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Omegon_GSO_RC One Step Forward and One Step Back!

astrophotography ccd collimation optics
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#1 Darth_Takahashi

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Posted 07 July 2020 - 07:50 AM

First step was to make a video of the optics and the current collimation. This all looked fine, however, you could see a lot of tubulence in the atmosphere, even above 60 degrees. Still I pushed on with the next steps.

 

1, Check collimations

2, Refine polar alignment

3, Tracking test with new mount CEM120 and the ZWO OAG

4, Image test run

 

I now have the polar alignment approximately less than 3 arcminutes. I'd like to get this back under 2 arcminutes moving forward. The ZWO OAG and CEM120 worked great together, however, I only realised later that I had inadvertenatly introduced another issue!

 

I will say that overall I'm happy with the test image of IC5146 in Ha. This is less than 1 hour, the stars are not great because there is TILT in the system!!! This comes back to what Timo has stated in his other RC collimation posts. Most issues lie with the camera's coupling to the telescope. I found my problem too late at night to correct it.

 

IMG_0390.jpg

 

The screw that I added to stabilise the ZWO OAG is now preventing the imaging train from seating nicely against the telescope! Squarely, that is!!!

 

IC5146.jpg

 

Now looking at the image you can see the shadow of the OAG marked with the angled line and the tilt it has introduced. Luckly I realised I had an issue and didn't touch the collimation to try and correct what is a camera to telescope connection issue.

 

So I'm making progress albe it slowly. These are the kind of issues you can expect when you buy a new telescope, its need some tuning / adjusting to be able to get the most out of it. Especally because this is my first OAG and I cheaped out and brought the first generation secondhand before realize why they changed it.

 

Regards

 

 

Neil


Edited by Darth_Takahashi, 07 July 2020 - 03:09 PM.


#2 Timo I

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Posted 07 July 2020 - 10:00 AM

Yeah, your example above shows quite perfectly how any RCT user could get into serious side-tracks with his RCT scope in no time. I guess now you will make it 100% sure, that no tilt exists anywhere in your imaging setup, before you touch those feared collimation screws in your setup... lol.gif  (Focuser levelling and OAG modifications are quite harmless actions compared to "huge" (=way too big) 1/8 turn twist in the secondary collimation screws.) Just try to keep your your RCT's optical setup as balanced as possible (=unaltered). Even minor tilting somewhere in your optical train could have as destructive effect to your images as that dreaded mis-collimation in optics. (I had only the standard 2" eyepiece adaption setup to my RCT over then and I needed to align my 2" AP CCD-T67 reducer + TS OAG-27 + QSI583ws camera combination several times into my focuser, before I was happy with the corner stars and was sure that no tilt existed there.)


Edited by Timo I, 07 July 2020 - 10:02 AM.

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#3 Darth_Takahashi

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Posted 09 July 2020 - 12:19 PM

OK, I had my focuser off and disassembled today and yes its a pile of junk. I was going to put the ZWO EAF on it but on reflection have decided against it. This is now on the list to be replaced at Christmas with something much more suitable.lol.gif

 

One of these should fit the bill.

 

https://www.primaluc...crofocuser.html

 

Regards

 

 

Neil



#4 Darth_Takahashi

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Posted 13 July 2020 - 04:49 AM

Success at last. This weekend has been great becuase its been a play weekend. Clear enough to collimate but not clear enough to do some imaging. Partially cloudy etc...

 

Well I couldn't get the scope collimated Saturday night, no matter what I did. So I decided to come back to it Sinday morning in the day light with a clear mind. With the laser collimator and cheshire I just couldn't get it to look right. Just slightly off no matter what I did. Having all of the measurements I took the secondary off and was shocked to learn that the laser wasn't passing through the middle of the bolt hole!!!

 

Step 1, Reset all my focuser tilt adjustments. This put the laser through the hole first time but slightly off. Made the absolute minimum adjustment possible to get the laser centered with the hole. racked the focuer in and out to confirm stability.

 

Step 2, Reinstalled the secordary and aligned with the focuser perfectly with the shadow of the donut on the laser. My central spring seems to be bigger and stronger than the one I have see in other posts. So nothing to fix or complain about here.

