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Old 16-03-2018, 08:38 AM
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Andy01 (Andy)
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RC8 star test - thoughts on Collimation?

First light for the RC8 + new Moonlite focuser combo.

Just a single Ha x 10 min sub of the Keyhole.
Stars in the bottom left look a bit mis-shapen to me?

I also have a GSO tilt ring but havn't installed it as yet.

Tips/thoughts on collimation of this beasty?

https://www.astrobin.com/337551/
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Old 16-03-2018, 09:00 AM
tim.stephens (Tim)
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Yes, a little off perhaps... Do you have a defocused starfield? I find the best way to check collimation on mine is with a bright central star and then an open cluster/rich field to check the 'balance' of the image. Intra and extra focus. doughnuts should be all squished the same amount and look the same as distance from the center.

Central star doughnut must be absolutely perfectly centralised. Usually the primary needs adjusting on these to get the airy rings balanced.

Also, what moonlite do you have? I have the 2 inch and my drawtube is off centre. You can't collimate the drawtube on this one, only the 2.5 inch version. I correct this slight misalignment with the collimation ring.

Have you also made the primary baffle tape-fix to stop the primary mirror moving?



Tim
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Old 16-03-2018, 09:18 AM
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Hi Andy,

Here's a map of FWHM and Eccentricity (values around 0.42 and less look nicely round) from your image.

A couple of interesting things: FWHMs are worse in the centre of the field which is the opposite of what you usually see and star roundness gets worse as you go from the right hand side of the image to the left. The FWHM thing could be caused by field curvature and focus. Maybe a little tilt contributing to the variation in Eccentricity too?

It's hard to say too much because that's not a very good FOV for making measurements. You'd be better off with a field with stars from edge to edge and no nebulosity.

Cheers,
Rick.
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Old 16-03-2018, 09:25 AM
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Tim's suggestion is a good one too - probably better for checking collimation but maybe not as useful for diagnosing tilt and field curvature.

Another variation on that theme is to take an image of a star field with a Bahtinov mask and look at variations in the pattern across the field.

Cheers,
Rick.
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Old 16-03-2018, 09:52 AM
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Quote:
Originally Posted by RickS View Post
Here's a map of FWHM and Eccentricity (values around 0.42 and less look nicely round) from your image.
Rick, how did you create this map? Is it something in PixInsight?
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Old 16-03-2018, 09:53 AM
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Thanks for your thoughts Rick & Tim.

I should have mentioned that I'm using a .8 flattener/reducer here as well,

Here's another starfield I shot same night that may be more useful, it's
60secs LUM only.

https://www.astrobin.com/337564/

Rick I don't have a Bhatinov mask - but I'll be ordering a laser etched one soon.

Tim, I'll try your suggestion with a defocused star field with a bright central star tonight (if it's clear at the star party at Snake Valley!) - I have the 2.5 inch focuser and the tilt ring.
I'm reluctant to fiddle with the focuser as it's meant to be collimated at the factory?

What's the primary baffle tape-fix to stop the primary mirror moving please?
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Old 16-03-2018, 10:20 AM
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Originally Posted by lazjen View Post
Rick, how did you create this map? Is it something in PixInsight?
FWHMEccentricity script, Chris. The "Support" button generates the maps.

Quote:
Originally Posted by Andy01 View Post
Here's another starfield I shot same night that may be more useful, it's
60secs LUM only.
Here are the maps for that one, Andy. It does look like there could be some tilt from top right to bottom left.

Cheers,
Rick.
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Old 16-03-2018, 10:29 AM
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Thanks Rick!

Some work to do it seems...
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Old 16-03-2018, 11:44 AM
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I assume you're using the the GSO FR/flattener? It should be good for apsc sized sensors - I've not tried mine with anything larger TBH.

I'd suggest taking the FR/flattener out for collimation as it will tend to remove some of the field curvature inherent in the uncorrected RC design and make it harder to balance the stars across the frame.

Bahtinov mask is very helpful when dialing in collimation - focus on a bright star in the centre and then check that star at each of the corners to measure the field curvature. Make sure the star at each corner is off focus by the same amount. Do this after you've got the central star collimation right and the airy rings/central obstruction perfectly centred (usually by adjusting the primary).

