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RC conundrum ?


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Well true to tradition - new scope and clouds. In fact it was clear until 5 mins before dpd delivered the TS 8 inch RC then it rained with hail...

Anyway I digress.

I set the scope up on a ridgid tripod and pointed it at a light blank wall to check out the collimation with just the focuser fitted, racked right in with a peep sight in the focuser. Quite pleased just a minor tweak on the secondary adjust bought the reflections into alignment. But then the problem. All the collimation reviews I've read, and what I've done with other RC's I've owned is to then adjust the primary to leave a thin ring of daylight around the secondary housing - concentric with the tube baffles. BUT no ring of light. I had to take out the peep sight and place my eye further in the focuser before I could see anything around the secondary housing. I found this odd but then I didn't know if TS had changed the design/layout of the scope making it impossible to see a ring. Perhaps they intend their customers to buy a laser collimator?

So I turn the scope around on the tripod to view a distant tree to establish I could at least get an image. I needed all 3 of the supplied extension tubes, plus a further 35mm extension tube and the focuser with another 30mm extension to get an image of the tree (well top branches) with my asi533 attached. I've never had that much extension behind the back plate before so I thought I'd check the secondary to see if I could ease it up the tube a little to bring the focus in further, getting rid of some of the extension from what is a back heavy scope anyway. The secondary mirror was right up against it's support so no outward movement was possible.

There was one thing I noticed (probably wrong) but the spider vanes are a shallow non symmetric triangle and I'm thinking that perhaps the vanes have been reversed taking the secondary mounting 2-3mm further into the tube. That would certainly push the focus further back requiring the extra extension. Forecast says clear tomorrow so I'll try it as-is first with the long extension. i will get a small inward shift when viewing stars as the tree whilst a good distance isn't at infinity. But to me I think I'm about 50-60mm further back that I should be. A plate solve should tell me what the native focal length is, it should be 1624mm...

I'll add some photos tomorrow to show what I'm thinking may be an issue.

TBC

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I'd be surprised if it really is an issue. All my Casses need a lot of back focus for terrestrial use. The only one that does not is the vixen 110 which uses a moving mirror arrangement. 

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Posted (edited)

Well after trawling through many varying RC collimation techniques on the web I found this one: https://www.dropbox.com/sh/1349fp34gqpbalm/AADPeMNonKZNk-1AkKUZ1dL4a?dl=0&preview=RC8+Collimating+Frustration+revision+1.4.pdf

Inital reading was a little daunting but definitely the most thorough how-to I've found.

Edited by fwm891
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Posted (edited)

Better not enter the messy RC collimation trap before you use it as it came from the factory.

As a well known professor wrote "Premature optimization is the root of all evil" 🙂

One of the most approachable documents I have read on the subject is below (at least, it promises a converging solution):

http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf

 

N.F.

 

Edited by nfotis
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Posted (edited)

As a base to start from here is my 1st light image of M51 (no adjustments fromm delivery).

Total of 14x 300s subs, processed in PI and scaled in PS.

PI had great problems stacking these subs as it was shot in two sessions each side of a meridian flip and the initial stacks failed to rotate the images post flip anf gave me two non rotated images in the same frame - very artistic but not what the doctor ordered. So I ended up stackingg the two sets separately, rotating one set and combining the two.

Lots of errors but the potential is there...

M51_RC8_1st-light.jpg

Edited by fwm891
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In which program do you stack? In AstroArt, which I use, you have to specify a maximum rotation angle so you opt for 180 degrees. There may be an equivalent in whatever you're using, perhaps a setting you hadn't noticed had changed?

The stars are a bit odd and seem to have 'bites' out of each side. Do they look any different if you stack only pre-  or post-flip subs? Most things would be the same either way but not something induced by tilt under gravity.

Olly

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Posted (edited)
59 minutes ago, ollypenrice said:

In which program do you stack? In AstroArt, which I use, you have to specify a maximum rotation angle so you opt for 180 degrees. There may be an equivalent in whatever you're using, perhaps a setting you hadn't noticed had changed?

The stars are a bit odd and seem to have 'bites' out of each side. Do they look any different if you stack only pre-  or post-flip subs? Most things would be the same either way but not something induced by tilt under gravity.

Olly

Hi Olly, The image above is a combination of the two stacks (pre and post flip). I tried for a long time last night to find the best focus: manually and via electronic means. Defocused stars were showing off centred dough nuts so collimation is a definite issue here at the moment. I'm just about to have another play with the two stacks using different software and see where that takes things...

Edited by fwm891
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Posted (edited)

Stacked all the subs as one stack in APP, colour has gone but stars are now more even elliptical shape...

Pixel scale on this is: 0.47571 sec/pixel and the scopes current focal length works out at 1630.3 mm, about 6mm longer than the 1624 mm in it's specs.

Playtime continues

M51_RC8_1stLt-RGB.jpg

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  • 3 weeks later...

Finally getting near with the collimation pispite having to find stars in the bright evening skies and patchy cloud cover to do it. After making a few final tiny tweaks to the main mirror tilt I've done a test on M13.

The integrated image has 4x 3=120s and 3x 300s. Processed in both PI and PS. Colour is off and stars at the edges are still slightly elliptical. Stars still have soft edges and my back focus seem short so mirror separation may be another factor here.

Comments welcome.

M13_3x300_4x120_533_RC8.png

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3 hours ago, fwm891 said:

Finally getting near with the collimation pispite having to find stars in the bright evening skies and patchy cloud cover to do it. After making a few final tiny tweaks to the main mirror tilt I've done a test on M13.

The integrated image has 4x 3=120s and 3x 300s. Processed in both PI and PS. Colour is off and stars at the edges are still slightly elliptical. Stars still have soft edges and my back focus seem short so mirror separation may be another factor here.

Comments welcome.

M13_3x300_4x120_533_RC8.png

Looking like your spacing is still slightly out stars are pointing from the corners into the center, only slightly so I think you're very close

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Posted (edited)

The colli quest continues.

I did find one unexpected problem though. One of the adjustment machine scrrew for primary tilt wasn't engaged with the mirror. I had to tilt the scope vertically and rotate the said screw anticlockwise till I heard the thread click then re tighten the screw. I was then able to do a visual basic adjustment with a peep/cap sight (Cheshire)? after unscrewing the baffle tube exension. Star test was reasonable so a few more minor adjustments and try out on M 27. diffraction spikes are still not as clean as I'd like them to be. Must make up the tilt rings!

Image 12x 300s processed in PI and PS

Cheers

Francis

RC8_Test_M27_300521.png

Edited by fwm891
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You seem to be getting very close now, Francis. I love the colours and contrast for just one hour of imaging. I had a similar issue with the back focus on my Tal 200K, the back focus was vey limited and trying to fit the CCD, filterwheel and OAG into such a short space was a challenge. Look forward to more images when you finally nail it.

Steve

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On 31/05/2021 at 10:25, sloz1664 said:

You seem to be getting very close now, Francis. I love the colours and contrast for just one hour of imaging. I had a similar issue with the back focus on my Tal 200K, the back focus was vey limited and trying to fit the CCD, filterwheel and OAG into such a short space was a challenge. Look forward to more images when you finally nail it.

Steve

Thanks Steve,

Just finished making and fitting a pair of tilt rings so I can reverse the tilt of the focuser when altering the primary. I have no problem with back focus now - probably too much...

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