 

Step 3, Aligned the primary using the cheshire and everything looked great. For the first time a saw a light grey ring around the secondary/primary baffle. I had previously never seen this because the shroud was on.

 

Step 4, Reread the DSI procedure especially concerning the ccd camera alignment and the clues to which bolts to adjust based upon the star shapes.

 

Step 5, adjust using a star field. No matter what I did I personally struggle to use defocuser stars. I used focused stars and MaxIm zoomed into 200%. I picked the wrong primary adjustment screw first!!! Returned it to its starting position (1/8turn back). found the correct adjustment screw which made my star shapes worse, changed direction and then the magic started to happen.

 

I probably could have gone a little further but I was making 1/16 adjustments and it was getting cloudy again. I break in the clouds came my way and I tested the adjustments i had just made on an open cluster NGC6819.

 

NGC6819-L.jpg

 

If there is a morale here it is to check and double check your own adjustments. When necessary go back to the beginning and build back up from there.

 

Regards

 

 

Neil


Edited by Darth_Takahashi, 13 July 2020 - 06:37 AM.

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#5 Darth_Takahashi

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Posted 13 July 2020 - 04:55 AM

Here is the final LRGB image, just under I hour taken under patchy sky's. I'm very happy with the stars in this image. I can live with this as is but will probably just try to dial in those last adjustments on another night.

 

NGC6819-LRGB.jpg

 

No sharpening, no decon, no complex image processing just as it is from the scope and camera.

 

These telescopes are a pain but it can be done, collimation that is! waytogo.gif

 

Regards

 

 

Neil


Edited by Darth_Takahashi, 13 July 2020 - 05:00 AM.

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#6 John Miele

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Posted 13 July 2020 - 05:36 PM

".... I can live with this as is but will probably just try to dial in those last adjustments on another night."

 

 

Regards

 

 

Neil

I warn you 'O Darth Takahashi...when you get this point..."many men have tried...they tried and failed?...they tried and died"...my apologies to Frank Herbert..LOL!!!


Edited by John Miele, 13 July 2020 - 05:38 PM.

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#7 Timo I

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Posted 14 July 2020 - 03:10 AM


Step 5, adjust using a star field. No matter what I did I personally struggle to use defocuser stars. I used focused stars and MaxIm zoomed into 200%. I picked the wrong primary adjustment screw first!!! Returned it to its starting position (1/8turn back). found the correct adjustment screw which made my star shapes worse, changed direction and then the magic started to happen.

 

I probably could have gone a little further but I was making 1/16 adjustments and it was getting cloudy again. I break in the clouds came my way and I tested the adjustments i had just made on an open cluster NGC6819.

Yeah, I guess here are the essentials for exact RCT collimation. You need to know when to stop adjusting (maybe due to clouds coming), but always need to remember there's tomorrow coming for the same task to continue. I can see some minor changes into your star shapes on the right side of this imaging area and a definite improvement in the latter image too. wink.gif

 

What you could test next is to get DSI view for the unfocused star field like this.

I used there 4x4 binning, because you could see everything necessary even with that resolution. Look for the corner stars and how they are "pointing" sideways (read your DSI guide here). Every four corner should be equally "balanced" to each other now. If this test shows some corner having more "pointy" stars , then you could make that 1/16...1/32 turn adjustment to the one secondary adjustment screw only handling that direction.

Please notice, that those "pointy" stars could point towards the image center too, but that relies on the focusing (inside/outside focus point views give that 90 degree turn there in this DSI test).

 

_small.jpg

 

After that you could take another 100 % focused star field and inspect with full 100...200 % zoom how the stars look like in the corners with 1x1 binning.

 

Then (if you are happy with the secondary adjustment), then align one medium bright star in the dead center of your imaging area and take this kind of unfocused 1x1 binned and 300...400 % zoomed-in test image from that star. Wait for seeing effects and average what you see there on your screen before you make any adjustments to your primary mirror. Remember the same principle as before ie. find the correct collimation screw, adjust one 1/16...1/32 turn to that screw and verify the result again by re-centering the star. This all takes several hours, because you might need to re-iterate between primary and secondary mirror adjustments until both give you as perfect results as possible.

