Colllimation of a fast newt.

[Tandum]

If you look at this thread (http://www.iceinspace.com.au/forum/showthread.php?t=81549) you'll see that I have pmrid's F3.8 10" newt with a view to giving the beast a tune up while he's tramping around Europe. I've had quite a few newts and peter had a set of great collimation tools so I didn't think twice to help him out as he just got his MX mount and has his hands full with that. I've found that this thing is hard to get right, I'm getting it closer and closer but I still have problems and maybe you guys can help.

I'm sure the problem is the secondary alignment. It has a prostar 3" secondary (http://www.fpi-protostar.com/) which has the focuser offsets built in. However I found that the 2 short legs of the spider where 2 mm different as well. I spent a lot of time getting the center bolt into the right spot, I have also center spotted the focuser however I had to alter the focuser tilt to get the secondary in the middle of the focuser sight tube which indicates to me something is screwed.

Doing a star test shows it is concentric inside focus but shows coma outside focus. This is giving funny star shapes. When aligning the secondary it always seems to have an apparent flat spot on the top left of the mirror. Does anyone have any tips on aligning the secondary? Should I center the focuser and move the secondary to suit instead of moving tilt on the focuser?

I also have reservations about using an mpcc at this F ratio. The info on opt says it's for newts from F4 to F6 with visual users reporting good results down to F3.7. I have a qhy9 with wheel hanging off it.

Here's a set of lum on tuc 5sec subs and 253 120sec subs to see how close it is. Check out the big stars. and I had fast downloads on to avoid the clouds hence the vertical bars on tuc.

[h0ughy]

Robin,

when i had my R200SS i found collimation a problem. it too is a fast f ratio. Looking at this link you can see things are not just centred but they also have to be orthagonal and aligned (http://www.vixenamerica.com/Pdf/Scopes/VixenNorthAmerica_R200SS_ColimnateN otes.pdf) not sure if you can glean something from this? i do conceed that there is still an alignment problem rather than just the secondary and the focuser. you may have to shim the primary mirror as well as the spider to get it centered in the focuser?

[bmitchell82]

Hey robin,

This is what im seeing. yes the stars are on the pi55 but and they are consistent problem is that its not consistent with coma. why do i say this. well coma is radial from the center out your either under or over corrected your stars arn't radial they are a bit all over the show. I think if you really wanted to get good results you would have to move to the keller corrector not the reducer but the corrector version.

I could well be completely wrong but with the secondary i thought that its alignment was to do with illumination more than anything. As there are 4 different types of alignments of which if you can nail down 2 out of 4 your system is perfectly collimated. there is a really good read about it on the cloudy nights forum.

I still think something is pinched or warped as even though im nearly 1 f number slower than you my secondary isn't quite aligned and i don't have these issues.

As for centering the secondary properly a proper sight tube does it for you something like the cats eye sight tube.

Keep us in the loop of what happens because ide like to know whats the cause behind this annoyance

Brendan

[Tandum]

Cheers Dave, I have sort of ignored the primary so far as he has it in a hi end cell and it always aligns nicely. All the triangles line up well. I'll knock up a tool to ensure the primary is 100% centered in the tube.

When I last aligned it before these images, I did it without the primary installed as I was re spotting it with the red triangle that comes with the autocollmiator. Secondary alignment still wasn't exact in my opinion. I thought I might get a spring and washers to go between the spider and the secondary, that would probably help a lot.

[Tandum]

Brendan, I thought I might remove the mpcc and wheel and just try it with a mono camera. If I can get the center right then the rest should follow with the right corrector. At the moment stars are all over the place :(

Yes I have peter's full catseye 2" kit. I wish I'd had one of these kits years ago, they are great tools.

[bmitchell82]

It just all seems odd because even you saying that your mirror is slightly out of center mine is too and i can see it quite visibly and its not a issue. if i was around your area ide be saying to you :) lets catch up because i love trying to solve issues.

Have you watched the cats eye video on how they use their stuff? if not cruise on over and have a look. the other thing that you will need is a focused beam of light to shine on the hot spot because its too hard to decern P1, P2 and P3 in my 4.7 your 3.8 would be even worse.

