#1  
Old 01-03-2018, 05:36 PM
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Camelopardalis (Dunk)
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Question Rescue my newt! (please)

Folks,

I recently "invested" in a fast newt (SW Quattro 8") to supplement my 4" Esprit for imaging, going in eyes open, taking on a challenge for those lovely clear nights while my Esprit merrily collects photons from around the night sky.

However, having done lots of reading and practiced collimation on the bench, my attempts so far appear to have been awry, as my test images look pretty horrid.

The secondary collimation screws on the Quattro are an exercise in futility and I recognise I will have to tackle them separately to make using them reproducible. I accept that this is just one area where an inexpensive scope needs work, and I may later discover other mechanical issues I need to tackle. I feel it's a bit of a project!

On the bench, I adjusted the secondary to be as circular as possible and concentric with the focuser tube, using a collimating cap (the Rigel one).

I then inserted the laser collimator and made the adjustments to get the beam in the centre of the primary. I made the assumption that, even if the primary is out of collimation, the beam should strike the centre spot if the secondary is collimated. Mistake number 1?

With the beam in the centre of the primary, I rotated the laser in the focuser and it doesn't move, so it appeared to me that the laser was collimated. I then racked the focuser in and out and, again, saw no shift in the position of the laser beam on the primary. I interpreted this to indicate that the focuser is correctly positioned with respect to the tube of the scope. Mistake number 2?

Then, I adjusted the primary to get the reflected spot centred in the reflective face of the laser collimator.

With the coma corrector (GPU) inserted and my usual imaging train added (ASI1600 + mini EFW), I pointed up at the night sky...and well, it was a disaster ...and this isn't the first time.

Having been happily collimating my SCTs for some time, I figured I'd approach with a similar strategy...centre a bright star, defocus, and adjust the collimation of the primary to centre the secondary shadow, as that was off-centre. With every adjustment, I re-centred the star, and repeated until I could collapse concentrically down to a focused point.

Regardless of the above strategies, the images don't lie and round stars are eluding me.

I've attached an image of my test target from last night...the Keyhole area of Eta Carinae nebula, cropped down to 1:1, so this is the central 1200x900 of 4656x3520 pixels. Further out from centre, the aberrations appear to get worse and larger. Just looking at the centre, it shows promise from a detail point of view, but obviously isn't performing its best given the issues there.

Is there any significance to the way the left-hand diffraction spike of Eta Carinae itself diverges, whereas it doesn't on the right (and up, down) ?

I don't have the experience with a newt and feel stumped at this point...if anyone has any clues as to where to start tackling this, either from scratch or by deconstructing the (likely multiple) issues in this image or others that I can post, I'd be very grateful. This wasn't first light for this scope and I'm beginning to lose hope with it... I suspected it wouldn't be easy, but there comes a point it needs to earn its keep.

Cheers,
Dunk
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  #2  
Old 01-03-2018, 05:57 PM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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I probably shouldn't offer advice, I mostly gave up and bought a smaller sensor so I couldn't see the badness... but

Can you try some test images without the coma corrector and verify that the best stars are in the center of the field? If they are, then I believe you're well collimated.

If you are well collimated and are getting poor stars across the entire field I'd first look to spacing of your corrector.
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Old 01-03-2018, 06:15 PM
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Camelopardalis (Dunk)
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Thanks Lee - I'll try anything! (although it's packed away tonight)

To be honest, I'm not certain how to determine I have good collimation. I assumed - possibly mistakenly - that if I followed the approach with collapsing a star in the centre of the field, then it shouldn't be far wrong

What spacing are you using with your corrector?
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Old 01-03-2018, 06:17 PM
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Camelopardalis (Dunk)
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Here are some 1:1 crops from the corners, clockwise from top left.

Maybe my expectations are too high, given my other imaging scope is a refractor that cost 3x as much
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Old 01-03-2018, 06:22 PM
ErwinL (Erwin)
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Seems like the spider blades of the secondary mirror are distorted and perhaps also slightly missaligned.
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Old 01-03-2018, 06:23 PM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Hmm. That looks like bad astigmatism to me. No direct experience, but from the research I did when trying to solve my own issues, bad astig will cause in focus stars to look like crosses. You can confirm by watching the stars as they go either side of focus... if they elongate in different directions on either side of focus it's astig.

Also looks like there's some residual tilt / collimation going on -- some of the stars are bigger than others.

As for me, I don't even remember what spacing I'm using any more. I tried so many variations with multiple correctors on multiple cameras... think I might be around 57-58mm with the Paracorr I?

