79 replies to this topic

[MikeECha]

Very interesting thread. I have  been playing with the collimation of my AT6RC but I do not know what is the actual focal length I have to collimate this scope to. The specs says 1370mm f9. That is a calculated aperture of 152.2mm/6". But as I read these scopes are designed in a country that uses the metric system. So I wonder if the exact fl is 1350 @ f9 with an apperture of 150mm and the 6" aperture is a nominal value just close enough (5.99in vs 5.90in.).

 

Currently, my scope platesolves at 1348.5 or so and the rings of the out of focus stars on the edge of the image look decently concentric. This is a secondhand scope so I would like to put it at design distance if possible.

 

Can anybody confirm what is the actual fl of an A6RC I shoul collimate to?

 

Thank you in advance for any help. 

[Timo I]

 I have  been playing with the collimation of my AT6RC but I do not know what is the actual focal length I have to collimate this scope to. The specs says 1370mm f9. That is a calculated aperture of 152.2mm/6". But as I read these scopes are designed in a country that uses the metric system. So I wonder if the exact fl is 1350 @ f9 with an apperture of 150mm and the 6" aperture is a nominal value just close enough (5.99in vs 5.90in.).

I think your guess for the nominal value is just correct. I have owned a 10" GSO F/4 reflector, which was supposed to have 10" (254 mm) main mirror. In practise it had 9.84" (250 mm) mirror diameter.

Look for this image, where I was setting up Catseye center mark into that mirror:

https://astrokuva.ga...k_assembly3.jpg

Catseye template shows quite clearly the true diameter for that so-to-call-10" mirror. Chinese factories are not that accurate with the aperture value, besides it saves a little bit money for them to manufacture somewhat undersized mirrors

 

I would not bother trying to get your RCT scope's focal length to exact value it's supposed to be. Just enjoy using your scope, if the image quality is good with it.

[MikeECha]

I think your guess for the nominal value is just correct. I have owned a 10" GSO F/4 reflector, which was supposed to have 10" (254 mm) main mirror. In practise it had 9.84" (250 mm) mirror diameter.

Look for this image, where I was setting up Catseye center mark into that mirror:

https://astrokuva.ga...k_assembly3.jpg

Catseye template shows quite clearly the true diameter for that so-to-call-10" mirror. Chinese factories are not that accurate with the aperture value, besides it saves a little bit money for them to manufacture somewhat undersized mirrors

 

I would not bother trying to get your RCT scope's focal length to exact value it's supposed to be. Just enjoy using your scope, if the image quality is good with it.

Hello Timo

 

Thanks for your response.

 

In this case I do not think they are cutting corners. I am speculating here but I just think they design for a metric country. They would have designed it and called it a 150mm @ f9 for their consumers. I really doubt the would have designed it 152.2". But to sell it here they would not call it 5.9" f9. A 6" makes more sense as nominal spec. Then, people like me, we find it "atrocious"   .

[akira]

I am having some trouble the past months in collimating my rc8 and i found this topic and i have some questions about david07's method. After i align the optics of the scope shouldn't i align the focuser, i have a moonlight focuser which i can adjust from its own flange (and i have adjusted it over the years) note that if i mount the focuser on the scope withount any extension rings i don't have access to the collimating screws of the primary mirror. Also david07 mentions the focal distance of the scope not being the proper one and he had a corrected image at 1663mm. How do i find the correct focal distance of my scope, currently my FL is 1607mm

Edited by akira, 12 June 2020 - 07:31 AM.

[David07]

Oh gosh. I've just realised that this thread has woken up again. Hi everyone.

 

Some of you are aware that there is another deep discussion going on on this topic on the UK forum, Stargazers Lounge, here:

 

https://stargazerslo...-howie-glatter/

 

I've published more information there, with photographs, on my exploration of the RC8 scope and suggest you pop over and have a look.

 

In the meantime, I'll have a careful read of where we're up to here before responding - I think I've answered some questions on the UK site already.

 

Here is a recent image for you to ponder. This is a low resolution image of M91 and three Starlink satellites captured on 20th April.

 

 

The RC8 scope has captured the faint outer spiral arms and the halo around the galaxy which saying something for the scope and the very clear Spring nights we've been having. During one of my subs, three Starlink satellites crossed the FOV and I've added their image on top of the image of the galaxy. Look at the star colours, especially the blue/orange pair top right. The colour rendition is one thing I really enjoy about this scope.

