Help fault finding with 8" f/4 Newtonian optics.

[Astroman]

Hi all,

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.

[glend]

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.

[Ken]

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:mad2:. 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. :lol:
Clear skies Ken.

[Astroman]

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.

[RobF]

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.

[Shiraz]

Hi Andrew.

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/showthread.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.

This might also be worth a try http://www.iceinspace.com.au/forum/showthread.php?t=82230

regards Ray

[Astroman]

a couple more things to look at, will check them out, thanks guys... clearing outside so fingers crossed I can do a star test

[Astroman]

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.

[Shiraz]

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.

[Astroman]

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 :D

[Shiraz]

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.

regards Ray

[Astroman]

it might pay to look at one that is offset to the one in the scope, that would be a good start. Thanks Ray.

[Merlin66]

The attached images may help.
The offset is even more noticeable in a f3.5!!!

[astroboof]

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.

[Astroman]

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.

[tlgerdes]

When you rotate your camera in the focuser by 90deg, does the problem rotate or stay in the same position?

[Astroman]

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...

[cometcatcher]

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?

[Astroman]

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.

Is your scope a BT-200 and does it have offset?

[cometcatcher]

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.

[Astroman]

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.

[cometcatcher]

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.

[Shiraz]

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.

[Jason D]

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.

Jason

[Astroman]

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.

[Astroman]

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 :D 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.

[tlgerdes]

So, what did you do? Just move the secondary away, or more?

Can you or Kevin post the picture he sent you? This has been a good discussion thread that will be valuable to others.

[Astroman]

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 :o

[DavidLJ]

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.

[DavidLJ]

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.

[Shiraz]

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 :thumbsup:.

[Astroman]

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...

[Shiraz]

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.

[Astroman]

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...

This is a crop.

[Astroman]

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.....

[astroboof]

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.

Be back soon...

[astroboof]

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.

[Astroman]

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.

[glend]

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?

[Astroman]

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....

[astroboof]

^ that's why I had to shim under the focuser of the R130sf, but it had a R&P focuser that needed 'the treatment'.

Well, it's beyond me other than that, sorry.

At these times it's a good thing to have a nice refractor to take solace in, or any old faithful scope as you suggest.

CS, and congrats on the Obs completion there. :thumbsup:

[Vermin]

Have you checked that your secondary holder is not pinching the secondary mirror?

[Astroman]

Yep, I did this, one of the many mods I have attempted to do. Although it is still in the plastic secondary holder, I have run a knife between the plastic and mirror. this is probably the best I can do without completely removing the mirror.

I will re-look at it at a later stage.

:thumbsup:

[Shiraz]

maybe that doesn't look too bad - suspect that most of the distortion could be due to tube currents - did you allow much time for thermal stabilisation? With a fan running, mine takes around an hour to settle down with typical temperatures. Tube currents could possibly also explain the pacman effect in your star images. apart from the distortion, the primary figure looks like it might be pretty good.

[Satchmo]

Hard to tell much from the defocussed star test except there is some strong aberration there amounting to waves. There is about 6 waves defocus and the out of roundness would be over a wave . A sequence of photos would show if those out of roundness was a transitory seeing thing or not . The collimation is also way off as well.

I would suggest collimate better and take more pictures which will help with a diagosis.

I can't help thinking provided the mirrors are ok that there is some mirror pinching going on there.

[jjz]

Are you using any filters? I recently tried using a cheaper UHC filter on my setup and ended up with triangular stars. Attached is a pic I took lastnight with and without the filter....

[Astroman]

Thanks for your input guys, no filters were used during this test... I did some minor reworking of the mirror cell over the last week or so. I siliconed the mirror to the cell and also added some stronger springs which seem quite adequate now. Time will tell when I get it back up on the mount. Looks to be good so far. I collimated using a cheshire, then flipped the scope over and rechecked, the collimation didn't move. I then flipped it again and it was still as I had left it.. I will wait till I can do a star test on it before saying it's fixed or now, as we all know the proof is in the image from it. Fingers crossed...