 

_small.jpg

 

You might also take some focussed star field test images between each and ever adjustment, but when you get routined with this method, then you can rely on DSI alone.

My end result from that time collimation attempt is here for my 10" RCT. As you can examine from there, center area stars are quite perfect there, but my corner stars were not. This was caused by my RCT scope's mechanical imperfections with these mirror and focuser alignments (lower left corner there). I needed to adjust those mechanically during daytime and test star collimation again with DSI after that mechanical adjustment. Anyway, as you can see it's quite possible to get decent results with this DSI method only.

 

_small.jpg

 

I suppose you won't see anything like that with your scope, but instead only perfectly collimated stars all around that test star field. (Please remember, your imaging scale will be much higher with your RCT scope (0,83"/pixel there in my RCT image). Good luck for your attempts!  smile.gif


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#8 Darth_Takahashi

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Posted 14 July 2020 - 03:59 AM

I warn you 'O Darth Takahashi...when you get this point..."many men have tried...they tried and failed?...they tried and died"...my apologies to Frank Herbert..LOL!!!

I wish you had told me this before last night!lol.gif Yes, I made it worse already...shocked.gif However, I know what I did wrong! I tried to make it better!!!

 

Seriously, when its clear I know how to get back to where I was. You never know what you have got until you lose it, right.waytogo.gif

 

Regards

 

Neil
 


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#9 John Miele

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Posted 14 July 2020 - 09:56 AM

LOL!

 

I can tell you I had an AT8RC and probably had my collimation within 95% of perfect. But I just HAD to tweak it to get that last little bit or performance. Well, somehow that started a massive spiral of miscollimation that I never recovered from. Never even got it back to that 95% level...I got it close but not as good as before I started...crazy.gif


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#10 Timo I

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Posted 15 July 2020 - 01:21 PM

John is absolutely correct there (or at least I have similar experiences from my previously owned 10" GSO RCT scope). Tweaking any RCT's collimation into over 95% is a bit like balancing two eggs on top of each other. If you can manage that without breaking the eggs, then you are fully qualified to turn those collimation screws! lol.gif  But seriously, let's hope Neil can present some good news next to this forum (=no pressure from us, because I can almost personally feel your pain in the night, when you noticed that happening).


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#11 Timo I

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Posted 18 July 2020 - 03:04 PM


Well I couldn't get the scope collimated Saturday night, no matter what I did. So I decided to come back to it Sinday morning in the day light with a clear mind. With the laser collimator and cheshire I just couldn't get it to look right. Just slightly off no matter what I did. Having all of the measurements I took the secondary off and was shocked to learn that the laser wasn't passing through the middle of the bolt hole!!!

 

Step 1, Reset all my focuser tilt adjustments. This put the laser through the hole first time but slightly off. Made the absolute minimum adjustment possible to get the laser centered with the hole. racked the focuer in and out to confirm stability.

Step 2, Reinstalled the secordary and aligned with the focuser perfectly with the shadow of the donut on the laser. My central spring seems to be bigger and stronger than the one I have see in other posts. So nothing to fix or complain about here.

 

One thing that bugs my mind here with this case so much, that I'd need to tell it aloud to this CN forum... confused1.gif

As you have removed your secondary and then put it back there, it's always possible to put it into somewhat (very slightly) different position, where your secondary was before (as it left from the factory).

I know there's a conical area in the center of your spider crosshairs and there's another conical screw head used there for tightening this secondary into its' spider support. So this structure should auto-align the secondary in the dead center of the spider, but in practise it gives your scope's secondary a very small sideways movement, where this secondary mirror could be located inside your scope tube.

 

I myself have learned this the hard way in the past, but when ever you remove the secondary from the scope, it's highly important to get it's optical mirror very accurately centered into the tube (ie. exactly the way as it was, when it left from the factory). Otherwise DSI method will NOT give you perfect results from your collimation attempts with it. The difference tolerance in secondary mirror's sideways movement seems to be extremely limited with these RCT scopes.

