Help please! Collimating an RC with a Howie Glatter

[davies07]

thank you very much for your reply (i also posted this question on CN so you don't have to answer me there) i think i'll look into a Ronchi eyeiece. Should i go for the Ronchi Okular Photographisch 10L/mm? https://www.gerdneumann.net/english/ronchi-okular-ronchi-eyepiece/photographic-ronchi.html

Yes, that is the one my friend bought. It seems to work fine.

[Fellside]

Hi

I have a RC10C and I normally collimate with a Howie Glatter Parallizer, Laser Collimator and a Cheshire Eyepiece. I am happy with the results.

To check the focal length I upload to Astrometry.net and then plate solve an image in Prism as a check. I don't own Pixinsight so I cant comment on it accuracy.

I am quite interested in the Ronchi eyepiece that has been mentioned. The one that is linked is a photographic version.

Dave07

Did you use it with a ccd connected or an eyepiece. Or can you just look through it once focused.?

The reason I am interested is I read an article that say the F/L of the RC10 should be 1970mm and not as you would assume 2000mm. Mine plate solves within 1mm of 2000mm. Collimation is supposed to be easier the closer you get to the correct spacing so I would certainly like to give one of the Ronchi eyepiece a try. When I have the instrument set I was never keen to go winding it in to 1970mm to check if the article was right, But now I may do that.

http://www.iceinspace.com.au/forum/showthread.php?t=148213

Thanks for a very interesting thread.

Graham

[SXBB]

I purchased the Photographic Ronchi to hone in on the correct distance for my RC10 but have had difficulty using it so I'm very interested in the experience of others.

As for the correct Focal Length, my understanding is that it can be slightly different for each batch of mirrors so a generalization of the correct Focal Length may be off the mark.

 

Thanks,

 

Bruce

[kookoo_gr]

5 hours ago, Fellside said:

To check the focal length I upload to Astrometry.net and then plate solve an image in Prism as a check. I don't own Pixinsight so I cant comment on it accuracy.

 

I checked both with astrometry.net and pix and they give the same results

 

For starters i will try to set the FL at 1624mm and then go from there, i may have to go to near 1660 or not, i ordered the Ronchi eyepiece and see what results i'll get

[Fellside]

8 hours ago, SXBB said:

As for the correct Focal Length, my understanding is that it can be slightly different for each batch of mirrors so a generalization of the correct Focal Length may be off the mark.

Bruce

You are probably correct. F/L varies between batches. The Ronchi eyepiece should allow the user to set the correct F/L to suit there instrument. Whatever it is.

What problems did you encounter when using the Ronchi?

Graham

[davies07]

17 hours ago, Fellside said:

Hi

I have a RC10C and I normally collimate with a Howie Glatter Parallizer, Laser Collimator and a Cheshire Eyepiece. I am happy with the results.

To check the focal length I upload to Astrometry.net and then plate solve an image in Prism as a check. I don't own Pixinsight so I cant comment on it accuracy.

I am quite interested in the Ronchi eyepiece that has been mentioned. The one that is linked is a photographic version.

Dave07

Did you use it with a ccd connected or an eyepiece. Or can you just look through it once focused.?

The reason I am interested is I read an article that say the F/L of the RC10 should be 1970mm and not as you would assume 2000mm. Mine plate solves within 1mm of 2000mm. Collimation is supposed to be easier the closer you get to the correct spacing so I would certainly like to give one of the Ronchi eyepiece a try. When I have the instrument set I was never keen to go winding it in to 1970mm to check if the article was right, But now I may do that.

http://www.iceinspace.com.au/forum/showthread.php?t=148213

Thanks for a very interesting thread.

Graham

I confess, I haven't used the Ronchi eyepiece myself but have worked with someone else who has.

I've used Es Reid's Ronchi grating with an optical eyepiece but that was on his optical bench.

Yes, there are two versions of the eyepiece, one for a camera and the other optical. Here is the optical one that FLO sell:

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html

They also have the photographic one:

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-photographic-eyepiece.html

The note on the FLO site says, of the photographic Ronchi:

The Photographic Ronchi has the same high-quality grating inside, which is made of evaporated chrome on polished glass. (254 Lines per inch or 10 Lines per millimeter). Contrary to the visual version it has no 1.25" barrel but a T-thread on one side an M52 on the other.  With the T-thread you may mount the photographic Ronchi on nearly all instruments. On the other side of the Ronchi you may install your camera (with a normal lens installed!!) at the M52 thread. (You might need a matching Step-Ring for your lens.)

You can download the manual here:

https://www.teleskop-express.de/shop/Bilder/shop/ts-okulare/ronchi-manual.pdf

I imagine the difficulties of using it on the sky are that the quality of the lines you see will depend on the quality of your focus and the seeing. If you need to adjust the secondary to change the focal length, then your scope will be out of collimation for the rest of the exercise. I found using Es's kit that by rocking focus back and fore you could see the pinched and barrel shapes well. At the point of correct focal length, the zooming in and out of straight parallel lines, indicating a correct focal length, was clear. 

 

 

 

[Fellside]

Dave

Thanks for the information.

I have ordered one from FLO.

