Rescue my newt! (please)

[Camelopardalis]

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 :sadeyes: ...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

[codemonkey]

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.

[Camelopardalis]

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 :shrug:

What spacing are you using with your corrector?

[Camelopardalis]

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 :shrug:

[ErwinL]

Seems like the spider blades of the secondary mirror are distorted and perhaps also slightly missaligned.

[codemonkey]

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.

[Camelopardalis]

Thanks Erwin, I'll check it in daylight.

[Camelopardalis]

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.

[glend]

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.

[Camelopardalis]

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 :lol:

[rbronca]

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/showthread.php?t=164860

[Camelopardalis]

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.

[RobC]

Suggest you solve all your problems buy donating the scope to a local land fill project.

[xelasnave]

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

[croweater]

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

[Merlin66]

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.

[alocky]

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.

[markas]

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

[rbronca]

Originally Posted by Camelopardalis http://www.iceinspace.com.au/vbiis/images/buttons/viewpost.gif (http://www.iceinspace.com.au/forum/showthread.php?p=1360731#post136073 1)
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 (http://www.telescopes-astronomy.com.au/bobs-knobs-bobsknobs-collimation-thumb-screws.htm)

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

[rbronca]

I have had that issue and the stars become triangles, I'm not seeing that with his image.

[Camelopardalis]

Thanks everyone for all the great feedback :thumbup:

The scope has certainly given me a taste of its potential despite the ugly stars. Of course, I’d like to nail it eventually but it’s certainly feels like hitting a wall :eyepop:

Today I’ve been looking at a couple of points raised and listed here...

I’ve been collimating in a horizontal position in its rings, before transferring to the mount (NEQ6). I added 13” Losmandy dovetails (top and bottom) to improve rigidity of the ring structure themselves, and to separate the rings farther than the included single Vixen dovetail.

I removed the mirror cell to check the holder clips. These were really tight and I had to use some force with a couple of the screws. I loosened them enough to be able to slide a piece of paper underneath and out, before reinstalling the primary cell in the tube. I forgot to measure the primary spot was centred on the mirror, but it’s a simple enough operation to remove it and check tomorrow.

I have removed the secondary mirror and the adjustment screws had been digging into the secondary holder. I picked up a stainless steel washer of the perfect size from a local hardware store and have installed this between the adjustment screws and the holder surface. This has improved the manoeuvrability of the secondary substantially. Such a simple fix, I can’t see why SW don’t do this from the factory.

While I’m the area, I measured the spider vanes. The vane in the axis of the focuser was dialled out 5mm more than the others (measured to the centre screw), so I’ve adjusted this to match the others. It appeared to be distorting the shape of the open end of the scope - when I noticed it, I was looking squarely at it from the front and it was quite obvious. The same vane was also not straight...it looks a bit warped. I’m a bit puzzled by this as I’d not touched them before, the closest I got was adjusting the secondary screws.

I don’t fully understand the significance of the offset. I understand why it needs to be so, and SW state in the manual that it has been offset at the factory. Since I have removed the secondary and refitted it, have I destroyed that and need to reset it? I don’t yet grasp whether I need to factor this in.

Looking down the focuser tube, I tried my best to centre the secondary under the focuser. However, when I insert the collimation cap in the focuser, the clamps on the outer edge of the mirror are not then all visible without adjusting the secondary some more. Does this indicate that I haven’t accommodated the offset, or that the focuser is not aligned with the secondary?

With the secondary adjusted (I’m not going to say collimated!), I then centred the dark dot from the collimation cap into the centre circle on the primary mirror relatively easily. I then racked the focuser in and out and the spot stays put.

The mirror is stuck to the stalk with a sticky pad, although it seems pretty secure.

It appears that my laser has lost collimation, as the rotation trick is no longer working. Rotating it in the focuser traced out a circle about 5cm in diameter. It has adjustment screws, so I’ll just have to learn how to do it. This could have been off the previous night, but I had worked previously.

[codemonkey]

Ooh. Interesting. Sounds like you may have found the cause of the astigmatism given the description of the primary. Hopefully that solves the bulk of your issues.

It's a bit of a drive but you're welcome to borrow my Catseye kit if you want to go down that path Dunk. They're the only collimation tools I have now but I don't use them often 'cause I leave my newt permanently set up.

One thing to note re Catseye is they take a fair bit of learning to use well, so are a bit of an investment in time. To be honest I still don't know if I "get it".

[RickS]

Hi Dunk,

Do you have a license for that thing? :lol:

I have a Catseye collimation kit if you want to borrow it.

Cheers,
Rick.

[peter_4059]

I think the main issue is he will need the catseye spot on the primary for it to work properly. The normal paper reinforcing ring is not sized correctly for the visual cues to work.

[RickS]

Yeah, think I might have one of them too... or we could use a Troy 180 out of phase selfie.