So if your collimation attempts constantly fail with the DSI method, then you might need to check how well your secondary alignment inside the tube is centered. This can be done by measuring secondary distance from 3-4 different points to the inside border of the tube. The differences should be very small ie. your secondary should be centered in the tube exactly (with a fraction of a millimeter).

 

In addition you're supposing there, that the secondary's center mark (non-aluminized part of the mirror) has been now been aligned very closely to the center of the secondary itself at the factory. In practise, there's also a very small play with the secondary mirror's own position in its' own holder. So it's quite possible, that your secondary needs to be rotated, before it can give you perfect results with DSI methodcrazy.gif  (= I'm supposing here that GSO factory has not centered the mirror perfectly in its' holder and thus it has now only one rotational position, where it can be successfully collimated with the same scope's primary).  As you can see, it's also possible, that the tolerances for the mechanical alignment of a RCT secondary mirror could give some average RCT user very serious issues with such collimation methods, which rely on the optical collimation of the mirrors.

 

After this longish prologue I'm asking from you have you managed to check this RCT's collimation with the suggested DSI method of has there been constantly too cloudy during nights to report anything?

Where do we stand with this RCT scope?


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#12 Darth_Takahashi

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Posted 19 July 2020 - 07:04 AM

So I wanted to give a quick update. We back to collimation, well 92% at least. The weather hasn't cooperated at all. When I get a good clear night I'm sure that we are only a whisker away from where I was or better!

 

Snip1.png

 

I'd like take a few more to make sure that I'm moving in the right direction.

 

The secondary was replace exactly where it was from the factory. How do I know? Two reason's, before I removed it everything was measured with a digital vernier and secondly the screw marks were all realigned. When your observant you can see that the bottom most screw mark is slightly worse than the other two. So no it wasn't rotated 120 degrees either! wink.gif

 

What I have noticed while playing around collimating it on sharp stars is that I can see the astigmatism rotating! we are now talking about very small adjustments of 1/16 of a turn on the primary, that's how close it is now.waytogo.gif

 

Regards

 

 

 

Neil


Edited by Darth_Takahashi, 19 July 2020 - 03:23 PM.


#13 Timo I

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Posted 19 July 2020 - 01:37 PM

Ok, thanks for the info!

Looking good over there then... laugh.gif

I migth be currently a little bit too much worried here due to akulapanam's comment here so I'd like to double/triple check every little error possibility, where my 1st choice "DSI method" could fail.

It's kinda exciting task to follow up your progress there, so please keep posting those reports and your findings with your RCT scope's collimation task.


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#14 Darth_Takahashi

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Posted 19 July 2020 - 03:21 PM

I also honestly think I'm a little over collimated but I was to put it buntly "p1ss1ng about" under patchy sky conditions. However, I'm confident given 1 hour under a clear sky I'm done.

 

CCD-Image-35.jpg

 

This is a crap image but it demostrates nicely the sky conditions I was working with. The telescope is very close I'm a little over corrected on the 'C' primary screw, I need to back it out by an 1/8 or so and re-check the extra-focus and in focus image etc...

 

Finally, some perpective from my side. I'm enjoying the Omegon RCT telescope. It has shown me a lot of promise. I have only myself to blame for adjusting it when I could be happily using it. Do I regret that, no! I have learn't so much about the way my telescope behaves approching collimation. People are talking a lot about mechanical issues??? There is really only one on the newer GSO RC telescopes. The primary bolts are too course(1.25mm thread pitch) and need replacing with a finer threaded bolt, that's it. Will this prevent me from reaching collimation, NO, but it could have been made easier with better bolts. Would I be willing to pay more for that, yes but only an additional 200$.

 

Monday is promising to be clear. Let's see what it brings.

 

Regards

 

 

Neil



#15 Darth_Takahashi

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Posted 21 July 2020 - 03:24 AM

One more iteration and I'm done. The smaller stars are a little pointed and I believe this is the secondary that needs a very small tweak. Since the same issue is also seen off-axis.

