I am slowly going through my 8" f4 newt, I bought from Bintel back in 2013. I have always had problems with the stars and sometimes I thought I had fixed the problem, but it may have just been seeing, masking the problem. I am not sure. I have collimated this scope more times than I would like to remember trying to solve this little issue I have. Using a Cheshire, sight tube and also went out and bought a cats-eye collimation centre spot and borrowed the collimation tools to get it right. I hope someone on here has seen similar and have a fix for me, because I am slowly losing my mind over it.
The problem is these two blobs that appear on the right hand side of the stars. I was thinking it may have been pressure on the primary, but I have loosened everything off and the mirror is quite free to move around but not so much to wobble about. I have even rotated the mirror in the cell but the blobs remain in the same spot. I removed the front ring that supports the OTA tube, just in case it was that. I have ran a scalpel around the secondary mirror plastic holder trying to remove it, but couldn't, it's stuck quite firm, I didn't want to be too heavy handed with it. I also blackened the edge of the primary mirror.
I am not sure what else to do. I was going to cut a couple of notched in the secondary mirror plastic holder, to remove any strain that may be on the mirror (saw this done on another website) I was also going to reduce the size of the three primary mirror clips, to see if that helps.
I am at a loss of what else it could be. Has anyone seen this sort of star image before?
The image is a single 5sec sub at iso800 of M7 taken in twilight. Canon 1100D, MPCC #1 Coma corrector. 8" f/4 GSO newtonian.
Forgot to add that the Diffraction spikes were put there by me in processing, to visually represent where the actual spikes are on the stars since they didn't show up too well.
It would appear that none of the stars are actually round but have tails pointing out to the say 2pm position if imagining a clock face. Fast newts can be difficult to collimate. Are you setting it up so the centre circle is slightly offset in the cheshire as per the photos of a f4 in Astrobaby's collimation instruction pictorial? You don't want the donut centred in the focuser tube for that scope. What alt angle are you using to collimate the scope? Some scopes exhibit collimation movement (due to flex and mirror movement) at different angles, good practice is to collimate at the angle you most often view, or image at. So 45 degress is a good starting point. You may also find the upgrading the GSO springs will reduce or stop any change in collimation through different angles. Good luck.
I spent about 4 hours trying to collimate my 18" f4.5 last week with offset secondary using a centering mask and holograghic laser grid only to find double diffraction spikes on my images. If your focuser drawtube is in the light path you will get a extra spike at 45 deg. to the main spikes.
Good luck who said newtonians are easy to collimate.
Clear skies Ken.
I will recheck again, but I have followed that Astrobaby's collimation page. Maybe I am not doing it right, maybe there is something I am missing... It looks in collimation according to the cheshire. Thanks for the info guys, I have another idea, will see how I go with it. So to most people it's looking like collimation so I will stick with that for now and stop modifications of the actual telescope... Thanks for the info.
Andrew, have you tried putting an eyepiece in an examining the defocused image either side of focus? Sometimes you can get a feel for possible sources of these issues. Might also get clues applying slight pressure to main or secondary mounts and see if star distortion changes on a bright'ish star.
I'm sure its not your issue, but I used to get very similar pattern of star distortion on my non-photographically optimised 8" Newt - the bottom of the focuser protruded down into the light path of the main barrel.
Looks to me like diffraction from something protruding into the light column, as others have said. This was discussed in a recent thread that might be worth a read http://www.iceinspace.com.au/forum/s...d.php?t=133830
The big problem with my 200f4 was that the secondary was not offset so the input light column ended up offset from the OTA centreline. I used to have stars like yours and was only able to improve them by:
1. remaking the secondary to incorporate offset
2. moving the mirror up and using a bigger secondary to get the focuser draw tube out of the way.
There are still some bits and bobs that cause stray diffraction, but it is now a lot more usable.
So what does it mean if I am looking at a slightly defocused star with central obstruction in the centre, then move to the other side of focus and it is off to one side? If I try and get it into the centre the other side is wrong. I would just be going around in circles.
that could mean mean that you are cutting off some light (vignetting) on one side of the incoming light column - probably it is running into the aperture on the way in. Back projection may show you what is going on. If the beam is truncated on one side, the apparent de-centering will switch sides as you cross focus. Try centering the Poisson spot so that it is the middle of the shadow region - if the outer bright zone then looks lopsided, you have uneven vignetting somewhere.
If that secondary is not offset (mine wasn't) I think that you will need to do that before you get anywhere.
Thanks Ray, I am not sure if I am getting this right or not so I snapped a quick image AFTER I had aligned the mirrors. Although the phone may show it slightly out, both sight tube and Cheshire say it's in line. So out of this image I can see the bottom of the focuser but it looks to be angled away, I would have thought the view would be looking directly up the Focuser tube, or is it.. I have moved the secondary back and forth along the tube and still get a similar image, some more some less... Anyway appreciate the thought on this image, except for the dirty optics
Thanks Ray, I am not sure if I am getting this right or not so I snapped a quick image AFTER I had aligned the mirrors. Although the phone may show it slightly out, both sight tube and Cheshire say it's in line. So out of this image I can see the bottom of the focuser but it looks to be angled away, I would have thought the view would be looking directly up the Focuser tube, or is it.. I have moved the secondary back and forth along the tube and still get a similar image, some more some less... Anyway appreciate the thought on this image, except for the dirty optics
FWIW my interpretation would be that your secondary does not have an offset. Therefore, when you look directly down the tube like this, everything appears to be skewed off centre (but it is still optically collimated). If you look down the tube from the position where everything appears optically centered, you will find that the ota axis is a fair way from the optical one. If you back-project some light and put a layer of baking paper over the aperture, you can see how much of the OTA edges etc the beam is running into (which causes the strange diffraction pattern). You cannot get rid of this input beam offset unless you offset the secondary to compensate (or accept partial illumination of the secondary, which has its own problems). If you would like, I will bring down the offset secondary from my 200f4 to try before you decide if you want to modify yours.
Andrew,
Possibly a long shot idea, but have you measured the focuser trueness to the tube. I only suggest the possibility because I had issues collimating a vixen 5" f5, turned out it came collimated with a 5mm positive offset partly on account of the focuser being a touch skewiff, I shimmed under it and that made life a lot easier. Your f4 is going to be a lot harder on the mechanics of an ota, and although the machined focuser should more or less fit perfectly angled onto the tube. I wonder how little a misalignment it would take to throw everything else out at a fast f4? Apologies if you have covered this idea.
Steve.
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.
in the same spot but 90deg on the frame, but looks different. I havent got a shot of that though, I turned the camera upside down and the flares are still there also...
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.
Following the thread with interest. My Bintel 8" F4 seems okay. Nice round stars.
What's yours like without the coma corrector? Just need to see the middle stars. Can you see any of the blobs visually on a bright star or do they just appear on images?
Last edited by cometcatcher; 04-05-2015 at 05:31 AM.
not sure what it is like without the corrector as I don't have another nose piece to attach to it to check. I had thought of this also but couldn't test it. I haven't been able to do a proper eyepiece test yet. I keep getting distracted.
Yes BT-200. It has some sort of offset. I don't understand the principle all that well even though I've read about it 100 times (I'm thick). Looking through the Cheshire it appears to have quite a lot of offset of some sort. I'll have to see if I can photograph the view through the Cheshire somehow.
. If your focuser drawtube is in the light path you will get a extra spike at 45 deg. to the main spikes.
