Tonight (hopefully) I will find out how much I've broken my Skywatcher 200PDS - Forums

As long as I get some clear sky I will hopefully find out the above.

* I've changed the focuser
* I've tried to re-collimate the secondary (big time). All axes altered (up and down the tube, rotationally around the OTA axis and tilted in relation to the primary).
* I've added Bob's knobs to make it easier if I have really screwed it.
* I've used my laser and cheshire to try and align it, but all during the day and inside.

Wish me luck!

Well it was supposed to be clear skies right now and there is not a star to be seen. I love the weather forecasters.

There is still some hope for the UK: Ventusky

Not much luck for Germany tonight ...

Fingers crossed
Wolfgang

OK, managed to get enough clear sky to polar align within 35 arc secs. Now capturing narrowband on the wizard nebula. Guiding is aroung 0.45 arc secs RMS despite the clouds. Stars, however, don't look round enough for my liking, so I probably need to do even more work on the collimation. I do think, however, that I have got rid of most of my diffraction spikes which is a good thing.

Step by step

NINA auto meridian flip worked perfectly with re-centre, so hopfully will get plenty of more frames to see where I am.

Aaaagh! Not a good result. My stars are a horrible shape, albeit I have got rid of most of the difraction stuff. Still a lot more work to do:

FWIW: This was just 10x300s Ha frames without matching flats through clouds.

Andy,
At first glance, it looks like trefoil. How did you realign everything with respect to the coma corrector? I think that you may have the optical axis either displaced or tilted WRT to the corrector.

John

John Hayes:
Andy,
At first glance, it looks like trefoil. How did you realign everything with respect to the coma corrector? I think that you may have the optical axis either displaced or tilted WRT to the corrector.

John

Thank you John! That suggestion is very helpful and I think may show me the blunder I have made. Would a secondary that is rotated about the axis of the OTA as opposed to being square-on to the focuser tube cause that? i.e. it would look more like an ellipse when viewed through the focuser than a perfect circle? That is the part I've found hardest to get right.

If you have issues in seeing that edge of the 2ndary against that of the sight tube then there is a problem. What is you 2ndary arrangement? Off-set centered, centered or tilted centered?

As for the previous question you would likely see one side of the field comatic and the other astigmatic, if your CC is anything like mines.

Andy Wray:
John Hayes:
Andy,
At first glance, it looks like trefoil. How did you realign everything with respect to the coma corrector? I think that you may have the optical axis either displaced or tilted WRT to the corrector.

John

Thank you John! That suggestion is very helpful and I think may show me the blunder I have made. Would a secondary that is rotated about the axis of the OTA as opposed to being square-on to the focuser tube cause that? i.e. it would look more like an ellipse when viewed through the focuser than a perfect circle? That is the part I've found hardest to get right.

Anything that causes the axis of the primary’s (a parabola) to not be coincident with the axis of the coma corrector will cause problems. Rotating the secondary about the axis of the tube would certainly be a problem. The view of the secondary through the focuser only affects vignetting so don’t use that view for anything but centering the secondary on the focuser axis. The tilt of the secondary then has to be adjusted to boresight the line formed by focuser axis so that it strikes the precise center of the primary. Then the primary tilt needs to be tweaked to retro-reflect that on-axis beam back on itself so that it passes down the center of the focuser axis (and the coma corrector…assuming that it’s well centered.) That’s about all there is to aligning a Newtonian!

John

John Hayes:
Andy Wray:
John Hayes:
Andy,
At first glance, it looks like trefoil. How did you realign everything with respect to the coma corrector? I think that you may have the optical axis either displaced or tilted WRT to the corrector.

John

Thank you John! That suggestion is very helpful and I think may show me the blunder I have made. Would a secondary that is rotated about the axis of the OTA as opposed to being square-on to the focuser tube cause that? i.e. it would look more like an ellipse when viewed through the focuser than a perfect circle? That is the part I've found hardest to get right.