[Tandum]

I'm convinced it's a secondary issue. I'm sure it's not square. It's too easy to adjust everything else to make it look right.

Pete's probably reading this on his iPad but I'm sorry I respotted the primary now. It was working ok with the original white dot. Those stupid red triangles should be white not red, they are too dark. It has to pointed to bright sky for it to work now but I'll try it on the light box flat out tomorrow and see :(

[bmitchell82]

I use the yellow one :) it works a treat. Don't be fooled by the not being square issue ive had some really bogus info before and i went around in circles trying to make sure everything was square. My Feather touch focuser does not have adjustments to "square" the focuser either so it cannot be that important.

I have found if you do a flat you can see where the center of the light cone is impacting your sensor.

if you have used the cats eye kit and have lined up the astrisims.... its right pretty simple :P

[jjjnettie]

Short focal length newts are a PITA.
I'm following this thread with interest.

[Shiraz]

Hi Robin. all looks depressingly familiar. I have an f4 newt with an MPCC and have spent countless hours trying to track down exactly what causes odd star shapes - with limited success.

Earlier posts are good advice (maybe JJJ's comment sums it up), but the following may also be useful - apologies if you have already been down these directions:
1. With fast Newts, if the secondary is not properly positioned and offset, the primary optical axis may not be aligned anywhere near the tube axis and you can get the end of the tube encroaching into the light column, causing flat spots on the out-of-focus images of stars and odd shaped in-focus star images. You have checked the secondary offset, but if you need to tilt the focuser to centre the secondary - and are still getting odd star shapes/vignetting - then either the secondary is actually offset away from the focuser by an incorrect amount or it is not in the right fore/aft position along the tube. try putting the focuser back to normal and move the secondary in/out to centre it. Also, it is always worth standing back and looking into the front of the OTA with nothing in the focuser - you can see if the secondary holder (or the edges of the secondary itself) looks tilted WRT the OTA tube and this will give some clues as to what way it might be misaligned.
2. A laser alignment tool can be very useful for initial alignment, since it is relatively easy to work out causes of gross misalignment - the passive tools are probably better for fine alignment, but interpreting what is going on is easier with a laser.
3. The MPCC is intolerant to misalignment. I would first try to get everything aligned without it - try to get a symmetrical coma pattern about the centre of the FofV. Then re-introduce the MPCC and see what it does. Even with the best alignment I could get, my f4 system cannot produce round stars out to the edge of the FofV. The MPCC probably meets the claim of "10 microns" when measured at half power width, but the PSF skirts are distorted and star shapes vary with brightness. The MPCC is definitely worth having, but not perfect at f4. And it is totally intolerant of misalignment - if you cannot get symmetrical star coma patterns without it, you will get bad results with it in place, so do your inital alignment without it.
4. I suspect that you may also have focal plane tilt in the camera, judging by the star patterns, but I would leave that as the last thing to check when you are convinced that everything else is properly aligned. A camera tilt that is fine at f6 may be disastrous at f3.8 and even a few microns of tilt will upset the star shapes. Make sure that the camera is perfectly seated in the focuser (without the MPCC) as a first step and then use any tilt adjustments that the camera may have to try to get a symmetrical pattern in the star coma patterns around the centre of the FofV. If the camera does not have tilt adjustment, check that tilt is the problem by rotating the camera in the focuser. The star pattern will rotate, but if the general nature of the star pattern distortions does not change (eg elongated in top left), then the chip is probably misaligned. Then you will need to adjust it - I spent many nervous hours with shims trying to get my QHY8 chip aligned with the camera body (at least to within about 10 microns) and it made a big difference. But only attempt it if you really need to.
Good luck - if you get it lined up, a fast optical system can be be great fun. Regards Ray

[pmrid]

I am wondering whether it would impact on this problem if the primary was not absolutely central in the OTA.That Optical Suports primary cell does have the means to adjust the 3 'legs'. Maybe the OTA itself has become slightly distorted and has thrown the mirror cell into a non-central position.
Peter

[bmitchell82]

Well i think that with everything thats been said my scope should be absolutely screwed but it isn't.

