Collimation on GSO f/4 newt

[Amaranthus]

I just received my new fast wide-field astrograph, a Guan Sheng GS-550 (aperture 153 mm, focal length 612 mm, f/4). It's a fantastically 'maneuverable' OTA (so light, even for a steel tube), and looks well constructed. The monorail focuser also looks well constructed, and should easily handle my light imaging train (1 kg). It's basically this model, except I got it from Andrews Comm. http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4762_GSO-6--Imaging-Newton---150mm-Oeffnung-f-4---2--MONORAIL.html

BUT... this is the first ultra-fast newt I've owned (my previous 8" newt [back in the day] was an f/7.5), and I'm just re-learning collimation on these babies. With f/4, it seems it's a whole new ball game. I currently have a Cheshire eyepiece, a collimation cap, and a Laser collimator, but I might need to invest in a Catseye kit?

Anyway, to my first question on this new adventure. I see six primary mirror collimation knobs on the GSO newt - three are relatively large, and 3 are smaller and have a flat-screwdriver-head (you can see them on one of the photos here: http://stargazerslounge.com/topic/196073-tsgso-150mm-f4-imaging-newtonian/). I presume the larger knobs are for collimation of the primary, and the smaller ones are the locking screws, but, when I tighten the locking screws up, I also get mirror movement. :shrug:

Also, in these fast newts, should I expect the secondary to be offset?

[graham.hobart]

Hi Barry, on my Vixen f 4 the instructions for collimation are quite specific about secondary offset. Certainly the collimation changed when I didn't have the offset.
I can send you copies of the manual if you like or its on the Vixen website under downloads or support I think

[glend]

Suggest you read Astrobaby's excellent information on collimation of newtonians, I think it is posted in the articles section here.

At f4 collimation is a bit harder and the spot at which perfect collimation lies is small so care must be taken.

Re the screws, three are lock knobs which push, and three are adjustment knobs which pull. It is possible to get movement when you tighten the lock knobs. The trick is to get the collimation as close as you can with the adjustment (pull knobs (the ones with the springs) and then tighten up the lock knobs (the push ones), you may have to loosed the pull knob slightly so that the mirror then gets pushed into perfect collimation with the lock knob. Is that clear?, sounds confusing but it works. Also I recommend a spring upgrade. GSO mirror springs in that scope size range have a reputation for being a little weak, and a stronger spring may help the pull knobs hold the collimation better by pushing the mirror harder so that the push knobs then don't push it out of collimation when tightened - even more confusing eh?

You can usually find a suitable replacement spring at Bunnings.

[rustigsmed]

Hi Barry,

Congrats on the purchase, looking forward to the view the small newt will give vs a refractor.

I have an f4 GS newt, I upgraded the springs and after reading something online regarding the nature of push/pull knobs I simply removed the 'pull' knobs... works fine with the stronger springs and saves some stuffing around - that f4 is sensitive.

Cheers

[Amaranthus]

Thanks Graham, I grabbed a copy of the PDF for collimating the R200SS and will have a careful read. The GSO scope does not come with any manual!

Thanks Glen for the advice on those screws. Should I loose the lock knows completely before I twiddle with the collimation knobs, or just ease them back a little?

I've armed myself with Astrobaby's excellent explanation, and a few others. A bit of reading to do!

Do folks think a cheap laser + cheshire is sufficient for tuning collimation at f/4, or do I need to invest more heavily in this kit?

[Amaranthus]

Russell, do you have to disassemble and remove the primary mirror to replace the springs? Sounds a little daunting, and I'm not sure what springs to get...

BTW, I also purchased a 10" Quattro CF f/4 as a job lot :) But I figured I'd 'do my learnin' on the smaller 6" OTA first! Until I'm happy I know the score, the beautiful 10" stays put.

[Moon]

Barry,

I have an 8 inch and 12 inch version of that scope. The 6 inch should be a lot of fun.