Anything that causes the axis of the primary’s (a parabola) to not be coincident with the axis of the coma corrector will cause problems. Rotating the secondary about the axis of the tube would certainly be a problem. The view of the secondary through the focuser only affects vignetting so don’t use that view for anything but centering the secondary on the focuser axis. The tilt of the secondary then has to be adjusted to boresight the line formed by focuser axis so that it strikes the precise center of the primary. Then the primary tilt needs to be tweaked to retro-reflect that on-axis beam back on itself so that it passes down the center of the focuser axis (and the coma corrector…assuming that it’s well centered.) That’s about all there is to aligning a Newtonian!

John

Thanks again! I was being very stupid and rotating the secondary about the OTA axis to get even field illumination and vignetting and then using tilt on the secondary (with a laser) to hit the primary centre spot followed by using the same laser to adjust the primary. All finished off by checking with a cheshire. What hadn't clicked with me was how stupid that was as it could mean the secondary still being out of alignment with the coma corrector and camera axes.

It does show up in ASTAP as a tilt of 10 degrees which is much more than my typical 3 degrees with this imaging train.

Thank you Andrea and John! I spent some time today aligning the secondary under the focusser tube and now my trefoil stars look almost round. I have ordered a TS Concenter eyepiece because I still find it difficult to get it accurately aligned just looking down the focusser tube, however those horrible triangular stars seem to have disappeared.

A lot of people were saying that it was pinched optics, however I think it was just my bad collimation.

It's not perfect yet as I have about 3% tilt according to ASTAP, however I can live with that for now.

Having bought this cheap low profile, low throw, focusser I have got rid of all those horrible off-centre diffraction spikes caused by the stock focusser that impeded into the OTA by over an inch at prime focus.

Guiding is around 0.4 arc secs RMS despite my mount being overloaded.

Slowly but surely getting there.

Good stuff Andy! Glad to hear that you are on the right track.

John

Well the saga continues. I've realised that the relatively cheap replacement focuser I bought is not up to the job. It introduces tilt because of its poorly designed telescoping mechanism and it's brass compression rings that only use one screw to tighten them. It is now destined for the bin or eBay and I have reverted to the stock focusser.

It did prove to me that a lot of my lopsided diffraction spikes were due to the incursion of the stock focusser tube into the OTA, however its tilt problems were even worse as it impacted a lot of stars.

So where does that leave me?

* Much more knowledgeable in terms of aligning a focuser with the secondary

* I now know what type of focusser I need to buy to handle my coma corrector/imaging train.
* Much happier that I understand the order and how tos of the three key stages of collimation. i.e. 1) get the secondary aligned properly to the focus tube. 2) Get the secondary aligned to the Primary centre spot. 3) Align the primary. It sounds so simple now.

So, nothing broken, but hopefully much better from a collimation point of view. The only way to learn is to try stuff I guess.

Glad to hear you didn't break your scope! Sorry to hear about the new focuser. Let us know which focuser you settle on.

Because of my illness, I have been mostly doing indoor astronomy with my new Newt from TS. Because TS ran out of their stock focuses, they swapped it out with a Baader steeltrack at no added cost. Still evaluating, but it seems OK, so far. There are a number of other good options, many pricey. One thing is try to find a focuser that allows for screw on adaptors to your optical train.

Alan Brunelle:
Glad to hear you didn't break your scope! Sorry to hear about the new focuser. Let us know which focuser you settle on.

Because of my illness, I have been mostly doing indoor astronomy with my new Newt from TS. Because TS ran out of their stock focuses, they swapped it out with a Baader steeltrack at no added cost. Still evaluating, but it seems OK, so far. There are a number of other good options, many pricey. One thing is try to find a focuser that allows for screw on adaptors to your optical train.

I will probably go with the Baader steeltrack ... the only thing that is stopping me going with it is that I have to take a drill to my OTA to fit it. That said, I don't think I have many other options apart from sawing off 20mm from my stock focusser tube.

Andy Wray:
That said, I don't think I have many other options apart from sawing off 20mm from my stock focusser tube.

Sounds like the issue is the stock focus tube impinges into the image. Sawing off that 20mm may be a good first step. After all, if it fails, you were going to get a new focuser anyway...