My Primary isn't centered in the tube its visibly out

My focuser does not give any tilt adjust (its a FT so it isn't a crap focuser)

I admit that it isn't a F3.8 but its classed as a fast scope at F4.7

What the cats eye gear does is align the optical axis with the focuser axis very effectively.

Be really careful when doing your alignments as at F4.7 i can barely see the P2 and from every thing i have ever read it gets worse the faster your scope gets so is it something that you may be using the wrong reflection. I know i was for a while untill i got a very bright led that focused down onto the hot spot. it was then that i realised that i had actually been using the wrong reflection.

Like i said before once you have those astrisims lined up .... you cannot be wrong unless your auto collimator is up the duff.

If you give me a few months ill be able to give you my thoughts and experiences as im just about to commence my wide field newt fast newt project.

[RobF]

Would be interesting to see some pics "down the barrel" of the tube and down through the focuser Robin. I would have thought with the Catseye gear you could get exceptional alignment of secondary and primary, even if they aren't perfectly positioned.

Sounds like optical abberation or pinching as Brendan has suggested, but I should really be listening rather than trying to help - no experience under F5.

[ausastronomer]

Hi Robin,

The F-ratio doesn't change things too much. A lot of people way overplay the difficulty in collimating fast newts. They are no harder to collimate than slow newts. The optical effects of poor collimation are greater, that is all.



There are 101 ways to skin a cat but I would have done this the same way. ie. Get the spider in the correct position with regard to the mechanical axis of the tube. Remove the secondary holder from the spider and square the focuser to the axis of the spider and the mechanical axis of the tube. I normally use some thin plastic shims to do this under the respective corners of the focuser, as needed.



I am pretty sure this is caused by the secondary mirror sitting too high in the tube. You should be able to pick this up in the sight tube.

Any misalignment of the primary "should" be minor as most of the telescope components are made on CNC machines and should be able to be fully compensated by adjusting the primary tilt only.



Brendan,

You adjust this by using thin plastic shims under the focuser baseplate adjacent to the mounting screws.

Cheers,
John B

[bmitchell82]

Thats some good insight John and useful information.

I guess with my scope i havn't worried at all about shimming because its pretty close anyhow and the collimation sucks up the rest.

Thankyou once again for dropping your view on things

Brendan

[troypiggo]

G'day Robin,

I've got a f/4.7 10", MPCC, and the Cat's Eye kit as well. I'm getting similar not-quite-right stars and am trying to pin down the cause of that as well. They look very similar to what you're showing in those shots.

Pretty sure it's not the collimation, as the Cat's Eye kit indicates it's all good, and I've had Peter (peter_4059) check it a couple of times to reassure myself I'm on the right track. He has a very similar scope to mine and his stars a something to aspire to.

Am convinced now it's a combination of the spacing between the MPCC and the camera, and a bit of the MPCC not sitting perfectly square in the focuser.

For the spacing, I had calculated the theoretical distance that should be between the camera and MPCC and it rounded off to 20mm. But given tolerances it could be plus or minus a couple of mm from there. Have ordered a set of rings that should allow me to now get spacings from 17mm to 23mm in mm increments so I can fine tune it.

For the squareness of the MPCC in focuser, still playing with ideas but either filling in that indent around the MPCC barrel so it doesn't cause a problem with my focuser compression ring, or also thinking of putting a ring on the focuser side of the MPCC's flange stop. My thread on this here (http://www.iceinspace.com.au/forum/showthread.php?t=81620).

The spacing rings should turn up in the next day or 2 and I'll be doing some testing (fingers crossed) this week. Will let you know how I go.

[Tandum]

Thanks for all the tips guys. I pulled it out of the dome yesterday arvo for another go and recentered the focuser again and got the secondary done before that storm rolled in. This time the secondary looks exactly as it should do in the sight tube. No flat spots and perfectly centered.

I'll make up a cardboard gauge today to measure the gap around the primary but a quick knuckle test in the gap shows it's pretty close. Rolling the tube on the workbench reveals no warping in the tube. My baader laser died earlier this week which is a shame, as barlowing it and adjusting the primary that way last week gave the best results in ccdinspector.

