hope Kevin doesn't mind
Hi Astroman. Have just noticed your thread. It seems that you have solved your collimation problem by achieving the proper offset of the secondary. Nevertheless for future reference you might be interested in what follows.
I also have an 8” f/4 Newt and when it arrived about a year ago it was well collimated. I had read that collimating a f/4 Newt was a black art and the temptation was to leave well alone. But I took the decision that I wanted to be the master so I deliberately de-collimated the 'scope with a view to re-collimating it myself. The learning curve was steeper than I expected but after much time, effort and practice I succeeded and collimation is no longer a problem. My 'scope is a Vixen R200SS but I would imagine that physically it must be much the same as your Bintel. Like you, I found that once the proper offset of the secondary has been achieved the rest of the process is quite easy and straight forward. My primary is centre-spotted and I use a laser collimator to good effect.
Vixen distribute a pdf document variously titled “Collimating the Vixen R200SS optical tube” and “How to collimate the R200SS”. You can find it at http://www.myastroshop.com.au/guides/astroguide.asp On the first page it provides a template for the secondary which can be photocopied and cut to size. The purpose of the template is to let you draw a very small dot on the secondary at the centre of the optical axis which is offset from the centre of the mirror by 3.8mm along the major axis of the mirror. Once placed, you use the dot to correctly place the secondary into position by sighting with a Cheshire eyepiece. This is very easy to do. The dot does not detract from the optical performance of the system and being permanently in place it is always there for tweaking the collimation when required. I thoroughly recommend it.
I realise that the size of your secondary may not be exactly the same as mine although I doubt that any difference can be great. But if you wanted to try dotting your secondary you could probably scale your photocopy of the template to suit your need. I found that it helped to include small tabs either side of the template so as to better hold it in position. Also I cut small indents at each end of the major axis of the template and drew a very small mark at each end of the secondary on the outer edge. Lining up the indents with the marks made sure that the template was exactly aligned with the mirror when drawing the dot.
Further to my last post I should have mentioned that when using a Cheshire eyepiece to align the dot with the focusers' central axis it helps to use some device that accurately defines that axis. In its pdf Vixen suggests using a white paper disk with a small central hole that is inserted into the barrel of the focuser and I find that that works very well particularly if cross-hairs are drawn on the disk and the disk is rotated so that one cross-hair lines up with the telescope tube. The cross-hairs are easily seen because in daylight the paper disk is translucent. If a paper disk is not suitable for your setup you could instead use one of the longer Cheshire eyepieces that incorporate cross wires at the bottom end. The paper disk method is arguably more accurate as it eliminates any error caused by any "play" between the Cheshire eyepiece and the focuser draw tube.
That's good news. The only thing left now is that the secondary may not be fully illuminated, but if the scope is working that well, who really cares .
You were right Ray, it wasn't fully illuminated an it was still leaving an odd shape to my stars, so I took everything out of the OTA agian, spider, primary mirror, secondary mirror etc...
From a bare tube, I centred the spider in the OTA, attached the secondary mirror and blocking the reflections with blue cardboard and a box blocking the rear view out the tube. Centred the Secondary mirror using a 100mm 2" extension tube and site tube in the focuser, this allowed me to get the edge very close to the secondary mirror with only a small amount of the blue cardboard showing. Once happy with that, I screwed the primary mirror cell back on, forgetting to take the box out of the tube, disassembled box and removed through the front of the OTA, carefully not touching the mirror. Checked the view through the focuser, looked ok... Put the cheshire into the focuser and aligned the centre of the primary mirror with the cross on the cheshire. Locked down the secondary mirror screws. Adjusted the cheshire black dot until it was centred in the primary mirror centre spot. Rechecked everything and tightened up the Primary mirror locking screws so they were finger tight. Re-checked the view, looked ok still.... Did I miss anything?!?!? FINGERS CROSSED! Must admit it does look much better, like what it is meant to... but the test on the stars has to be done yet...
I think that looks much better Andrew - but a star test will tell the whole story.
My understanding is that, with a central secondary, having the light column aligned with the OTA tube is only possible if the secondary is not fully illuminated - I am reasonably sure that the only way to get both a fully illuminated secondary and an aligned light column is by offsetting the secondary away from the OTA centreline, but I could be wrong.....
I feel the pain - I spent over a year trying to understand why my **$%#@ scope was not behaving.
Although I have not had a chance to do a proper star test yet. Conditions were quite unstable and the cloud kept moving in and out, I managed this sub, and a bout 10 others exactly like it. You can see that there is still something wrong here... Wont know of course until it is properly collimated. Anyone seen stars like this before?
I am sure it cannot all be collimation otherwise 90% of people with the same scope would see the same thing... Will get around to do a star test one day, when the clouds go away...
Not sure if this helps or not, but I did a quick "star" test using an artificial star and a philips 900NC in the scope. Left is inside focus, right is outside. Bit of turbulence due to ground heating. Took a 500 frame avi and processed in registax. Still something not looking right there.....
So your secondary is perfectly cross-centered between vanes, you have secondary mirror longitude axis offset to compensate for unequal reflection lengths into your focuser, focuser axis is good, there are no intrusions into the tube apart from draw tube, And if you spin the tube around in the rings the effect follows?
Had your mirror tested?
Sorry no more ideas here Andrew without being there, can't ride down twice lol.
Ok re-read and rotating your primary does not have the effect follow it.
mirror alignments aside, how are the primary coatings? good or soft? are you getting light back up the kazoo or bounce back? flocking of all light active components?- reflections off spider vanes mirror mounts etc. I know this is elementary stuff for an experienced guy like yourself but there are still varibles or combinations of.
GSO are known for variations in mirror figures, seondary too. I've had three now over ten years and all seem to be a little different in general-principle optical quality.
LOL Steve at Riding down, yeah I still think it's the secondary now. BUT! This has me stumped still... I have collimated this thing so many times and end up with similar results. So it is either the Primary mirror or the offset. I even went as far as making a mask to cut the edge off the primary. I have set it aside for now, it's annoying me too much, so going back to old faithful for a while.
Not sure if this helps or not, but I did a quick "star" test using an artificial star and a philips 900NC in the scope. Left is inside focus, right is outside. Bit of turbulence due to ground heating. Took a 500 frame avi and processed in registax. Still something not looking right there.....
Don't know if this is related to the problem but it sort of looks like your focuser is not plump on the tube (perpendicular to the light path). The offset to the right on the image to the right shows a shift as the focuser is wound out. Did you check with a laser to insure that the light spot always stays in the same place as you wind the focuser in and out?
Don't know if this is related to the problem but it sort of looks like your focuser is not plump on the tube (perpendicular to the light path). The offset to the right on the image to the right shows a shift as the focuser is wound out. Did you check with a laser to insure that the light spot always stays in the same place as you wind the focuser in and out?
One thing I couldn't check Glen, I don't have a laser handy, hmm there's a thought, could have checked it with the cheshire....
Focus was a touch out too I think, going by the double diffraction spike when zoomed all the way in.
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