John Hayes:
Andy,
At first glance, it looks like trefoil. How did you realign everything with respect to the coma corrector? I think that you may have the optical axis either displaced or tilted WRT to the corrector.

John

@John Hayes A big thank you John! You were spot on I believe. Having really focussed on my collimation/focusser alignment etc. and thrown out the cheap focusser and reverted to the original Skywatcher one that actually holds quite well and is pretty predictable I now have round stars and it looks like little tilt. This is the result from just one sub tonight, but it bodes quite well

N.B. I don't have a tilt adjuster, so this is just collimation + a fairly sturdy image train.

and here is an abberation shot of 14 Ha frames ... still more work for me to do, but not too shabby now for a $400 F5 200mm Chinese OTA. I will get rid of those horrible diffraction spikes one day. I'd also welcome any feedback on what you might see here ... I know my backfocus is close, but not sure I have it nailed yet as an example.

Andy,
Oh, yeah! That's looking a LOT better. However, you still have funny shapes in the bright stars. When everything is right, the stars should look perfectly round. Can you post the same analysis using just a single frame? Don't sell that scope short. Just because it's relatively inexpensive, doesn't necessarily mean that it won't produce nice round stars. Let's see if we can figure out if we can make it even better.

John

John Hayes:
Andy,
Oh, yeah! That's looking a LOT better. However, you still have funny shapes in the bright stars. When everything is right, the stars should look perfectly round. Can you post the same analysis using just a single frame? Don't sell that scope short. Just because it's relatively inexpensive, doesn't necessarily mean that it won't produce nice round stars. Let's see if we can figure out if we can make it even better.

John

Thanks John: here is a single frame. One problem I know I have is the Skywatcher focusser tube which impedes big time into the OTA at prime focus. That's why I bought the cheap low profile/low throw focusser. It did get rid of all the lopsided diffraction spikes, but also introduced far too many tilt issues. I think I need the best of both worlds, but would welcome any advice.

Also, my RA guiding was pretty poor tonight, s not sure if that is compounding the issue.

Andy,
Thanks for the single frame. The field performance looks really good but something still isn't right. When you get a chance, can you point at a bright star and put it in the center of the field and take three exposures for me? First take an exposure just to show the in-focus star shape using a relatively short exposure. Next defocus inside of focus to make the defocused star pretty big–say around 100 px. I'm thinking that you might have to rack the focus forward by maybe 5 mm. Then do the same thing outside of focus. Try not to totally blow out the diffraction pattern in the defocused star images. The best situation is if the star is bright enough to require a 10-15 second exposure. With the right exposure, you should be able to see a clean diffraction pattern in the linear data (i.e. no stretch). Don't use something like Vega that might require a 200 msec exposure. I'm guessing that a mag 3 or 4 star might be about right. Take 5-10 frames at each location and post the best ones cropped to show the star.

Let's see what that looks like. Maybe we can figure out why the stars aren't round. I suspect that you still have a small, but significant alignment error.

John

John Hayes:
Andy,
Thanks for the single frame. The field performance looks really good but something still isn't right. When you get a chance, can you point at a bright star and put it in the center of the field and take three exposures for me? First take an exposure just to show the in-focus star shape using a relatively short exposure. Next defocus inside of focus to make the defocused star pretty big--say around 100 px. I'm thinking that you might have to rack the focus forward by maybe 5 mm. Then do the same thing outside of focus. Try not to totally blow out the diffraction pattern in the defocused star images. The best situation is if the star is bright enough to require a 10-15 second exposure. With the right exposure, you should be able to see a clean diffraction pattern in the linear data (i.e. no stretch). Don't use something like Vega that might require a 200 msec exposure. I'm guessing that a mag 3 or 4 star might be about right. Take 5-10 frames at each location and post the best ones cropped to show the star.

Let's see what that looks like. Maybe we can figure out why the stars aren't round. I suspect that you still have a small, but significant alignment error.