Here are some images (http://deepspaceplace.com/images.php?sort=&filter=AT8IN) from the 8 inch version.
If you are having trouble with the collimation / locking screws then stronger springs might be a good idea. If you put in 6 springs (instead of 3) you might not even need to use the locking screws, they just stop the springs popping out.
Collimation and focus is going to be critical. If it's off, the images will look terrible. I'm of the view a full secondary offset is best - i.e. both away from the focuser and towards the primary.

I have a tip that has helped me and might be of some use: cut 4 wooden blocks that fit exactly between the secondary mirror and the tube wall. One for the top, bottom, left and right. This will do 2 things:

1) it will help you make sure the secondary is in the exact center of the tube (left / right) The left/right secondary collimation screws tend to swing the secondary mirror through an arc and will move the secondary mirror away from the center of the tube. Only use those 2 screws to get the secondary into the center of the tube. That will also place it directly under the focuser. Rotate the secondary to align it with the primary and use the remaining secondary screw to get the tilt correct.

2) it's a great way to check the offset is correct. The top block should be 2 x the offset longer than the bottom block.

There are lots of good resources out there, such as http://www.catseyecollimation.com/mccluney.html

My advice is to take your time, and don't skip any steps. I have seen scopes where the factory installed center spot was off and also where the focuser was not square to the tube. You would think this could not happen, but it does.

Once you are done, rotate the tube and see if the collimation holds - if not investigate why.

The catseye tools are a good investment - the "radiation" center spot is awesome when paired with the INFINITY XLP and it's real eye opener to see low little movement is required to loose colimation. In reality the Teletube XL is probably all you need. A laser won't help much sorry to say. A final tiny tweak of the primary under the stars with CCDInspector can help you get the last little bit. You will also need to find a way to get perfect focus - for me it's a robofocus and FocusMax.

Happy imaging.

James

[Amaranthus]

Thanks James, that's really useful. Much appreciated.

I'm approaching collimation of my fast newt like everything else in astrophotography - a steady and careful journey, not a quick fix. This philosophy has worked well for me so far - it's amazing to me now how confident I am (and how easy things seem) in so many matters that, when I started, seemed incredibly daunting. I'm sure this will be the same :)

[Shiraz]

I spent many happy hours with one of these, mainly trying to figure out how to align it. Also occasionally managed to do some imaging.. EDIT: no I didn't, yours is a 150..:P

might get a few ideas from these:
http://www.iceinspace.com.au/forum/showthread.php?t=113291
http://www.iceinspace.com.au/forum/showthread.php?t=111441
http://www.iceinspace.com.au/forum/showthread.php?t=105392http://www.iceinspace.com.au/forum/showthread.php?t=82230

[Amaranthus]

Thanks Ray, they're useful threads to read. I figure with the light 6" model, the mechanics of the scope might be quite acceptable, including perhaps the springs. It seems the problems tend to arise when these are scaled up to larger versions. Anyway, time will tell!

[Shiraz]

mine was definitely too floppy in native form, but the 6 inch may be better. I think that the mechanical components are just scaled from whatever works in their dobs - a little flex is OK in a Dob, but not for imaging. However, the optics are not much short of excellent and they can work really well if the seeing is good. Have fun. regards Ray

also, I find this tool to be essential for aligning the secondary, but others do not seem to like it, since what it shows is not immediately obvious - might be worth a look. http://www.iceinspace.com.au/forum/showthread.php?t=82230

the secondary in my 200 was not offset, so I remade it with proper offset - reduced stray scatter a lot.

[rustigsmed]

Sorry barry I just checked out the CN thread the back of the scope looks quite different to my model (previously I looked at the TS link and it didn't show the back).

therefore it may work well enough in your design perhaps best to ignore my comment and test it out first.