Looks like it will be clear enough tonight to get another look.

[edit]
So much for the knuckle test, I found the primary mirror was easily about 4mm off center axis.

[Tandum]

Starting to look the goods now. I think the axis offset I found on the primary was causing most of the grief. I had this scope textbook aligned on the bench, as soon as I put it back on the mount and double checked, it had moved.

I believe that is another issue to be looked at later as my back was also complaining today so I didn't want to spend much time adjusting alignment on the mount. Just did a quicky so it was close. The light cone is off to the left in these images so even though alignment is not perfect the stars are way better than they were in earlier images. FWHM is down as well.

I notice that inside focus a star test shows a lump on one side while outside focus the lump is on the other side. I'm hoping that this is normal and correcting the alignment will have them both concentric.

Here's 253 and tuc, without and with the mpcc. There is obviously some camera tilt going on in there as well so concentrate on the center :)

Also added the 253 center star close up, last week and today. I got some 5 minute colour 253 coming for a look see.

[Tandum]

and one for pete's ipad :)

253 newt colour 5minute subs 963mm FL from here (no flats) and 253 RC8 colour 10minutes subs 1624mm FL from 10chain.

[pmrid]

Thanks!!!
Peter

[RobF]

Certainly looking the goods in those 253 images Robin. :thumbsup:

[Moon]

Robin
I've just gone through the process of learning how to collimate a 200mm f/4 and now 300mm f/4 Newt for AP.
You have had plenty of good advice here already. There is also lots of info on the internets but the majority applies to collimation for visual, which is quite a different process, and has different objective.
Here are two of my learnings:

1. Analyse the collimation by looking at the shape of the stars. It sounds obvious, but it's the name of the game. You might get lucky and the collimation with the laser or cheshire is still ok when you put your camera in, but this would be good luck. Tools will get you in the ballpark after you rip the scope apart and put it back together. I find that I can only get it dead nuts by pixel peeping. When you do nail it, it will 'snap' like magic and then you'll be afraid to touch it!

2. Make sure nothing moves. That includes the focuser and primary. If you're in the process of collimating, and something moves without you realising, you'll be going in circles unknowingly.
If you have a robofocus, make sure it holds focus all over the sky and through the night. Holding focus is critical - good focus makes good collimation look better. If you're slightly out of focus, it will really accentuate all collimation issues. Initially I was loosing focus **during** a 10 min exposure. This weekend I took the same drastic action I did with my 200mm. There could be an earthquake directly underneath my observatory and the primary mirror is not going to move one micron. Tonight was the first proper test run - so far it looks perfect.

Hang in there...

James

[troypiggo]

James, could I ask for more detail about how you're ensuring focus, what drastic action etc? To do with primary, focuser, or both?

[Moon]

I need to store my scope horizontally so I can close the observatory roof. I was finding that the mirror was tilting forward during the day and then settling back down into the cell as the night went on (and also jumping around at times). I knew it wasn't thermal contraction because it was going in the wrong direction. The cell had small pads on the back that can expand and contract. Also only one of the 3 points on each triangle was stuck to the mirror, the other 2 were free to separate from the mirror, allowing the mirror to fall forwards. To make it worse, the mirror clips on the side are angled forwards too, so when the scope is horizontal, the mirror really tilts forward a lot. The solution was to glue the mirror to each triangles with 3 blobs of silicone. Simple and easy. I also "tightened" the mirror clips, so they just start to grab when I slide a piece of paper between the clips and the mirror. Since this a standard GSO newt, I suspect the all have the problem.

To my amazement, I found the tension screw on the bottom of the focuser works loose every night. I couldn't believe it was true (or possible) so I drew a little arrow on it, and sure enough, in the morning I could see that it had unwound itself about 1/4 of a turn. I still can't figure out the mechanics of how that is possible, but it happens. So I used some sticky tape to hold it in place.

These are both simple things really, but it takes time to sort them out one by one.

I wrote a little program to extract data from my focusmax logs so I could see what is happening using excel. The attached graph shows the before and after of how the scope holds focus. Yes there is still room for improvement, but at least I have a chance now.

James

[Tandum]

I'm pretty sure everything is on axis now Rob, just needs a little more time but looking at the forecast it won't be this week.

That's the result I'm looking for James. No robofocus on this one, Moonlight focuser on a prostar tube and the tube has been strengthened around the focuser. You have a keller corrector on that tube don't you?

Troy, have a look here (http://deepspaceplace.com/at12in.php) for some info.

[Moon]

Yes, it's the 3 inch corrector from ASA - I can't fault it.

Do you have a motor on the Moonlight? If so, it would be interesting to see how well the scope holds focus. If you can confirm nothing is moving, then it's just a matter of turning some screw one way or the other to get it collimated.

I just realised there is a picture of the offending tension screw on my site (before I drew the arrow) pic (http://deepspaceplace.com/images/at12inFocuser2.jpg)

I'll need to update that page soon..

James

[Tandum]

No motor, just a normal 2" tri knob with shaft lock. And I've just been dropping my 8" mask in the front of the tube to focus it :)

[ausastronomer]

Hi James,

Why do things become different for imaging and visual purposes?

Collimation of a newtonian reflector focuses solely on aligning the optical axis of the telescope. If you wish to take collimation to the ultimate level, it also involves making the mechanical axis of the telescope coincidental with the optical axis. Making the optical and mechanical axes of the telescope coincidental does not affect optical performance in any way. It affects the pointing accuracy of an DSC system employed and the ongoing tracking accuracy of the telescope only.



A laser in itself is not adequate for collimating a telescope properly. A laser is only suitable for adjusting the "tilt" of the primary mirror, after the secondary mirror has been correctly positioned in the telescope and its axial rotation properly adjusted.



This is not correct. Someone who knows how to collimate a telescope properly can collimate a telescope with a very high degree of accuracy using high quality collimation tools like the Catseye or Tectron Tools. Both of those collimation sets include an "Autocollimator" which is very accurate indeed. The only time these tools will not be 100% accurate is when the mechanical centre of the primary mirror (where the dot is) is not in fact coincidental with the optical centre of the primary mirror. Due to modern mirror production techniques this is a pretty rare occurence these days.

Cheers,
John B

[Moon]

It depends on your definition of collimated. My definition is the stars on a full frame chip are symmetrical - and hopefully round. The view I see with my Catseye is not the endgame. It's the second last step. Final tweaking is still required after the Catseye is removed and the camera is inserted.
James

[bmitchell82]

Im with John on this one James, the right tools in the right hands will get it just about perfect, you cannot not be right when your looking at 3rd or 4th reflections eg correct me if im wrong the image has gone from your eye piece down the tube and back again 3 times now on my 1200FL F4.7 newt thats nearly 7 meters do some quick trig on that and you will find you are in the micron region of accuracy. I believe that auto collimators are readily able to get within 10 micron of optical alignment (assuming your reference spot and the like are right).

Pixel peeping can be hit and miss unless your seeing is BRILLIANT eg your stars are virtually still and even so can still hide optical collimation as if your seeing is better than 1 arc second (which it generally is not) then depending on your camera/FL set up your pixel is actually under the seeing limit! My F4.7 + qhy9 mono produces a 0.93 arc second per pixel so you see its kinda close ish. Then you have to contend with your mounts guiding/tracking and hopefully you are 100% guiding on a pin point if it wobbles your star has a whoopsy in it are you able to detect the difference in a guide error vs optical error from looking at a pixel?

I know its getting nit picky and all theoretical but at the end of the day if you take a image especially with newts you will see the ones who have nailed their collimation look at the diffraction spikes they generally have clearly seperated rainbows in them. I know that when i get my collimation right they are very distinct when i am out.... they are just whiteish.

Brendan:thumbsup:

[Moon]

Don't get me wrong, I really like my autocollimator thingie and I'm not saying you don't need one - you do. But we are not replacing it with an eyepiece here - it's being replaced with a camera. Lucky for me, my Catseye has two holes, so I can also view the alignment off centre and this helps a lot. But remember the sensor might not be square, the weight alone is probably enough to push the scope out of collimation - as per the video on my website.
It's also a practical matter - for the same reason lots of people use the CCD to do the polar alignment rather than swap the camera out with a reticle and then swap back to the camera, it also sometimes easier to use the CCD to do the the final collimation. Removing the camera to insert the autocollimator and getting it back in the perfect position is not always easy.
I'm not talking about the endless theory of collimation here which we all know back-to-front and no-one can despute that. It's the practical side that we all have to come to grips with and find a way that works for our own setup. For me, its easiest to use the CCD Sensor as the reference plane, and work back from that.

Lets say you collimate with a catseye and get it all perfectly stacked. Then you take an image and notice that the stars are wonky on the right hand side only. What do you do? Put the catseye back in and start over? Maybe. The alternative it to center a star and push it left or right using the collimation adjustments and see if it fixes it. That's what I would do.
James

[bmitchell82]

I guess you are right with the focuser thats why I countered that with utilizing screw down fittings all the way down to the focuser and a FT 3" focuser because i knew that the standard Skywatcher 2" sucked big ones and with the addition of a 3" OAG the QHY9 mono plus associated filters in the filter wheel there is not a hope in hell the Skywatcher can hold onto that with any reasonable accuracy.

Secondly if your moving your collimation that much from when your autocollimator out to when you put your camera in start fixing! When i do my collimation i have a reference screw which i always screw down first and then in a clockwise fasion tighten down the remaining two this makes sure that the camera is sitting virtually in the same spot as the auto collimator and hence my collimation routine is still valid.

The other thing that you have to remember that its well and good to collimate to your focuser if you have a stationary focuser but if you want to frame your object to take full advantage of your sensor you are screwed big time because you will have to not only have to do a focus routine on every object you will have to collimate on every object which is not really feasable and in the case of my FT it has a 360 degree rotatable focuser which holds focus. So at the end of the day you want to get your collimation perfect so to speak that way you have the best flexibility.

You can see from my images that i have round stars across the FOV admittedly this isn't a 11k like yours but then again if im getting pin point stars to the edge of FOV now there will be no difference if i increase the sensor apart from if there is any residual error it will be more evident but still under the seeing celing.

In relation to the tilt of the ccd sensor, if it has tilt then utilizing the cats eye gear will give you a big piece of mind and a base line that the imaging plane isn't tilted. this will then show up inadiquacies of other components and allow you to fix them as you go.

I guess at the end of the day it all comes down to your equipment, if your trying to get ultra fine and precise alignment using the bottom of the line gear you will always struggle to push 5hi7 up the hill. on the other hand if you get the top of the line gear... it just works seemlessly. Most of us spend our lives somewhere inbetween and as you say you do what works for you but for me I am a tinkerer and i wont stop till i have figured out how to solve my issues not band aid them for the long term.

Brendan.

[ausastronomer]

Hi James,

IMO the Catseye autocollimator is capable of getting you much closer than "in the ballpark". I always check my collimation on a defocused star at high power (200X plus) after using the autocollimator and invariably my Tectron autocollimator has me almost spot on and I don't need to tweak anything. If I do need to tweak it, it is about 1 time in 5 and about 1/10th of a turn on 1 screw, which 95% of people wouldn't detect or wouldn't worry about anyway.

I think there are 4 possible problems causing the autocollimator to not be giving you the desired results. These are in order of probability IMO.

1) Your triangle or spot has not been correctly positioned on the primary

2) You have flexure in your focuser drawtube and housing, or play in the drawtube rollers, which manifests under the load of the camera.

3) Your focuser is not squared to the optical and mechanical axes of the telescope and the camera focuses in a significantly different focuser position to that which you are using the autocollimator.

4) The optical and mechanical centres of the primary mirror are not coincidental.

That all having been said you need to use a method that works for you with the equipment you have. However, that doesn't mean it is what the "majority" needs to do.

Cheers,
John B

[Moon]

John
For visual you are 100% correct, but I thought we were talking about imaging here.
James

[ausastronomer]

James,

I asked you in an earlier post "what is the difference between the two". I don't see any, other than a need to compensate for flexure or slop in the focuser, due to the weight of the camera and that IMO is not a collimation issue, it's putting a bandaid on a focuser issue.