[billdan]

" I don’t fully understand the significance of the offset. I understand why it needs to be so, and SW state in the manual that it has been offset at the factory. Since I have removed the secondary and refitted it, have I destroyed that and need to reset it? "

Hi Dunk,

What Skywatcher means is that the secondary mirror itself has the offset built into it. In this image attached your secondary should look like the one on the right hand side.

Cheers
Bill

[peter_4059]

That might work ok with the Brounaye colimation tools.

[Camelopardalis]

Thanks for the offers and explanations, guys - much appreciated! Hopefully all this instruction will advance me to a “P” before the winter goodies arrive ;)

I’m hoping loosening the mirror clips will help...but with the weather forecast I might be in for a long wait :sadeyes:

[gaseous]

I don’t know how your laser got so badly out of whack, but 5cm over the length of your tube is pretty substantial. I wish you luck in colllimating your laser - in my experience this was possibly one of the most frustrating things I’ve ever tried to do. A proper V-block would possibly be a better guide to how aligned it is - I found that anything less than a perfectly snug fit in the focuser will present apparent mis-collimation. Good luck!

[Camelopardalis]

:lol: yeah the problem multiplies :eyepop:

[glend]

Yeah, laser fitting into focuser tubes, we could have a thread on that alone (and have in the past). Not all focuser tubes, or adaptors, are the same diametre, and the various type of compression fittings can push the laser to one side. I suppose they allow some slack in the bore for various EPs that might all be the same diametre, and for EPs that might not matter too much. I found on my GSO laser that I had to shim it by wrapping a thin piece of plastic film around it to get it fit snug in the focuser and not push the beam off to one side when I tightend the knob. The self-centering adaptors are great but of course scopes don't come standard with those.
My Moonight focusers can't be faulted but stock ones are just too loose.

This slack in the bore can also be a factor in lasers showing collimation changes as scopes are moved on their axis.

[Shiraz]

Hi Dunk. lots of good advice here. FWIW, maybe a little more?

1. If the mirror assembly is like the 250f4, these scopes cannot be collimated while horizontal. The mirror is free-floating on cork pads and can also slide very slightly sideways - it can flop around alarmingly if the OTA is horizontal. It can hold collimation OK, but only if you initially do it with the mirror held back against the pads by gravity - suggest that you collimate on the mount with the scope pointing above the horizon by say 30+ degrees.

2. Collimation through the CC seems to work best - the CC can have enough wedge to throw off the collimation, so leave it in.

3. a simple jig to collimate your laser can be made with a bit of wood and 4 nails (image). Project on the wall and adjust the offset screws to fix any misalignment when you rotate the tube. A well aligned laser is plenty accurate enough to get these scopes working properly - if the centre spot is right and you clamp the laser tightly in the drawtube adapter.

3. your weird stars suggest that the light column is being truncated by the OTA, because the secondary is misaligned in-out. This might just be due to collimation varying when you put the scope up on the mount, but it could also be a more fundamental problem with the secondary alignment. It is nigh on impossible to get the secondary in the right place by looking down the focuser - your eye needs to be in the focal plane, which means that you cannot see the edges of the mirror (the drawtube gets in the way) and you must guess how well it is aligned. If you find that the stars still look crook after you have fixed the collimation problem, the back-projection method woks OK to get the secondary tidied up. Will put a summary in another thread, rather than clog this one up. Cheers Ray

[Camelopardalis]

Thanks Ray, great little jig btw. I think my level of handiness might even be able to manage that :)

The mirror is held in the inner metal cell by 3 rubber/plastic clamps, then the inner cell attaches to the outer metal cell by the 3 pairs of collimation push/pull screws. This whole unit is attached to the tube by half a dozen screws.

I'm going take the cell out again to check the centre spot is actually centred...

Certainly lots to learn with this scope :eyepop:

[andyc]

I think Richard (and possibly others) mentioned the primary mirror clamps - I'd definitely be checking those, as I had star shapes quite like yours when I tightened mine too much after cleaning the mirror one time - as Richard said these need to be barely touching the mirror with minimal pressure. If it's not that, then I'm not sure, others will know more than me on other issues. And collimate at roughly the tube elevation of your target as it can shift around a bit (I learned that the hard way on planets after over a year of lazy horizontal Newt collimation).

[Camelopardalis]

Thanks Andy, I need to take the mirror cell out again anyone so will see if I can find a sweet spot.

Clouds and rain have stopped play, however :sadeyes:

[Camelopardalis]

So I've just been idly poring over one of my seemingly futile captures from that night and having centred the secondary shadow in the doughnut, I took a short capture of the Moon in video mode.

With the FOV of the scope/camera, the Moon was occupying about 1/4 of the frame.

Stacked and lightly sharpened. I expected worse... :shrug:

[xelasnave]

Good one Dunk none of the stars are elongated☺
Alex

[Camelopardalis]

:lol: :lol: :thumbsup: