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Old 21-08-2016, 04:14 PM
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Grimmeister (Anthony)
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Seeking some help for a GSO RC10 Scope

Hi Everyone,

Calling all GSO RC10 owners, I am seeking some help from you all! I have had to dismantle my GSO RC10 due to what I thought was a cracked mirror but turned out to be bug poop . Of course mirror spacing is critical to these optical units and I have not got this correct after the process . As such I am asking for some assistance from others that own these units.

The history so far: during my testing and posting of Astrometry results (0.444 arc seconds per pixel scale) I can determine the focal length of my scope to be at 2002.25mm based on the following maths:

(206.2648 x [Pixel Size in μm]) / [Arcseconds Per Pixel = Focal Length in mm

I have a CDS600D with 4.3 micron pixels

Giving me the following: 206.2648*4.31/0.444 = 2002.25mm Focal Length.

Now I have searched around and from what I can see focal lengths are listed anywhere from 1970mm to the full 2032mm. From my understanding the focal length should be 2032mm based on the 254mm Mirror and F8 Focal ratio but I am way off that number.

I however either need a real world example or the exact focal length as designed by the manufacturer as the basis with which to work from. Once I have this detail I can then tune according to my optics and scope specifics to get an exact set up functional.

If there are any GSO RC10 owners out there could you let me know the following details:

1) what your image scale is by testing it here: http://nova.astrometry.net/upload
2) What is your camera pixel size in microns?

I can work out the rest from there using the above formula and have an indicator if my scope mirrors are too close or too far apart.

Thanks in advance for your help .

Cheers

Anthony
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Old 21-08-2016, 05:11 PM
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lazjen (Chris)
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I've got a GSO RC10CF.

I uploaded a file to the astrometry site and got these details:

Size: 34.4 x 22.9 arcmin
Radius: 0.344 deg
Pixel scale: 1.35 arcsec/pixel

I'm fairly certain I was using a 0.67 Reducer at the time (it's been a while, for reasons)

Camera is 9 microns.

Here's the astrometry annotated file, if it's accessible: http://nova.astrometry.net/new_fits_file/1689968
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Old 21-08-2016, 07:14 PM
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Grimmeister (Anthony)
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Hi Chris,

Thanks for posting, if it had the focal reducer it probably negates helping in this instance as it will change the focal length as designed and add an assumption the the .67 * reduction is exact.

I will double check the math on the scale and FL and see what it looks like.

Cheers
Anthony
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Old 21-08-2016, 07:36 PM
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lazjen (Chris)
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Yes, I thought that might be the case, but I posted it anyway. I can't remember the last time I took images without it. I'll see if I can find something that doesn't use it.
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Old 22-08-2016, 10:33 AM
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I should have some FITS files from my old RC10 configuration. Will take a look when I get home. The usual method for setting the primary/secondary distance in RC scopes is with a Ronchi grating.

Cheers,
Rick.
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Old 22-08-2016, 05:43 PM
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Plate solving on one of my 2012 subs gives: 0.561 arcsec/pixel, 1984.99 mm focal length (5.4 um pixels). I tried a second one and got 1984.51 mm focal length. A third sub solved to 1984.73 mm.

Cheers,
Rick.
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Old 22-08-2016, 06:11 PM
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byronpaul (Paul)
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Anthony,

drop LucasB a PM ..... he is an absolute expert on this topic with an RC10.

I went through this exact exercise with my RC8 after I got it through the IIS classifieds. The previous owner had adjusted the focal length and I wanted to get it back to standard.

Regards,
Paul
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Old 27-08-2016, 02:01 PM
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Grimmeister (Anthony)
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Hi Everyone,

Thanks for posting results, I also reached out to GSO and they state the Focal Length should be 2000 mm so I am curious now as to how flat a field people are getting.

To give you an idea of my field flatness (or lack thereof) I have attached an image of it. It would seem that the 2002.25 mm I have set is not correct and I suspect that the 2000 mm is also not correct

So where to from here? Well thanks to Rick I believe the value of 1985 mm is probably more the ball park but is also dependent on the optics and each scope. I am likely faced with a lot of trial and error to get this right.

Paul, I will reach out to LucasB as suggested as any help available will welcomed to get this back to spec.

Thanks again for your posting results.

Cheers
Attached Thumbnails
Click for full-size image (Field Curvature Map.JPG)
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Old 27-08-2016, 02:04 PM
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Grimmeister (Anthony)
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Oh I also forgot

the thread of the center screws holding the Secondary Mirror is M6x1.0 so I should be able to work out the angular rotation to distance to get very very close to the correct spacing.