The hardest part about this is isolating all the contributing factors, thus why I suggested first confirming (without the corrector) that the best stars are in the center of the frame. I think that confirms primary / focuser alignment.
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Old 01-03-2018, 06:39 PM
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Camelopardalis (Dunk)
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Quote:
Originally Posted by ErwinL View Post
Seems like the spider blades of the secondary mirror are distorted and perhaps also slightly missaligned.
Thanks Erwin, I'll check it in daylight.
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Old 01-03-2018, 06:56 PM
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Camelopardalis (Dunk)
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Quote:
Originally Posted by codemonkey View Post
Hmm. That looks like bad astigmatism to me. No direct experience, but from the research I did when trying to solve my own issues, bad astig will cause in focus stars to look like crosses. You can confirm by watching the stars as they go either side of focus... if they elongate in different directions on either side of focus it's astig.

Also looks like there's some residual tilt / collimation going on -- some of the stars are bigger than others.

As for me, I don't even remember what spacing I'm using any more. I tried so many variations with multiple correctors on multiple cameras... think I might be around 57-58mm with the Paracorr I?

The hardest part about this is isolating all the contributing factors, thus why I suggested first confirming (without the corrector) that the best stars are in the center of the frame. I think that confirms primary / focuser alignment.
Yeah the astigmatism is awful...I'll have to chuck it on the mount next clear night and check visually.

I never expected it to be easy to tame a fast newt, but astigmatism is a wildcard I hadn't hoped for.
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Old 01-03-2018, 07:07 PM
glend (Glen)
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Well Dunk, this is a new scope right? I would start with some basics, like making sure the secondary holder is properly centred in the tube, ie all the vanes are identical lengths and the centre lock screw is exactly in the middle. Check the vanes are vertically aligned in the tube, some can be tilted if they are just held by a screw through the tube wall. Also don't assume your laser is in fact properly collimated, i recommend checking that by rotating it in a jig. You want it to stay within a circle of 10mm over say 4 M.
Don't mess with the secondary offset, just yet, it should have been set ok at the factory. Read Astrobaby's paper on Collimation and secondary offset.
I don't know anything about that coma corrector, i use the Baader MPCC and never have a problem (it uses 55mm backfocus). You should be collimating without anything else on the scope, i assume that was obvious. Try sticking your DSLR on it, use Liveview to star test.
Don't over tighten the secondary collimation screws, this could distort the shape, ie = astigmatism. Also check the tightness of the primary adjustment and lock screws, sometimes they are overly tightened by the factory for shipping security . All of this can be done on a bench, or on the lounge, as long as its properly supported. Good luck.

Last edited by glend; 01-03-2018 at 07:26 PM.
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Old 01-03-2018, 07:41 PM
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Camelopardalis (Dunk)
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Thanks Glen, yeah it’s relatively new, I’ve been trying to tame it on and off but getting nowhere fast.

I’ll need to get the measurements checked in daylight. I’ll also have to back-off the secondary screws, they seemed to need to be tight to stop the secondary flopping around like a rabbits ear when slewing.

Having almost relied on the laser in early attempts, last night I had ditched it and resorted to the cap for aligning the secondary, but to be fair I can’t tell the difference once darkness falls. I had been using Astro Baby’s - and others - material as reference, to try to figure out where I could have gone wrong.

This is my first newt, but at the same time collimation isn’t a new concept...just there are double the adjustments! I wouldn’t rule out it being a nice scope if I could be sure the collimation was good, but it might just take a little investment in time and money to get it right. I’m resigned to that. Despite the stars, the detail in and around the Keyhole was the kind of upgrade I was looking for over my apo.

It’s a journey! There just aren’t enough clear nights to try every combination
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Old 02-03-2018, 01:23 AM
rbronca
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I have a 10" version of your scope and I have seen what you are seeing a number of times.

A laser simply isn't accurate enough for F4. I have a Hotech which is fine for F5, but not F4.

For this scope, I would use a Catseye collimation triple pack set.

Center the secondary mirror holder - make sure the vanes are tight. Use a small ruler.

Replace the useless secondary screws with Bob's knobs, make sure you get the long versions.

I would add two circles of plastic milk bottle between the screws and the secondary (create a hole for the center screw). It allows you to rotate the secondary and avoid the pits you will dig in the back of the secondary stalk. (Hard bolts into alloy).

Use the telecat to center the secondary within the focuser.
(A sheet of one colour paper behind the secondary and another across the tube makes this easy to see).

These two videos will help.
https://www.youtube.com/watch?v=wcvHOHwg1j4&t=2s
https://www.youtube.com/watch?v=3LbR1nIx-jw

Respot the primary accurately with a yellow hotspot. (who doesn't like a radiation symbol).

Use the blackcat to align you primary.
Infinity to align your secondary. Both primary and then offset.
Iterate between the two steps.