John

I will do. I'll try and find a gap in the clouds tonight, otherwise it will be tomorrow when it is due to be clear.

Just out of interest, is it possible to overtighten a CC in a brass compression ring fitting? I have been tightening it up really hard recently.

Andy Wray:
Just out of interest, is it possible to overtighten a CC in a brass compression ring fitting? I have been tightening it up really hard recently.

Andy,
Yes!
Just did that very thing a month ago when I had my Newtonian out for the first time. I had been trying a number of different optical configurations that are off topic for this conversation, but I had originally had my CC directly threaded to the focus tube and took a few shots to see if I had the backfocus correct. The stars seemed very good for a first shot. Later, using a different configuration where I had put the compression clamp back on the focuser, I thought I would actually collect some data for real that night. I had things set up pretty well, but was concerned that the CC and camera would fall out, so I tightened things up a lot. I got sqare stars where the normal coma would be and even the stars dead center were not great. I hadn't even thought of this as I read the thread until you just mentioned it in your last post. I haven't been able to get out under the stars since. I too have lots of work to get things up and running. Maybe next month... I will attach a reduced sub from the pinched optic. Needless to say, I will not be using the clamp ever again! I just hope that I have the backfocus to allow me to work with the threaded on CC. I think I gained a few mm by moving my main mirror forward with the slack I had in the adjustment screws. Good luck. If this is not the solution, I am sure John will track it down.

Alan

Alan Brunelle:
Andy Wray:
Just out of interest, is it possible to overtighten a CC in a brass compression ring fitting? I have been tightening it up really hard recently.

Andy,
Yes!
Just did that very thing a month ago when I had my Newtonian out for the first time. I had been trying a number of different optical configurations that are off topic for this conversation, but I had originally had my CC directly threaded to the focus tube and took a few shots to see if I had the backfocus correct. The stars seemed very good for a first shot. Later, using a different configuration where I had put the compression clamp back on the focuser, I thought I would actually collect some data for real that night. I had things set up pretty well, but was concerned that the CC and camera would fall out, so I tightened things up a lot. I got sqare stars where the normal coma would be and even the stars dead center were not great. I hadn't even thought of this as I read the thread until you just mentioned it in your last post. I haven't been able to get out under the stars since. I too have lots of work to get things up and running. Maybe next month... I will attach a reduced sub from the pinched optic. Needless to say, I will not be using the clamp ever again! I just hope that I have the backfocus to allow me to work with the threaded on CC. I think I gained a few mm by moving my main mirror forward with the slack I had in the adjustment screws. Good luck. If this is not the solution, I am sure John will track it down.

Alan

You just tilted the CC, poor thing, out of kelter with all that clamping, as it is shown in the right hand side of the image (out of focus stars showing coma). Nothing to do with pinched optics as I think the force required is far too great to apply without amplifying tools (for normal people, that is).

andrea tasselli:
You just tilted the CC, poor thing, out of kelter with all that clamping, as it is shown in the right hand side of the image (out of focus stars showing coma). Nothing to do with pinched optics as I think the force required is far too great to apply without amplifying tools (for normal people, that is).

I wouldn't trust my setup enough to say that my clamp forced a tilt to the CC. What I mean is this was basically first light, and I am not sure I even got the collimation done correctly. This after changing out my secondary a few times. I do see that the stars (square, as they may be) are certainly more out of focus in the lower right hand corner of the image. So likely some tilt or collimation issue. But the stars are sure square in all four corners! There may be more than one thing going on here.

I wanted to be sure things would not fall out, so I tightened quite tightly. Used a torque wrench with a 1 meter extension!

Whether Andy's issue is tilt or pinched optics, it is super easy to diagnose next time he gets clear skies. Either way, I would recommend a threaded connection, or lacking that, some sort of adaptor in the tube that can get him threads to use.

Andy Wray:
Just out of interest, is it possible to overtighten a CC in a brass compression ring fitting? I have been tightening it up really hard recently.

YES! I would be very careful not to over-tighten components. That definitely could be a source of your funny shaped stars. Loosen it up and look at the star shapes again.

John