I have a 12" skywatcher dob and found their factory springs fine to use - so you may not need to fiddle at all with springs when you get to the quattro. :thumbsup:

[cometcatcher]

I have the Bintel (GSO) 8" F4. The only thing I collimate it with is a cheap Cheshire eyepiece that I got from Astro shop. The 6 mirror screws are a silly way to do it but they work. Once roughly collimated, I tighten the locking screws to tweak final collimation and lock it in place. Surprisingly, doing it this way my scope has held collimation since I got it in December. I want the 10" F4 next. :P

[Jason D]

Secondary mirror offset is one of the most misunderstood alignment in collimation.
There are two secondary mirror offsets:
1- First offset is towards the primary mirror. This is an easy one. Centering the secondary mirror under the focuser will take care of this offset automatically -- no need to be preoccupied with it.
3- Second offset is away from the focuser. This is the most misunderstood offset. This offset is required only when the reflector comes with a corrective lens located the OTA opening. Manufacturers will take care of mounting the secondary with the proper offset for these scopes -- no need to be preoccupied with it. In addition, this offset "might" be needed when the scope comes with tight baffles and/or tight OTA opening. Again, manufactures take care of this offset for such scopes. The 3rd and last reason why such an offset might be needed is to improve the reading accuracy of DSC though this is a weak reason.

In general, do not think too much about the secondary offset. If it is needed, the manufacture will take care of it.



Hi Graham, can you elaborate? What was specific about their instructions? Is there a web link to their instructions?



Hi James, I do not believe these steps will help much. Following the typical collimation steps should suffice.



Hi Ray,
In your first post of the link you provided, you stated "it seemed that most Newt alignment procedures get the mirrors and focuser properly harmonised, but largely ignore the alignment of the optical system with the tube itself"
Why do you think it is important to align the optical system to the tube after a successful (or harmonized) alignment between optics?
Interestingly, scopes are designed to get the mechanical parts out of the light path. It does not matter if the OTA is square or round or bend or even irregular. As long as the OTA is out of the light path and as long as the optics are well-aligned with respect to one another then you have achieved good collimation.

Jason

[glend]

Secondary offset is important in 'fast' newts and it is shown well, as are the comparative differences with slower newt, in Astro-baby's instructions.

[Jason D]

Hi Glen,
Which offset are you referring to? Is it the one towards the primary mirror or the one away from the focuser? The first one is important and as I mentioned is taken care of when the secondary mirror is centered under the focuser regardless of the scope's F-ratio. The second one is not important except for the few cases I have listed.
Jason

[Shiraz]

Barry, hope you don't mind me responding in your thread. also, my apologies for not reading your initial post properly.

Hi Jason. Agree that the manufacturers should take care of offset if needed and make sure that the light clears the OTA, but they don't necessarily do so.

My 200f4 has a tube with 220mm free aperture, so, with a 20mm ccd, it is guaranteed that some of the incoming light will impact with the ota on the way in when it is trying to get to the corners of the chip - even if the central light column is perfectly centered. But it wasn't centered anyway, because the secondary was not offset away from the focuser - so after collimation (particularly the act of centering the secondary under the focuser), the central light column was necessarily offset away from the OTA centreline. This ensured that even more of the light that should have formed the image, hit the OTA on the way in. The light that got there had to run past the rough lip of the OTA, several little casting bulges and the bolts that hold the spider vanes. The result was very messy, with asymmetric diffraction patterns around bright stars, that varied with their positions in the image.

The problems were partially fixed by remaking the secondary with some offset to bring the central light column back into alignment with the OTA when collimated, but ultimately, the thing needs a bigger OTA diameter - but that brings with it problems with back focus and requires a larger secondary.....

I agree that these issues are non-events with visual scopes or even with imaging scopes of about f5 and above. But they can have a very deleterious effect on image quality with some fast scopes, particularly those that have been assembled using components that were initially designed for slower visual scopes (eg, the (no offset) secondary in my 200f4 was interchangable with that in my 300f5 Dob). If you have a fast scope, it is worth doing a back projection exercise to see how much effect the secondary position/offset can have - it can be a bit scary when you see how far out a properly collimated fast scope can be and a lot of the woes of fast scope imaging can be traced back to this as the cause.