[Darth_Takahashi]

Oh gosh. I've just realised that this thread has woken up again. Hi everyone.

 

Some of you are aware that there is another deep discussion going on on this topic on the UK forum, Stargazers Lounge, here:

 

https://stargazerslo...-howie-glatter/

 

I've published more information there, with photographs, on my exploration of the RC8 scope and suggest you pop over and have a look.

 

In the meantime, I'll have a careful read of where we're up to here before responding - I think I've answered some questions on the UK site already.

 

Here is a recent image for you to ponder. This is a low resolution image of M91 and three Starlink satellites captured on 20th April.

 

M91&Starlink-web.jpg

 

The RC8 scope has captured the faint outer spiral arms and the halo around the galaxy which saying something for the scope and the very clear Spring nights we've been having. During one of my subs, three Starlink satellites crossed the FOV and I've added their image on top of the image of the galaxy. Look at the star colours, especially the blue/orange pair top right. The colour rendition is one thing I really enjoy about this scope.

I have read the thread and beleive that these people have bought into the "MAGIC" of the Howie Galtter collimator! These tools are just that, tools that get you very close to collimation. Nothing substitues collimating on a star under a clear sky. Well maybe and artifical star! Hopefully, at this point the tools will have done their job and you will be close to collimation and just some minor tweaks will be needed.

 

Regards

 

 

Neil
 

[Jaspalchadha]

Can someone remind me... by unscrewing the centre bolt one whole turn adds or reduces the focal length by how many MM

Thanks


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[David07]

Can someone remind me... by unscrewing the centre bolt one whole turn adds or reduces the focal length by how many MM

Thanks


Sent from my iPhone using Tapatalk

Hi Jaspal,

 

The centre bolt on my RC8 is an M6 bolt. Unscrewing that one turn (1mm) lengthens my focal length by approximately 20mm. This is a diagram I constructed from adjusting the number of turns on the centre screw and measuring the resultant focal length.

 

 

As delivered, the focal length was 1604mm. It turned out that the 'corrected' focal length (as measured by a Ronchi grating) is 1660mm, which is where my scope now works.

 

David

Edited by David07, 21 June 2020 - 05:46 AM.

[Jaspalchadha]

Lovely thanks. My 14 inch GSO is shown on many sites as 2854mm.

It was 2879mm before and I’ve got it down to 2859, one full turn has taken it down 10mm.

Should I bother with the 5mm


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[David07]

Lovely thanks. My 14 inch GSO is shown on many sites as 2854mm.

It was 2879mm before and I’ve got it down to 2859, one full turn has taken it down 10mm.

Should I bother with the 5mm


Sent from my iPhone using Tapatalk

My only question would be: did Es set the focal length at 2879mm because that was where he got a properly corrected image with his Ronchi grating? If so, then you should leave it at the original 2879mm focal length where the scope is fully corrected.

 

I made the mistake of having Es adjust my scope to a fully corrected condition but at a focal length of 1660mm, only for me to then adjust it back to the advertised value of 1624mm where it was overcorrected. I did this twice! The mirrors on my scope now work correctly together but give a focal length of 1660mm. I have a Ronchi eyepiece on order so that I can check the setting for myself.  

 

Also, the 2854 number is interesting because the theoretical focal length is 2848mm - 356mm at F/8.

[Jaspalchadha]

That’s a good point, we didn’t use the Ronchi eye piece, we changed the primary mirror bolts, as one had snapped right through.

When the scope did come back it wasn’t working well on a real star test.

Es did recommend getting it close to its MM ( + or - )

Having spoken with GSOs customer service it was recommended the correct distance mirror to mirror should be 790mm, I might be very close.

I’ve spend too much time and money on this particular RC :/

Edited by Jaspalchadha, 21 June 2020 - 11:12 AM.

[akulapanam]

One thing I have never understood is how some RC manufacturers allow for secondary focusing which inherently changes the mirror distance.  This would seem to imply that what they are really doing is finding the best focus for a given focal length instead of the best focal length for the optics?

[skybadger]

I guess you have to ask, is secondary focusing intended to provide focus control for a fixed back focus point or to support all the different back focus distances your kit May drive you to ? I suspect the former because of that focal length change.