 

NGC6819_LRGB.jpg

 

However, this is very useable now and with a little bit of pixel manipulation I could very easily fix this minor issue so that you would never know it was there.waytogo.gif

 

So we are back to approx 93% and are now looking at very very small adjustments. How much the actual focus point and seeing has to do with it is also unknown. I will at least double check the focus before making any changes to the secondary.

 

Regards

 

 

Neil

 

 



#16 Timo I

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Posted 21 July 2020 - 10:00 AM

That looks very good indeed (at least in the size, which CN allows images to be attached here). cool.gif

 

If you want to share 100% star shapes to this forum, then this kind of copy/paste work is quite useful for any forum usage.

https://astrokuva.ga...image_zenit.jpg

(=copy for example 400x400 pixel areas from each corner and the center of your star field and put all those into one forum sized image to be attached here).

It's kinda fun to get the maximum performance out of any RCT design, but with forum size images no-one can see the difference after some spot.


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#17 Darth_Takahashi

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Posted 21 July 2020 - 11:14 AM

Here it is as requested.

 

NGC6819_mosaic.jpg

 

Made with PI's aberator inspector script.smile.gif

 

Regards

 

 

Neil



#18 Timo I

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Posted 21 July 2020 - 01:53 PM

I'd say your collimation is now almost there (a bit over 95% ?). I can see a very minor "flaws" in star shapes with the left side (all three areas) of your star field.

There your stars look very slightly oval in horizontal direction and stars are also not as perfectly in focus as in the right side of that star test image. But it's quite hard to tell, if it is secondary or primary you'd need to adjust.

I would be interested too see the unfocussed star fields, which I informed in my answer #7 above. Do these DSI views tell you the same story ie. your star field seems to be tilted now sideways a little bit?

 

I'd say you're now quite far away from this kind of very unbalanced DSI view seen here (image #1), but maybe your DSI unfocussed secondary image looks like this (image #2) or this (image #3) or even better?

As you can see, there are quite minimal changes between images #2 and #3 there, but I have touched ever so slightly to my RCT's secondary collimation screw handling that horizontal direction between those images. Anyway, when you're judging your adjustment needs by those DSI rules, then it might be so that you cannot get it right with the secondary adjustment alone, but you need a minor adjustment to your primary mirror too, before secondary screw adjustment(s) sort of locks down to the correct position. You need to iterate between those two to know, which mirror has the worst aberration and needs to be adjusted first. So for the sake of common knowledge, it would be great to see where your collimation stands now (judged by the DSI method), before you do any minor adjustments to your scope. So if you could publish those images too, that would be highly appreciated here waytogo.gif


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#19 Darth_Takahashi

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Posted 21 July 2020 - 02:10 PM

The reason that I think it's the secondary is because this was the last interation that I made before effectively moving the primary slightly back and the DSI method recommends making adjustsments to opposite bolts to correct the on-axis versus off-axis stars.

 

Primary (A,B & C) I'm not touching the A bolt and the majority of my adjustments have been on the C bolt. (clockwise, top to bottom)

 

Secondary (X,Y & Z) I'm not touching the Z bolt and the only adjustment that I made was a very slight tightening of the opposite X bolt.

 

I will of course, double check everything before changing anything now. I'll make the DSI images but honestly they are not telling me much right now.

 

What did happen and was very obvious to see was the star shape in the OAG dramatically improve from almost being a miss-shapen lines to a more bloated star shape!!! Of courses, its further out an see a more aberrated field but it so very noticable.

 

Finally, what could be tripping other people up is the focus and adjustment point this needs to be at extra-focus and peferrable at near to the same point - repeatable. The reason is I can clear see a star coming to focus and the aberation rotating to a point. I'll also try to take some example images of this too.



#20 John Miele

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Posted 22 July 2020 - 01:14 PM

Darth...this is the RC collimation Police speaking...hands up..sweaty.gif .slowly back away from the collimation screws...I said slowly now...nice and easy...that's good. Now squirt loc-tite all over those screws and never touch them again lol.gif​ ...lol! 

