Ongoing RC10 issues - need help/suggestions

[lazjen]

My GSO RC10 (carbon fibre tube) has had some problems for a while now - I have a go at trying to fix it occasionally but with no luck. A long time ago, it was great, but Something Happened (maybe during our house move) and it hasn't been really usable since then.

Here's a cropped image from the middle of the image:

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Jpeg compression has ruined things a bit, but I think it's still possible to see the bad star shapes.

Here's a near focus image of a star:

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Out of focus star, inwards focus:

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Out of focus star, outwards focus:

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Bright star, uncropped image (just massively resized), 5 sec exposure for the donut. Am I right in assuming that the donut should be centered on the star and not offset?

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I've tried a number of things now, with some making a bit of difference, but others I suspect may have made things worse. I need suggestions or ideas on where to proceed from here.

What I know or have been able to eliminate in the process:
* It's not a camera issue - I've tried 2 different cameras.
* It's not a focuser issue - I've had 2 different focusers that give the same result, plus I can use the tak collimating scope with or without the focuser and it gives the same result
* Fairly certain there's no other real tilt that can come into play. And if so, with all my adjustments taking things on/off, I doubt I could replicate things 100%.
* PA is good, certainly good enough for at least 30 sec unguided at this focal length.
* For all these images, there's no flatteners/reducers in the image train.

What I have done (for good or bad):

* It's been partially taken apart. I go desperate enough to do it to check if there were any issues with the primary, so I took it off the tube to look. Nothing obvious. The secondary has also been unscrewed and checked - nothing obvious there.
* I got in contact with GSO for the exact length required between primary and secondary. I think I am very close to that figure - maybe a few mms either way. (It's difficult, because of the stupid way the distance is measured.)
* I have used a tak collimating scope and it seems the secondary was "nicely" aligned.
* I have done adjustments using the stars as well (taking images - my eyesight sucks a bit for visual).
* There's a few other things that have been tried as well, but overall, no luck.

Any ideas? Maybe I need to get a "professional" to look at the scope and fix it?

[multiweb]

Don't want to state the obvious but it looks to me like a simple collimation issue? Tweaking the secondary tilt should resolve that. :shrug:

If you have difficulties doing it visually stick a webcam or other planetary camera at the back and look at a screen instead.

PS: did you play with the primary tilt at all by any chance?

[glend]

As a previous RC owner (RC8), I see bad coma. Good chance the primary is out of alignment. The history of the GSO RCs is pretty clear, never touch the primary if you can avoid it. Your nice alignment in the TAK scope just means you messed up the secondary to make it appear aligned to the messed up primary.
There are numerous RC collimation threads, heaps on Cloudy Nights forum, and a number of illuminated tools to help you re-establish a base position. A new tool recently released is the Teleskop-express RC illuminated collimator,

https://www.teleskop-express.de/shop/product_info.php/info/p11188_TS-Optics-2--LED-Collimator-for-RC-Telescopes-and-all-other-Types-of-Telescopes.html

Btw, I have one of these TS devices, which i have used on my Classical Cassigrain and it works well.

I always found the TAK scope difficult to use, mostly because it was not illuminated nor throwing light out the front to align to a target.

There is a thread on the Net that shows you how to make an illuminated target box, a La the ultra expensive Hotech system, which works very well. It uses a white LED in the target to bounce back from the primary to align the primary centre spot to a wall mounted target. If I find the link I will post it here, I had one but threw it away when I sold my RC.
Good luck.

Edit: Found it. Look here,


https://britastro.org/node/16167

Note that the article contains a pdf file link to do you can print the target.

[lazjen]

"Play" would not be the total sum - I think the incident(s) I referred to might have put it out of alignment. Then, compounding the problem, after taking apart/putting back together, I did try some adjustments at the back of the scope (the 3 points which seem to have 2 adjustments at each point?). I quickly gave that up though since I couldn't see any real change no matter what I tried. No doubt I made it worse.

[multiweb]

Ok then if you can't collimate it's the primary tilt. If it's off you will get on axis oblong stars. You need to fix your primary tilt first then everything else will fall into place. Do you have access to a Hotech laser board?

[lazjen]

Thanks for the reply.

That TS device looks good, and I agree the Tak scope can be annoying because of the lack of a light source.

I might try the illuminated target box first to get the primary fixed. Then use the tak scope for the secondary. If I didn't have the Tak scope, I think I'd jump straight to the TS device now. I found it easy to adjust the secondary and see the feedback via the Tak scope and via images, so for now it should do.

[lazjen]

No, but it looks like Glen's link is to something equivalent?

I note in the instructions it mentions up to 200mm / 8" - not sure if it would need adjustments for 254mm / 10"?

[lazjen]

Marc: Is this the Hotech system you were talking about: http://www.hotechusa.com/category-s/23.htm?

[multiweb]

Yes. You take the secondary out. Stick a laser at the back in the focuser and center it on the board. Then you use the board cross laser on the primary and tilt it so it is centered on the board. That gets you ball park then you collimate on a star.

[lazjen]

Well, lots of reading later... yeah, I really need to sort out the primary first. Get it lined up properly and the tilt fixed. I think the rest should be easy from there.

[lazjen]

Just put an order in for the TS device Glen mentioned as it's got a major advantage over the Hotech - I don't have to remove the secondary, which is a major pain to do.

[multiweb]

Looks like a pretty simple device. You could whip one out yourself with a piece of PVC tube. The only potential issue I can see with that is the distance between the device and the secondary is quite short. You get a more accurate registration with the hotech board the further it is from the primary. See how you go.

[atalas]

Ok this collimating RC's comes up a lot and I'll just layout the routine to follow.

1: Collimate focuser to secondary mirror using laser (centre in secondary)

2: Collimate secondary to primary with Tak scope(read instructions)

3: Collimate primary to secondary with Tak scope(read instructions)

A few points: make sure you don't pinch mirrors by tightening too much and use a strong little torch on the Tak scope.

Also any tweaking to be done via star should be done on primary .

I've owned an RC10 and a RC12 and found both very easy to get good collimation with a good laser and the Tak collimating scope when done correctly.

Ken Crawford put out a little video showing the routine to collimate RC's maybe you could look for It or someone could share that with you.

ps. Apologies If this has been covered in this post already

Ok found the link to the video: https://www.rcopticalsystems.com/support/index.html

[lazjen]

Yes, it did cross my mind that could be the case with the accuracy.

I was also having difficulties sourcing a Hotech board anyway (the dealer for Australia doesn't have it listed on their site), so the convenience on the secondary and the I can order an item now tipped me.

[lazjen]

I probably should get a laser sometime...

How do I tell if the mirrors are being pinched?

[atalas]

Loosen off collimation till they just grab a little and then collimate.... do the same with the secondary.

The easiest way to centre the focuser is a laser.....If you've got a mate who has one borrow It till you can buy a good one like a Howie Glatter Laser Collimator - 2"

Get one of these too: https://www.bintel.com.au/product/bintel-gso-m-lrc-focuser-collimation-ring-rc10rc12/?v=322b26af01d5

Their easy to install ....this focuser hanging of the primary cell was a bad idea.

[lazjen]

Only the primary has the 2 screw system - the secondary just has a total of 3 screws (plus the centre screw that holds the assembly)



Got the ring already. I think I can get a loan of one of these: https://www.bintel.com.au/product/farpoint-1-25-inch-collimation-kit/?v=322b26af01d5 <-- that laser should be ok?

[atalas]

On the primary one screw adjust and one screw locks ....from memory I think the small screw was the locking screw.

That's right, three screws on secondary just make sure none are too tight when you reach collimation or It will pinch.

Oh yeah I'm assuming that you know how to collimate your focuser with the ring right? they have two screws as well If memory serves.

[lazjen]

The device from TS turned up yesterday (https://www.teleskop-express.de/shop/product_info.php/info/p11188) and I've just had a go at using it. Very easy to use once I got it installed. Something to keep in mind - it's not threaded, so you have to put it into a 2" holder - and it's a very tight fit. That's useful because it's not going to move once inserted. In fact, I'm leaving it inside the holder "forever", since I can't remember the last time I used it for anything else.

After I made the adjustments, I put the tak scope on, and it seems to agree with the TS collimator. Proof will be in the star images, which I am hoping to try tonight.

[pmrid]

I was impressed by this device and have ordered one for myself. They seem ideal for daylight collimation. Glad to read other positive impressions.

[pmrid]

Chris, did your device come with any kind of instructions? I'm wondering what the "R" and "W" stand for on the 2 switches.

[lazjen]

Alas, after an afternoon and early evening of nearly clear skies, night brought almost complete cloud cover. Can't report any results. :(

[lazjen]

Red and White "light". :)

No instructions. The page has details for use: https://www.teleskop-express.de/shop/product_info.php/info/p11188.

I found using the Red light as per instructions for the secondary worked well. And using the White light helped with the primary. The white light is actually 4 pinpoint leds.

[lazjen]

Managed to get a bit of time on the scope tonight to check my results. It looked very good to me, but I'll check the images a bit closer tomorrow.

[pmrid]

I’ve had several sessions with this TSRCKOLLI and have mixed feelings about it.

1. I can’t fully insert it into my 2” Moonlite focusers.
2. I can’t clearly identify the primary mirror shadow.

But I am happy to report a cheap and simple daylight method that looks a lot better.

An article in the British Astro Org web site entitled Kitchen table collimation ....” http://articles.adsabs.harvard.edu/pdf/2015JBAA..125..281S
looks to me to be even more accurate than the artificial star technique. I plan to try it this weekend.

Peter

[lazjen]

Just FYI, that's exactly the same details as the link Glen posted early on in this thread.

I understand the difficulty and I wonder why there was no threaded option? I know it's plastic and it's good to have a tight fit to ensure no slippage, etc, but really it was a bit excessive. Also, even without threading the "technology" to lock in place for these holders is well known, so again a bit strange.

If you can get past the above, I think it works well enough.

[glend]

Yes, as posted before the DIY illuminated collimation target gave me great results in tuning my Classical Cassegrain. I did not have to buy anything to build it. I used a bicycle LED headlight module (removed the mounting strap), a cardboard box, and a printed target. Just mount the target on the box, place a 1mm hole in the centre of the target, and duct tape the LED light to the insude of the box (facing outward of course).
Using it is very simple, just set everything up on a kitchen table, with the scope pointing at the targrt make sure it is square on. Slide the scope forward or backward to get the distance right, you want the rings to line up with your tube, and you may have to raise or lower the box to achieve this. Now turn on the LED, and you will see a white donut reflected onto the target, this is the reflection off the primary mirror. You can slide the scope backward or forward to get the donut as small as possible. Now do your primary adjustment to centre the donut around the LED source on the target.
Your primary mirror is now perfectly optically aligned on-axis with the scope.
Secondary alignment still needs to be checked too, but unless severely tilted(creating a slight shadow) it will not affect the primary setup, as you have setup the primary already.
Of course follow this up with a star test, for final tweaking if required, but don't touch the primary again.

I only bought the TSRCRolli after my kitchen bench primary setup.

Good luck.

[lazjen]

An advantage of the TS unit (and tak scope) is that I didn't have to take the scope off the mount.

[glend]

True that, and I appreciate that the RC10 is not easy to move around. If you have an observatory it would be possible mount the light box target on a wall., just as you might do for a Flat target.

[pmrid]

Well, (and with acknowledgement and thanks to Glen), I gave this a try today. All I need is a few stars!

Peter