[SXBB]

There’s really only one correct distance between the primary & secondary that will be both in focus & also minimize aberrations. If you move the secondary in order to focus, it’s necessary due to expansion or contraction of your scope which has changed the distance between the 2 mirrors. By moving the secondary until focused, you’re restoring that ideal spacing which also brings it into focus assuming that the focal plane of your camera is constant & correct.

Best,

Bruce

[akulapanam]

The thread I started in ATM was helpful.  Worth pointing out that you do have some leeway here per this http://interferomete...en-gso.html?m=1 .  6 to 8 mm or 60 to 80mm of focal length to stay diffraction limited.

[Timo I]

There’s really only one correct distance between the primary & secondary that will be both in focus & also minimize aberrations...

I think I could agree fully with Bruce here.

In the past I have done some extensive testing for the distance between primary and secondary RCT scope mirrors too.

CCD Inspector results show those RCT aberration changes as "curvature changes" between my test images seen here.

 

 

This is an image showing CCDI "curvature" differences, when only this distance between primary/secondary mirrors has been altered very slightly (and my RCT scope has been collimated between each mirror distance change):

In the past I have done quite a lot of different practical tests between every possible factor in RCT scope, which could have some effect on the scope's picture quality.

This collimation progress (and several backsteps too) have been documented fully (in Finnish language only, sorry!). If you would like to get a shortcut to the middle of that l-o-n-g collimation thread, then you could hop into this part, where my RCT scope had received a brand new carbon tube for itself in the beginning of year 2015.

 

After that phase, there are several pages of Finnish forum discussion with plenty of detailed test images and CCDI test results for the same test images including that distance test. I had over then lots of issues with my RCT scope's mechanical alignment of hyperbolic mirrors (and scope's focuser placement compared to this optical train alignment). If you don't understand Finnish in that message thread, then please just click on the photos and scroll fast forward with that RCT collimaton progress story (maybe only stopping where some interesting bits and pieces exist).  

 

As a conclusion I would even say that "There’s really only one correct position between the RCT primary & secondary mirrors" for these parts to work with optimal results.

Getting one's personal RCT scope from zero-colllimation into that optimal state of nearly perfect collimation is quite doable (at least with lots of Finnish persevarance called  "sisu" ).

Proof for that fact lays here in this 1,83 arc sec star field (which actually tells us, that there is no such thing as "perfect 100% collimation result" with these cheap do-it-yourself-by-tinkering Chinese RCT scopes).

Well advanced CNers have found that fact too already in year 2015, I think

https://www.cloudyni...for-masochists/

Quote:

"There is a 3rd problem that RCs suffer from: spacing issues between primary and secondary. Improper spacing can manifest as spherical aberration. The best way to diagnose this is with a Ronchi eyepiece and test on a bright star just inside of focus and verify that the Ronchi lines are straight.  Even better is to do a null Ronchi test on an optical table set up for autocollimation. This step will magnify errors by a factor of 2 and help diagnose issues nicely during the daytime.This step is usually not required because significant despace (> 4 mm?) before it starts becoming obvious. But I've heard of cases where tweaking this helped tremendously.

 

I think these RCs are ideal scopes for tinkerers and DIY sort of people. You could probably luck out and not have to do anything to your scope and get consistent sub 1.8" FWHM stars edge-edge. But in general you have to ENJOY tinkering with and tweaking your RC to get the best performance."

[pterodyne]

I'm the original poster.  I have listed my scope

 

look on astromart if you want a deep discount on an AT14RCT

 

That being said, likely it can be fixed.  Just with someone who has more patience, time and access than me.  My observatory is 2 hours away and setup for remote imaging.  The best evidence I have for this is Astrotech's comments to me.  Each mirror set is interferometriclyallywisbang tested.  Beyond that the truss setup, mirror cell etc are all very sturdy and adjustable. 

 

Anyway, thought Id post my own personal closure on this issue.  Side note Im looking at the 12" GSO Truss newtonian.  

[Darth_Takahashi]

When you have decided to sell OK, but its a real shame. Here are all my measurements for an Omegon 14inch RC which should be the same as yours!

 

 Copy of Omegon RC355.xlsx   22.93KB   57 downloads

 

All measurements made clockwise from 12 o'clock around both sides of the primary adjusters and to the back plate of the secondary. There looks to be a 4~6mm gap between the holder and the push screws/backplate etc...

 

One last try?

 

Regards

 

 

Neil

Edited by Darth_Takahashi, 30 June 2020 - 10:46 AM.

[andysea]

I'm the original poster.  I have listed my scope

 

look on astromart if you want a deep discount on an AT14RCT

 

That being said, likely it can be fixed.  Just with someone who has more patience, time and access than me.  My observatory is 2 hours away and setup for remote imaging.  The best evidence I have for this is Astrotech's comments to me.  Each mirror set is interferometriclyallywisbang tested.  Beyond that the truss setup, mirror cell etc are all very sturdy and adjustable. 

 

Anyway, thought Id post my own personal closure on this issue.  Side note Im looking at the 12" GSO Truss newtonian.  

If you are set on the RC design I think looking for an RCOS/DSI may be the best way to go. They are mechanically much better than the GSO scopes.

[Darth_Takahashi]

If you are set on the RC design I think looking for an RCOS/DSI may be the best way to go. They are mechanically much better than the GSO scopes.

Andy,

 

Is this actual information or anecdotal information? When its the former I would to learn more. Here in Europe there is a company placing the GSO optics in their own RC's and charging at least 10k more for them. All I will say is that's a lot of extra money for Polish Aluminium.

 

For me and I actually own an Omegon 355mm branded RC, there is one main weakness that's simple to fix and doesn't cost an additional 10k. Change all of the adjustment screws from metric (1.25mm pitch) to precision fine threads. Especially for the primary adjustments.

 

 

I will do this myself in a couple of years from now the first time that I need to remove the primary for some TLC. Then I will have micrometer adjustments on my RC as well.

 

Regards

 

 

Neil

Edited by Darth_Takahashi, 30 June 2020 - 02:39 PM.

[Timo I]

I'm the original poster.  I have listed my scope

look on astromart if you want a deep discount on an AT14RCT

 

That being said, likely it can be fixed.  Just with someone who has more patience, time and access than me.  My observatory is 2 hours away and setup for remote imaging.  The best evidence I have for this is Astrotech's comments to me.  Each mirror set is interferometriclyallywisbang tested.  Beyond that the truss setup, mirror cell etc are all very sturdy and adjustable. 

 

Anyway, thought Id post my own personal closure on this issue.  Side note Im looking at the 12" GSO Truss newtonian.  

Sorry to hear that AT14RCT decision. I googled your ad on Astromart, where you told this:

"In full dislosure I have not been able to collimate it.  No matter what I do I can't get my star sizes smaller."

Here comes just a kind reminder for my posts and test images linked above, because I feel sorry if those writings make someone frustrated enough to sell their expensive RCT scope. That was certainly not my intention there... 

Techical details from my 10" RCT imaging setup are such, that my imaging scale was deeply in the Goldilocks zone (10" RCT at 0,83" / pixel scale, 0.67 reducer and KAF-8300 sensor, 1x1 bin) compared to average seeing conditions here in Finland. You can calculate your own imaging setup scale easily with the help of this web page: http://astronomy.too...ccd_suitability

 

If someone wants to compare my test image results with their own RCT imaging setup, then they should always try to get near that 0,83" / pixel imaging scale I have used there in my test images. That's the only way of seeing real differences in actual star sizes between different scopes (supposing seeing conditions are equal foir both scopes). For example AT14 RCT @ 2850mm FL and ASI 1600 Pro camera would need 3x3 binning in order to get similar imaging scale to be comparing similar star sizes with my 10 RCT scope (2x2 binning would be comparable with 0,67x reducer in that AT14 RCT).

But despite of that reducer that's still quite a long FL you will get there from your scope to compete with the existing seeing conditions (compared to my 1340...1400mm FLs used with my 10" RCT).

A kind reminder: please try always to compare "apples to apples" there, when doing such direct image comparisons...   

 

Also please remember, that I have always used short 15...30 second exposures on those test images. Normal deep sky imaging uses much longer sub exposures, which will blur and bloat star shapes in any case.

Longer subs will blur the star shapes quite a lot and I have been doing real DS imaging quite a lot with quite poorly collimated RCT scope too.