 

Seriously, those stars look very good even into the corners. And the diffraction spikes are getting that nice thin long symmetric look to them. I say 95.263% there...grin.gif


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#21 Darth_Takahashi

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Posted 23 July 2020 - 02:17 AM

John, your post is well timed. I'm calling it done. Maybe even 96-97%. I have the loc-tite ready to seal the deal.lol.gif

 

Test_Integration.jpg

 

I had to move away from NGC6819 since a tree was starting to get in the way. I'll post the CCD_Inspector image and DSI image in a follow up posts so that I can post the biggest resolution image that the forum allows.

 

Chased collimation around all night and repeated the DSI method twice. To me that means each secondary adjustement is followed by a primary adjustment. I was literally just cracking the stiction on bolt C clockwise in the end to get this result.

 

Next the CCD_Inspector_File... To be continued.


Edited by Darth_Takahashi, 23 July 2020 - 02:41 AM.

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#22 Darth_Takahashi

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Posted 23 July 2020 - 02:24 AM

Here is what CCD_Inspector showed me after all of my adjustments had been completed.

 

Collimation N23P7H20.PNG

 

Perfection achieved, right!!! When I saw this I had a little sigh of relief followed by a big smile of joy knowing that this is it. With the current focuser and bolts I'm done. waytogo.gif

 

I have said this consistently, the primary bolts need to be changed for better more precise threaded bolts. But I'm not disambling a new telescope. Not until the primary needs cleaning, then I will do this job myself. GSO, if you are listening, thank you for making my dreams come true with this excellent RC telescope. Now please change the bolts on them to a fine thread pitch, please.smile.gif


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#23 Darth_Takahashi

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Posted 23 July 2020 - 02:40 AM

Finally, here it is the last DSI image. Essentially the above image defocused to show you all the donuts, how they looked at the end of collimation. I big thanks to everyone who commented and helped advise me alone the way in reaching this result.

 

Last_DSI_Image.jpg

 

I must admit that I found interpreting these donuts to be difficult. Perhaps that was in part because I was initially using too low a resolution (binned 2x2). Moving the binning back to 1x1 for these extended stars really hepled, especially on the brighter stars where I could see the spider viens and more details on the primary mirror, some concentric rings etc. Still they move around a lot under the seeing and transparency changes during the night.

 

So I now pronounce this telescope collimated.waytogo.gif  I hope some of you got some insights from these posts. Good Luck and regards

 

 

Neil



#24 Darth_Takahashi

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Posted 23 July 2020 - 04:04 AM

I just realised that I probably should share this as well. You should log all of the adjustments that you make. It been stated in another post on this forum as well so I just reconfirm it here once again.

 

Omegon_RCT.png

 

I made myself an Excel file to log the major changes to the adjustments. The delta is from the original settings as received. All measurements made with a digital vernier.

 

Regards

 

 

Neil


Edited by Darth_Takahashi, 23 July 2020 - 04:05 AM.


#25 Timo I

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Posted 23 July 2020 - 12:54 PM

Finally, here it is the last DSI image. Essentially the above image defocused to show you all the donuts, how they looked at the end of collimation.

I must admit that I found interpreting these donuts to be difficult. Perhaps that was in part because I was initially using too low a resolution (binned 2x2). Moving the binning back to 1x1 for these extended stars really hepled, especially on the brighter stars where I could see the spider viens and more details on the primary mirror, some concentric rings etc. Still they move around a lot under the seeing and transparency changes during the night.

So I now pronounce this telescope collimated.waytogo.gif  I hope some of you got some insights from these posts. Good Luck and regards

Neil

Congratulations for that kind of end result! smile.gif

I think you could stop reading here and skip the rest of my reply, because it could spoil your day...

 

Anyway, here comes some explation for the DSI guide and those defocussed star donuts.

They say this in the DSI guide:

In a well collimated, rotationally symmetrical optical system, any resulting images should also be rotationally symmetrical. We call this “balanced”. In a balanced system, off‐axis stars will converge to
focus uniformly to produce pinpoint star images.

(...clip...)

All of the perimeter star images on the right are roughly round and similar. They are rotationally symmetrical. That is, they all look about the same from the perspective of the center of the image indicating good balance. The on‐axis star image 5 is uniformly illuminated indicating no obvious on‐axis coma. This system is well collimated.

 

You can watch all that happening in this DSI image below: 

(I think your image was not unfocussed enough to see all that, but let's forget that at this stage)

 

_small.jpg

 

I have drawn some lines into your DSI image (attached here as the last image). You could draw (virtually in your own mind) similar lines into image above and notice how those are rotationally symmetrical and the center area stars are uniformly illuminated (just like DSI guide tells us). You don't need a high resolution image to see this (the above image is 4x4 binned only, because I had a very slow image downloading QSI 583ws CCD camera over then and 4x4 binned images downloaded quickly enough for my collimation purposes). What I have done for that image is this: I have altered it's screen stretch function too so that I could see only brighter stars and their unfocussed areas more easily from those oval stars' peripheries. By unfocussing more and altering screen stretch in my imaging software, that kind of DSI image becomes easier to understand.

(BTW, you can see there the center area star's Arago spot too.)

 

In practise, there is a radical difference between Neil's image and its' center area (latest image here) and my image above for collimated secondary/primary mirror. In my image all unfocussed center stars over there are round, but in Neil's image those are slightly oval. That tells us that his RCT scope is not fully collimated by the principles of DSI method. I'm sorry to tell you this, but DSI method is quite ruthless in this perpective and it will tell every tiny error in RCT collimation. This gives us also more understanding to akulapanam's comment here saying that "GSO scope collimating that way is virtually impossible".  

As a result I could say quite frankly that Neil's scope is not a rotationally symmetrical optical system like they describe there in DSI guide.

 

What explains this? For example your focuser axis might not be perfectly aligned with your RCT scope's optical axis and light goes into zig-zag ways, before it gets to your camera in this RCT focuser. Or your primary could need some minor adjustment combined with secondary re-adjustment etc. etc. etc. (which could never be fully completed with DSI principles, unless he does some minor adjustment to his RCT's mechanics (link)). So its' quite likely that Neil has run here to the fact that GSO RCT scope's mechanics might be just a tiny little bit deficient (incomplete) for this DSI method to work out as it supposed to work. I don't want to spoil Neils party here, but he's quite right there saying that his RCT is now collimated up to 96% (=not fully 100%, which is a very rare achievement with these RCT scopes).

 

Conclusion:

I'd suggest you should just continue using your RCT as it is right now, because it can give you such good looking stars all over your imaging field. wink.gif

Forget what I have told you above and enjoy from your scope, because going forward from here is going to be very time consuming and extremely arduous task. And this will require doing more mechanical checks for all optical parts ie. perhaps disassembling your RCT scope. Come back to DSI method, when you change your focuser for this scope, unless you're willing to spend more night hours maybe fruitlessly jogging around with DSI method. I have been there, so please take my advise.

 

I'll show below an image, which shows different star shapes from my own RCT scope during several years of collimation adjustments for it (stars are in no particular order there):

 

_small.jpg

 

When someone gets' his RCT scope into over 98% perfect collimation, he could achieve such pinpoint stars as seen in the lower part of that image (seeing is one variable there together with RCT collimation accuracy).

Second row of stars shows good examples from a similar RCT stars we have seen in this thread. But chasing that kind of perfect pinpoint collimation from a GSO scope could literally spoil your astro life, so I do not suggest this to anyone wanting to use their RCT into normal imaging (etc.) activities. If you want to get your RCT into better than your current collimated status, that could mean that your imaging hobby becomes a hobby of collimating a RCT scope... lol.gif

I have been there and I could say, that it really gives you pleasure to watch those 96% accurate RCT star shapes suddenly settle for over 98% accurate RCT stars with a less than tiny adjustment to some random collimation screw in your scope. In that situation your stars almost suddenly "improve your seeing" and snap into focus. But depending on your RCT mechanics, this does not necessarily mean that all four corner areas in your imaging field will have round stars...

Says a man who has achieved this 98% collimation status for his RCT scope, but never that 100% tongue2.gif

 

PS. My desire for perfect stars was achieved by getting a high cost TMB 152/1200 APO LZOS refractor with its' really pinpoint stars.

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