Here is a short version of what I do:

Do you have a cheshire to check your collimation in the daytime? I found that I can get almost perfect collimation with this tool but the trick is to remove the primary baffle by reaching in and unscrewing it (let it rest on the outer baffles) and keep the telescope horizontal. You can then see all the internal reflections and secondary baffle easily - adjust the primary as necessary to centralise all the reflections and baffles and adjust the secondary so the black center mark is... well... center to the cheshire! You have to be extremely particular when centralising the reflection rings, have a play around with it to be comfortable with shifting both mirrors. When you think it's close, check the collimation from the front with the 'hall-of-mirrors' technique - go right up close to the front on a bright day and actually count the number of internal reflections of the black secondary baffle at all points around the mirror. You should see exactly the same number of reflections, if not, then something isn't quite right and you have to go back to the cheshire to diagnose.

Now here's the thing - these scopes are poorly designed in several ways. When you remove the primary baffle for collimating in this way, you can accidentally unscrew the primary mirror retaining ring because it is one unit. I found out the hard way - all of these scopes have this design flaw and the result is shifting collimation when the scope slews as the primary becomes unscrewed. To diagnose - with the baffle in place, pick yours up from the spider end and hold it vertically. Shake it gently from side to side and see if you here a 'thunk' in the OTA. If so, then your mirror is loose and any collimation you do inside is pointless.

This might seem a little extreme but what I suggest people do is pull off the back mirror cell (black metal bit) to access the primary. Unscrew the primary baffle, then the retaining ring, then pull the primary mirror from the central holder. You might notice that there is a LOT of lateral play in the primary and this is the cause of many difficulties in collimating these scopes.

Pull the white plastic teflon off the holder and add a thin piece of tape around where the mirror sits. I used black electricians tape as it was the perfect thickness to increase the circumference enough to hold the mirror in with a little more force. reassemble everything including the teflon ring, making sure your primary isn't too tight and is seated properly against the cell holder. Then collimate again. I did mine last year and it hasn't moved at all.

Hope that helps a little...

As with the focuser, they are supposed to be collimated but mine was not. I probably should have sent it back but it took over a month to get here one way, the postage was really expensive and I fixed the problem easily with the GSO collimation ring. Check yours with a laser - rotate the focuser and see if the laser makes a circle on a far wall or one the secondary. I should have bought the 2.5 inch...

Tim

Edit: Just had a look at your astrobin picture. Your collimation does look a little off... How long was your exposure? Looks to be the primary needs to be moved at the 1-2 and 7-8 o'clock positions

Also, don't listen to people on the forums when they say 'don't adjust your primary'. You have to when collimating these scopes properly :-)

Quote:
Originally Posted by Andy01 View Post
Thanks for your thoughts Rick & Tim.

I should have mentioned that I'm using a .8 flattener/reducer here as well,

Here's another starfield I shot same night that may be more useful, it's
60secs LUM only.

https://www.astrobin.com/337564/

Rick I don't have a Bhatinov mask - but I'll be ordering a laser etched one soon.

Tim, I'll try your suggestion with a defocused star field with a bright central star tonight (if it's clear at the star party at Snake Valley!) - I have the 2.5 inch focuser and the tilt ring.
I'm reluctant to fiddle with the focuser as it's meant to be collimated at the factory?

What's the primary baffle tape-fix to stop the primary mirror moving please?
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Old 16-03-2018, 01:04 PM
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FWHMEccentricity script, Chris. The "Support" button generates the maps.
Excellent. I'll give it a go later.
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Old 16-03-2018, 01:59 PM
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Thanks so much for taking the time to post all that comprehensive info Tim!
I’ll absorb all of that and try again 😊
Cheers
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Old 16-03-2018, 02:40 PM
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Just to put my 2c worth into the conversation.

First up, is the secondary centre spotted? If so then collimation is a breeze, if not it is a little more difficult but can be done with a Howie Glatter laser and the appropriate pattern lens.

So, if you have a centre spotted secondary, buy yourself a Takahashi collimation scope, or borrow one from someone. There are numerous pages on this forum where I have described a sure fire method. The bottom line is you want to get everything concentric including the very tiny hair like circles in the centre of the centre spot. This gets me collimated in 5-10 minutes and no pulling apart. Yes you have to move all the collimation screws including the primary and you need the collimating ring for perfect collimation. If you get stuck, drive over here and I will show you how.