IMO a defocused star at 200X is a lot more telling on miscollimation and any other optical issues (to someone who knows what they are looking for) than what you see on your CCD chip.

Cheers,
John B

[bmitchell82]

James tonight I am going to take a few photos of my telescope to show you that even with the mirror not perfectly centered, and the focuser not perfectly aligned to the mechanical center, the cats eye equipment will yield perfectly round stars at the focuser as last night I finally got the scope under the stars for the first time in nearly 6 months (grr weather gods and uni and work and life). I didn't use any form of collimation with the scope nor did I do a defocused star test it is purely auto collimator and stacking the required astrisims (when they all dissapear and your left with P1 amen).

I just want to make sure that when people who are starting out for the first time start reading these posts which they enevitably will that the correct information gets out there. From there people can tweak the methods to suite themself.

Brendan

[Tandum]

Looks like we might get a couple of hours with stars in the sky tonight, it's been crappy weather all week, so I have it back on the bench.

Last run I again got concentric stars inside focus but non concentric outside focus. The secondary offset is 3mm but it should be 5mm. That offset is built into the spider however the 2 short legs, the ones away from the focuser, are 2mm different. I think one has been stretched somehow and I've had to shim the other one by 2mm to get the bolt centered in the horizontal plane.

Is this discrepancy in the offset going to cause me grief? I seem to be having a lot of trouble keeping the secondary centered and round in the sight tube and pointing at the primary center at the same time.

Also these red triangles are rubbish, even with a halogen lamp stuffed down the tube, refection 3 is almost impossible to see. I notice they sell white and yellow center marks so maybe one of those would be better? I should mention that to focus an eyepiece in this scope you need about 80mm worth of extension tubes in the focuser so the autocollminator is not being used at the eyepiece focus position.

[adman]

Robin are you talking about the red catseye triangles? I never had any problesm seeing 4 with mine - but thean again I had the you-beaut, super duper, catseye clip light - have you tried illuminating it with something red - a super bright LED maybe?

[Tandum]

I saw that LED light on their web site. No, I don't have anything like that here.

[adman]

for what it is its hideously overpriced (solidly built though), but it is nice and bright, and it just clips on the end of the tube - hard to beat. I also use mine as a headlamp (clips onto the brim of a cap nicely, and with the little ball joint can point wherever you need it) and keyboard illuminator - clips onto the laptop screen.

Adam

[alistairsam]

Hi,

don't meant to detract, but I once observed collimation on an SDM where he placed a yellow magnetic disc under the focuser from inside the OTA and used a laser to illuminate the reflected triangle. can anyone clarify what this is?
I've been thinking of getting the catseye kit for AP.

Thanks

[ausastronomer]

Hi Robin,

Those two short spider vanes should be exactly the same length. Unless the spider vanes are partially rotated for some odd reason. To be honest I really hate spiders with a built in offset. There is ZERO need for them. With enough clearance at the side of the telescope tube and a spider whose vanes are not too long, it is an infinitely better system to use a spider which has 4 vanes of equal length and then to adjust any required secondary offset by proper adjustment of the spider vane mounting screws only.



Generally the secondary offset does not cause any optical problems unless the secondary is almost undersized to begin with and then only with the longest focal length eyepieces, where you "may" notice some very slight vignetting. Most scopes used for imaging generally use an oversized secondary to begin with. In these cases the secondary offset is needed for mechanical reasons moreso than optical reasons. The need for an offset increases as the aperture increases and the F-Ratio of the telescope gets faster. With most scopes the advantage of an offset is to properly align the mechanical axis of the telescope with the optical axis. This affects pointing accuracy with DSC's and ongoing tracking accuracy. With short exposures the 2mm offset error would be immaterial. As exposure times increase it may or may not become noticeable with non circular stars. I am not an imager so don't know all the answers in this reagrd, but an autoguider may of itself compensate for this issue.

Cheers,
John B

[ausastronomer]

Hi,

This is a barlowed laser system. Those supplied with SDM's are from Astrosystems which are very good. However, they are not suitable for adjustement of the secondary mirror in any way. They are only suitable for adjusting the tilt of the primary mirror. They do an excellent job of this and are very fast and accurate.