I also have some diagrams they provided of the scopes specs.
Attached Files
File Type: pdf RC10-L3.pdf (57.3 KB, 135 views)
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Old 04-06-2018, 05:10 PM
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jwoody (Jeremy)
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Hello Anthony.
So in the end what was the magic number you came up with?
I am in a similar boat to you having just cleaned my primary in the RC10 and want to get it perfect.
Thank you
Jeremy

Last edited by jwoody; 04-06-2018 at 06:12 PM.
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Old 04-06-2018, 05:39 PM
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Grimmeister (Anthony)
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Quote:
Originally Posted by jwoody View Post
Hello Anthony.
So in the end what was the magic number you came up with?
I am in a similar boat to you having just cleaned my primary in the RC10 and want to get it perfect.
Thank you
Keremy
Hi Jeremy,

To be honest I have let this one sit for a while, I just made an adjustment and recollimated but also got a new Esprit120 so ended up playing with that instead.

I believe 1970mm is about right but need to confirm. I did reach out to Lucas many moons ago and this is what he suggested I try.

I will aim to get this set up next weekend to have a test and play. The easy part is that now thw primary morror is straight it doesn't take long to adjust the secondary and recollimate, in fact I can do this at the mount out the back to get it exact.

Once I have I will get you the Astrometry details.

Re: GSO RC10 Help
Hi Anthony,

I have a quick and easy method for getting the spacing right for the GSO RC10. I know for a fact that the back focus distance from the end of the GSO focuser (with the 1.25" adapter inserted) is 123.33mm. I have a schematic diagram from GSO which shows this. As I use the 2" barrel when using the focuser I remove the 1.25" adapter (which from memory is 10mm thick) so the back focus becomes 133.33mm from the 2" barrel on the focuser. Obviously 133mm is close enough.

What I do then is use the live view mode on a DSLR to focus. I use the large 50mm spacer and my DSLR with t-ring is 55mm. So with 105mm spacing from the DSLR and large spacer I know the focuser needs to rack out 28mm to equal the correct back focus.

Now the rule is for each 1mm the secondary is brought closer to the primary the back focus increases by 10mm.

For example with my DSLR and 50mm large spacer (105mm), if the focuser only racks out 18mm to reach focus on the live view I need to move the secondary 1mm closer to the primary to create another 10mm of back focus. I then do a quick cheshire alignment of the secondary after moving it and hopefully the focuser racks out to 28mm when focusing again with the DSLR.

If the opposite occurs and the focuser racks out too far then the secondary needs to move away from the primary.

When you have this sorted and your back focus is 133mm from the 2" barrel you can then fine tune by using plate solving with MaximDL to calculate the focal length. You will find the focal length of the GSO RC10 is approximately 1970mm.

When you use ccd inspector with the correct spacing you will still see a very slight hump in the centre of the field but not like the one you have currently.

I hope I have explained this so it is understandable? Let me know if you have any questions. I will try and post the RC10 diagram I have or if you send me your email i will send it direct.

Kind Regards,
Lucas
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Old 04-06-2018, 05:42 PM
glend (Glen)
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Quote:
Originally Posted by jwoody View Post
Hello Anthony.
So in the end what was the magic number you came up with?
I am in a similar boat to you having just cleaned my primary in the RC10 and want to get it perfect.
Thank you
Keremy
If you cleaned the primary from the front by removing the front ring which holds the secondary, then you should never have to worry about distances changing.
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Old 04-06-2018, 06:09 PM
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jwoody (Jeremy)
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Quote:
Originally Posted by glend View Post
If you cleaned the primary from the front by removing the front ring which holds the secondary, then you should never have to worry about distances changing.
Your dead right, but muggins here had brain fade and undid the adjustment screws unnecessarily

I know before the mirror clean my focal length was 2014mm so I can get back to that easily enough and collimate with the tak scope. I haven't touched the secondary.
Thanks
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Old 04-06-2018, 06:10 PM
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Thank you Anthony
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Old 04-06-2018, 07:05 PM
Star Catcher (Ted Dobosz)
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A bit late to this discussion. Been there done that. The correct separation distance for a GSO RC10 is:

463.64mm.

As communicated to me by GSO themselves.

Ted
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Old 20-06-2018, 03:45 PM
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jwoody (Jeremy)
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In the end I settled for a F/l of 2014mm. Nearly flat to all 4 corners
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