If you don't have a centre spot, you can use a Howie Glatter laser. Hard to find now, but if you know someone with one again it is quite quick once you have the method down pat. I don't believe it is as good as using a Tak scope but it's pretty good and way better than you current collimation.

You definitely have collimation issues at present and also some tilt. Sort the collimation first and then sort the tilt.
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Old 16-03-2018, 03:13 PM
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Geez Andy, just buy an 8" apochromatic refractor and put all these worries behind you.

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Old 16-03-2018, 03:52 PM
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Thanks Paul, good advice there.
I have a mate nearby with the Tak microscope collimating gadget, so I’ll pay him a visit next weekend!
Cheers
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Old 16-03-2018, 05:46 PM
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If you don't have a centre spot, you can use a Howie Glatter laser. Hard to find now, but if you know someone with one again it is quite quick once you have the method down pat.
You can purchase Howie Glatter lasers from Starlight Instruments.
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Old 05-04-2018, 03:29 PM
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Update 5/4/18

Thanks to my ASV mate Clive We managed to better collimate the beast with his Tak microscope.

It needed a couple of tweaks to the secondary - and it's certainly an improvement!

This is a test on the Gem Cluster - around 3 hrs of 3nm Ha.

I played with the spacers I have and 5mm was bad, 6mm was pretty good (seen below) but maybe it needs 7mm?

Now we're also wondering if the Stellarvue SFFR-70APO flattener/reducer is right to use with the RC8?
It's designed to be used with the SV70T which is only 420 mm FL, the RC8 is 1625mm FL. - Hmmm...

Anyway - it's still not 100% but it's getting there!

There's also a generic starfield (60sec Lum) and a couple of images of Sirius in and out of focus either side - and also way out of focus to see the donut.
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Last edited by Andy01; 05-04-2018 at 03:41 PM.
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Old 05-04-2018, 05:03 PM
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T
Now we're also wondering if the Stellarvue SFFR-70APO flattener/reducer is right to use with the RC8?
It's designed to be used with the SV70T which is only 420 mm FL, the RC8 is 1625mm FL. - Hmmm...
Good to see you've made progress, Andy.

What you want is a flattener/reducer that matches the field curvature of the scope and that depends on the scope design and f/ratio. I wouldn't worry about the FL in isolation.

In the absence of an optic designed specifically for the RC8 it's a bit of a crap shoot I was pretty happy with the AP 27TVPH on a RC10 but anything bigger than a KAF-8300 showed its limitations!

Cheers,
Rick.
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Old 05-04-2018, 05:24 PM
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I agree with Rick. There is a chance that your flattener / reducer won’t matchup the field curvature correctly. Then again, it might if you get to 7mm too! Only one way to find out
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Old 05-04-2018, 05:42 PM
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I like the latest image Andy.

I also agree with Rick as for a dedicated corrector, although the guys from APM reckon that Riccardi that was designed for refractors, also works well with RCs.

EDIT: There is a 0.75 RC reducer for sale on classifieds. Made in Taiwan, same as the Riccardi.
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Old 06-04-2018, 11:17 AM
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Quote:
Originally Posted by Andy01 View Post
Thanks to my ASV mate Clive We managed to better collimate the beast with his Tak microscope.

It needed a couple of tweaks to the secondary - and it's certainly an improvement!

This is a test on the Gem Cluster - around 3 hrs of 3nm Ha.

I played with the spacers I have and 5mm was bad, 6mm was pretty good (seen below) but maybe it needs 7mm?

Now we're also wondering if the Stellarvue SFFR-70APO flattener/reducer is right to use with the RC8?
It's designed to be used with the SV70T which is only 420 mm FL, the RC8 is 1625mm FL. - Hmmm...

Anyway - it's still not 100% but it's getting there!

There's also a generic starfield (60sec Lum) and a couple of images of Sirius in and out of focus either side - and also way out of focus to see the donut.
Your primary is still off , if you look at at fainter stars , their rings are slightly pinched on one side you need to adjust the primary till they all look evenly round , the adjustment will be very small increments.
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