[Amaranthus]

Thanks everyone for this info. Much useful knowledge/experience to digest. Got a bunch of material printed out now, giving different perspectives on collimation of fast newts, which I plan to read during a long flight I'm about to take. When I get back to Oz in a week, I'll try it out.

Ray, I'm using an ICX694, so I suspect I'm not going to get a lot of vignetting. But I'll test, see, adjust, etc. I'm in this f/4 newt game for the long haul (haven't even unwrapped my 10" Quattro CF yet!), and am willing to work to get things just right - eventually.

[Jason D]

If the OTA opening is too tight then I can understand why you would want to offset the secondary mirror away from the focuser -- I did include this reason on my list.

However, only stars that fall close to one edge of the frame might be impacted -- not the ones in the central vicinity and the opposite edge of the frame.





I did it years ago with a laser and a holographic attachment. But you need to be careful. When the source of light is below the focal plane the reflected light off the primary mirror will diverge and gives you worse reading.


Jason

[Shiraz]

enjoy the trip. The main problem with my system was diffraction effects - doubt vignetting will be a problem for you. The 10 inch f4 CF is a great scope - look forward to seeing what you can do with it.


with my scope and sensor, the worst diffraction effects covered about half the field from memory (ie it was significantly compromised) - now I get some issues across much of the field, but less noticeable. Barry's scope is the same brand, so may share a similar design approach (and possibly problems).

That's a good point re the placement of the source. Also, it is often not mentioned that your eye must be in the focal plane when using a peephole to position the secondary - for similar reasons.

[Jason D]

If you do not apply the away-from-focuser secondary offset then the incoming light for the central star will shift my around 2-3mm closer to the OTA opening edge (the edge above the focuser). You should not see issues for the central area since you have 10mm margin around your primary mirror.




In theory, the proper placement of the peephole is above the focal plane -- at the point where the reflection of the primary mirror coincides with the edge of the secondary mirror regardless of the location of the focal plane.

Jason

[Shiraz]

It's not that clean in practice Jason. My scope has numerous intrusions into the light column (7x casting ridges about 5mm high, 4x bolts/screw blocks for the spider and a screw to hold the safety clip, which clip also intrudes onto the secondary). With my QHY8, only the central 20% or so is free of noticeable input-aperture-related excess diffraction on my 200f4 scope (with the offset secondary). The offset required was about 4.5mm from memory (used formula in Suiter's book). Without it, I guess that the input column would have been about 4mm closer to the edge of the OTA.

You can see similar spider support intrusions and clip screw on the 150mm scope in the front-on image in the link that Barry posted http://stargazerslounge.com/topic/196073-tsgso-150mm-f4-imaging-newtonian/ That image also shows that the secondary on the 150f4 does not appear to have an offset and it also shows the other potential diffraction source that commonly appears with a small fast scope - the intrusion of the focuser into the light column. The test image in that link shows a bright star on the left hand side with a messy diffraction pattern around it. It looks a bit like I used to get before I added secondary offset (and repositioned the primary and increased the secondary size to get the focuser out of the light column). I came to the conclusion that getting the secondary offset and placement right is mandatory if you want nice looking stars from a small fast Newtonian and found that back projection is the best way to get optimal longitudinal and skew alignment of an offset (and slightly oversize) secondary. It will be very interesting to see what Barry finds with his new scope.

excellent graphic explaining optimal eye placement for secondary alignment - thanks for posting :thumbsup:

[Jason D]

Sounds like the interior of your OTA is an obstacle course for the photons :)
You did a good job in explaining why your scope needs the offset. Good luck to you and Barry.
Jason

[Amaranthus]

Thanks Ray and Jason. I'll report back here when I get some results!