(No one will look into image's collimation errors, if someone wants to present nice deep sky photos, especially when the images have been downsized to web size. )

 

Here's one 1:1 resolution image from my 10" RCT about just before I sold it (I got fed up with the poor imaging conditions here in Finland, which results into only 2-4 finished deepsky imaging projects per season (=autumn, winter and spring in total, no night stars seen here in summer time between May to August):

 

 

M27_10_RCT_LRGB_35min_30min_30min_30min_total_2h5min JPG imege there, which tells what kind of image quality could be achieved with that kind of 0.83" / pixel imaging scale with 10" RCT and KAF-8300 sensor (5 min subs there for each LRGB channel for combining that LRGB image in PixInsight).

 

On the other hand, I can understand perfectly you telling this:

"Just with someone who has more patience, time and access than me.  My observatory is 2 hours away and setup for remote imaging."

 

Free time in our hands and lifes is quite precious and I too wanted to make the best out that time. So I decided doing deep sky imaging was after all not my case of having fun, nor was staying awake in the early hours of morning and trying to collimate and re-collimate my RCT scope (I'm glad, that I did learn the process, but to be honest - it all took way too much time from myself! 

So, someone will get from you a very nice 14" RCT  imaging scope and I wish you very good luck with the sales process!

Edited by Timo I, 30 June 2020 - 04:16 PM.

[andysea]

Andy,

 

Is this actual information or anecdotal information? When its the former I would to learn more. Here in Europe there is a company placing the GSO optics in their own RC's and charging at least 10k more for them. All I will say is that's a lot of extra money for Polish Aluminium.

 

For me and I actually own an Omegon 355mm branded RC, there is one main weakness that's simple to fix and doesn't cost an additional 10k. Change all of the adjustment screws from metric (1.25mm pitch) to precision fine threads. Especially for the primary adjustments.

 

Precision Thread adjusters.jpg

 

I will do this myself in a couple of years from now the first time that I need to remove the primary for some TLC. Then I will have micrometer adjustments on my RC as well.

 

Regards

 

 

Neil

I used to own the AT10RC truss tube. The optics were indeed very nice.

Then I bought the DSI RC10C. For me the big difference, besides the construction being more robust, is that the DSI focuses by moving the secondary. So as long as your camera is at the correct back focus your mirror spacing will be correct. The Paul Jones optics are also top notch.

The main driver for me at the time was that the RC10C can produce a fully corrected field that matches the 16803 sensor but besides that the DSI was superior to the AT10RC as far as precise machining and construction. The DSI scopes, and I think the RCOS too, are tested one by one and they are tweaked for optimal performance when they are assembled. Of course the cost is very different but they are completely different instruments.

[Tom Gwilym]

Just skimming through this thread for ideas.  I see so many opinions on lasers, Takahashis, DSI method...etc.  I see the original poster gave up and sold his scope.  If the 16 inch GSO scope was mine and not the club's, it would be on the market.  I've put probably 100 hours into this thing and still can't get a decent star.  I won't get into details just search my profile and see all my postings. 

I tried again last night and just made it worse again.  I'm a nearly a complete loss now what to do.

 

Tom - beaten by the RC. 

[plane48]

Here is a great thread on RC collimation:

 

https://www.cloudyni...rc-distortions/

[Tom Gwilym]

Good thread.  I've met the original poster, Cedric, on a Facebook thread.  We have been working with a guy in Poland who has given us some good tips to collimate the RC without any lasers, Takahasi or Cheshire EPs.  It's basically what he calls the "eyeball" method.  Remove the focuser and all rings and look through the end toward the secondary.  Line up your pupil reflection on the secondary dot and move just enough back and forth until you see the edge of the baffle and the edge of the secondary gap.  If the gap is uneven, move the secondary.  Line up the primary (spider vanes line up), back to the secondary until nothing changes. 

Then do a star test.  I have fairly round stars that look pretty good now.  I just need another clear night to do some tweaks to the primary mirror to do a little adjustment to one side of the image.  I --think-- I may have figured this out.  I need another clear night to see if this all works and not just one of those "one night" things.  The method is very basic and just makes sense. 

We are working on a document describing the steps, so if this all works I'll have to share that.

 

Tom

Here is a great thread on RC collimation:

 

https://www.cloudyni...rc-distortions/