Cheers,
John B

[Moon]

2mm error doesn't sound too bad. According to the theory (http://www.telescope-optics.net/newtonian.htm), it should be 'easier to collimate' but not improve 'image quality' when you have the right offset. Personally I found my 8" f4 much easier to collimate after the correct offset was applied.

The other thing you can have a look at is the vignetting: test a high quality flat field with CCDInspector, and also have a look at the vignetting impact on defocused off-axis stars.

Good luck.
James

[Tandum]

Cheers guys, I've got it looking pretty good on the bench so back in the dome it goes till the stars show up.

Having a defocused star concentric inside focus and showing coma outside focus is what I saw last time although FHWM took another step down which is good. The secondary looked like it needed to be moved up the tube a little this time, we'll see tonight with a bit of luck.

[Tandum]

A bit stop start tonight as clouds passed over but got a result. It seems I may have been chasing my tail a bit. The mpcc appears to be distorting the star shapes and giving the weird in/out of focus results.

First shots showed FWHM under 4 but stars where out of round. I used ccdinspector to tune collmination but it wasn't till I removed the mpcc that it started to come good.

Here is without mpcc and with mpcc, then the 2 central stars without and with plus a tuc shot for colour. 4x120sec on 253 and about 40x10sec on tuc.

I'm pretty sure my distance to mpcc is within 2mm or less so I'm not sure if the mpcc has issues itself or if the low F ratio is a problem for it. I'll have a look at it's lenses in the daylight tomorrow.

Also the primary cell can move on the lock screws, they are a real pain. This scope originally had a cell with 3 captive screws moving the mirror and if you still have that cell Peter, you'll save yourself some hair if you put it back in :)

[pmrid]

It's a bit late to be saving on hair, but OK.We'll whack the captive-screw system back in. Need to replace the screws though. The originals are rubbish - Phillips heads are stripped. Sounds like a new coma corrector too?

Peter

[Tandum]

Small parts and bearings have moved to the old harley dealers shop at Capalaba. They'll have everything. I'll visually check the mpcc tomorrow. Best to try and borrow one before buying another one though.

Here's a half dozen 2minute Ha subs :)

[pmrid]

Be good to try for something like Bob's Knobs - screws with heads that you can turn by finger. Might get Some made if need be.
P

[Tandum]

This newt seemed to snap into place tonight. I tweeked the primary a bit on a star without the mpcc then put the corrector on and shot a few images. Still a few odd shaped stars in there but the figures look ok.

[Tandum]

I thought I'd finish this off instead of starting another thread.

What we found was one leg of the spider had been stretched by 2mm. To offset this I added packing to the other leg to center it in the tube. However, this introduced enough slack in the spider to allow it to twist and this twist changed depending on the altitude and because the secondary was so heavy the twisting then distorted the tube which gave us crazy results.

Peter got a new spider for the newt. Straight away it collminated at elevation and started to work however there is still a flex problem. I stuffed a camera with an OAG down it's throat and the flex issue was gone so it was external. I then removed the guide scope from the top of the newt and put it on a SBS bar with a tak fs60 as a guider and still saw intermittent flex even at 120sec subs. I tried wrapping a strap around the tube and the guider and this reduced flex a lot but it is still there. I think those GSO rings with the single mounting bolts are just not good enough for big newts.

Here's a couple of snaps under a bright moon.

[pmrid]

The Eta Carina shot seems pretty sharp. I can't see any elongated stars in it. The Omega Cent however is different - there does seem to be an elongation in the horizontal - or is it my astigmatism?
Peter

[Tandum]

Yes it's there and it's in DEC. All subs are 120sec, I got none of this with the OAG, not even in 600sec subs. However, I can't run my oag with the mpcc or the wheel. Either the distances are too big or I run out of backfocus :(

[pmrid]

Well, my SX rig ought to work OK with the MPCC since I don't have to use any nosepieces on the camera to get the OAG parafocal with the CCD - it is designed to be right without it. I'll go ahead and replace those rubbish GSO rings I think - can't hurt anyway - and see what happens.

Peter

[Tandum]

I measure the OD of the tube as 297mm.