The bible for the Catseye collimation process:
https://www.youtube.com/watch?v=Vj12cx3tnsM

You will likely find the factory focuser suspect for imaging. I ended up with a Moonlite 2.5". The cool kids are going Litecrawlers now. (Sit down when your pricing).

We haven't discussed tilt yet. Depending on your coma corrector this can be finicky. I'm now in the GPU camp.

At F4, your dealing with sub millimeter tolerances. You will get there.

A recent test image of mine to show it is possible. (I'm still playing with tilt).
http://www.iceinspace.com.au/forum/s...d.php?t=164860
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Old 02-03-2018, 09:03 AM
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Camelopardalis (Dunk)
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Thanks rbronca.

That’s a lot of not inexpensive accessories, and really makes me think that I’ll be cutting my losses and gunning for another refractor.
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Old 02-03-2018, 09:38 AM
RobC (Rob)
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Suggest you solve all your problems buy donating the scope to a local land fill project.
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Old 02-03-2018, 09:54 AM
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Dunk do you leave the scope in the rings when on the bench.
Just a thought that there could be some flex in that area.
Anyways I am confident you will sort it out...all part of the fun.
Dont let it beat you.
Good luck
Alex
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Old 02-03-2018, 10:14 AM
croweater (Richard)
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Hi Dunk. Have you checked the bolts/screws that hold the mirror in its cell (from the sides). This will cause astigmatism if too tight. Also the same goes with the hold down clips. There must be no pressure on the mirror from these clips.They are just to stop your mirror falling out. Cheers Richard
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Old 02-03-2018, 10:25 AM
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Merlin66 (Ken)
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Not sure if it's the same... but on my 6" f3.6 Schmidt-Newtonian the secondary off-set is very noticeable and needs to be considered.
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Old 02-03-2018, 10:55 AM
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As well as all the other advice in here, have a look at how the secondary is mounted. If it is held on using silicone I would suspect your ‘stig may be caused by a strained secondary. Also, what mount have you got it on, and have you tested how stable the collimation is as you point the scope in different parts of the sky?
Lots of things to try, and although f4 is fast, a barlowed laser collimator should be perfectly adequate in your scope.
Cheers
Andrew.
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Old 02-03-2018, 04:05 PM
markas (Mark)
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Dunk,

There is a lot of good advice already on this thread.

But here is another consideration: perfect axial collimation of a fast Newtonian does NOT guarrantee absence of tilt of the image plane. The only condition where zero tilt is possible is that the focuser axis and the primary mirror axis intersect on the secondary mirror surface and the secondary long axis is at exactly 45degrees to both.

It is possible to achieve perfect axial collimation without the above condition being met.

With the size of your camera sensor, the problem is not as severe as with a large sensor, but you need to bear it in mind. I have seen distortions similar to what you have shown caused by tilt errors alone, although you probably have more than one problem here.

Mark
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Old 02-03-2018, 04:23 PM
rbronca
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Originally Posted by Camelopardalis http://www.iceinspace.com.au/vbiis/i...s/viewpost.gif
Thanks rbronca.

That’s a lot of not inexpensive accessories, and really makes me think that I’ll be cutting my losses and gunning for another refractor.


Bob's knobs are cheap. ~$40. You will put more in the swear jar playing will allen keys in the dark.
http://www.telescopes-astronomy.com....umb-screws.htm

I have seen several Catseye sets in the classifieds recently.
http://www.iceinspace.com.au/forum/s...d.php?t=164384

If your looking at newts being a long term thing, then that cost isn't huge and everything will last for ever. You will only need a new center spot for each scope you purchase.

If you know someone with Catseye tools then your only down a center spot and whatever bribe / inducement required to borrow them. If your a member of an astronomical society I'm sure someone will be more than happy to assist.

Getting it correctly setup once may be enough to allow your laser to provide the final tweaks each time. The secondary mirror setup is a one time thing.

Someone mentioned the secondary offset. The stalk should be centered, the offset is between the stalk and mirror. (Hopefully they glued it in the correct spot).

The focuser is more problematic. Replacement options aren't cheap, especially if you want to auto focus. Manual 2" Moonlites come up from time to time second hand. You just need the matching mounting for your scope.

This is exactly the same issue you will have on the refractor side.

I would suck it and see with the factory one for the time being. I used the factory one for a couple of years on mine.
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Old 02-03-2018, 04:27 PM
rbronca
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Quote:
Originally Posted by croweater View Post
Hi Dunk. Have you checked the bolts/screws that hold the mirror in its cell (from the sides). This will cause astigmatism if too tight. Also the same goes with the hold down clips. There must be no pressure on the mirror from these clips.They are just to stop your mirror falling out. Cheers Richard
I have had that issue and the stars become triangles, I'm not seeing that with his image.
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