I only put my Esprit on the mount a couple of weeks ago. I will replace it with the RC10 when it gets dark again and see what results I get.

Many thanks

Graham

[davies07]

I've just ordered the photographic one. We can compare notes later.

[Fellside]

The same photographic one. Thanks for the manual  link.

I've just had a look back through the thread. It a pity your F/L v`s Turns graph dos`nt go up to 2000mm😉

Graham

 

[kookoo_gr]

10 hours ago, davies07 said:

I imagine the difficulties of using it on the sky are that the quality of the lines you see will depend on the quality of your focus and the seeing. If you need to adjust the secondary to change the focal length, then your scope will be out of collimation for the rest of the exercise. I found using Es's kit that by rocking focus back and fore you could see the pinched and barrel shapes well. At the point of correct focal length, the zooming in and out of straight parallel lines, indicating a correct focal length, was clear. 

 

 

 

While i wait for my Ronchi eyepiece, and i don't have experience with these tests, i would like to know that how a bad collimation will affect this test? Also  for making this test do i need a star or can i use a bright light during the day?

[SXBB]

On 13/06/2020 at 03:33, Fellside said:

Bruce

You are probably correct. F/L varies between batches. The Ronchi eyepiece should allow the user to set the correct F/L to suit there instrument. Whatever it is.

What problems did you encounter when using the Ronchi?

Graham

Hi Graham,

I found the instructions lacking regarding how to position and use it with a camera. I attempted to use it but didn’t have a DSLR with a normal lens at my disposal which I suspect was the problem. I’d greatly appreciate someone illustrating how to set it up with a ccd or cmos Camera! A thread on setting up a basic optical bench would also be wonderful!

Best,

Bruce

[kookoo_gr]

I redid my collimation with the howie glatter but this time i pointed my scope vertically down, needless to say everything were off and i had to redo everything form the start. Last night i did a star test and things were improved i include some photos form m13 and the ring nebula along with some FWHM and eccentricity tests for these images obviously the collimation and the corrected focal length since i measure it at 1605 mm needs work and i will get to them once i get my Ronchi eyepiece. Now if possible could you help me interpret the FWHM and eccentricity diagrams?

 

Edited June 19, 2020 by kookoo_gr

[davies07]

7 hours ago, kookoo_gr said:

I redid my collimation with the howie glatter but this time i pointed my scope vertically down, needless to say everything were off and i had to redo everything form the start. Last night i did a star test and things were improved i include some photos form m13 and the ring nebula along with some FWHM and eccentricity tests for these images obviously the collimation and the corrected focal length since i measure it at 1605 mm needs work and i will get to them once i get my Ronchi eyepiece. Now if possible could you help me interpret the FWHM and eccentricity diagrams?

 

 

 

Hi Konstantinos,

Eccentricity is a measure of the how oval a circular shape is. It is defined as =SQRT(1 - y^2), where y is the minor diameter of the oval and the x diameter is assumed to be unity.

Your eccentricity readings are maximum at the tops of the image (~0.6) and indicate that at the top of the image, the minor axis of your stars is around 80% of the major axis. I downloaded your images and detect that the stars are wider in the horizontal direction than the vertical. Stars are at their roundest at the bottom of the images where the value of e falls to around 0.4 to 0.5, indicating the minor axes here are 85% to 90% of the major axis.

In a collimated scope I would expect to see the roundest stars (minimum vales of e) in the centre and would hope to achieve e values around 0.3 (minor axis of the star better than 95% of the major axis). 

I cant make much comment on the FWHM (full width half maximum) measure of star diameter other than I would hope to see the sharpest stars (smallest FWHM) in the centre. Your smallest stars seem to be off to the right.

In absence of a Ronchi eyepiece - I'm also still waiting for mine - I would unscrew the secondary centre screw one turn anti-clockwise. This would push the secondary mirror in towards the primary by around 1mm. This in turn should change the focal length to around 1625mm - at least close to the advertised value. Then retighten each of the three collimation screws by around 3/4 turn each, to just snug to the secondary and then recollimate.

I would also put the Howie Glatter laser aside and make yourself a card disc with a hole in it as I've described, above, and work through my instructions. I'm willing to bet a glass of ouzo that you will achieve a better collimation result.

A couple of weeks ago I put my HG laser on my own scope and it said the scope was out of collimation! I didn't touch anything because I know that it is, in fact, collimated. 

Best wishes, 

[Melitastro]

I'm the owner of a GSO RC6 and have just come across this discussion. One point that stands out is that the consensus of opinion for adjusting the secondary mirror to achieve the optimal focal length is by adjusting the centre screw on the secondary support. I'm not convinced this is the best way as the mirror distance can be adjusted by slacking the locking ring (see photo) and screwing the housing in or out as needed.

 

It's much easier than using the centre screw and in theory will not mess up the collimation.  However @davies07's post says that Es Reid used the centre screw adjustment method so I'd really like clarification on this as I must be missing something.  A better view of the secondary mirror housing construction can be found on Han Kleijn's site http://www.hnsky.org/RC_collimation.htm

Tony

 

[kookoo_gr]

So you rotate (screw - unscrew) the mirror and you use the ring to lock it into place, but with rotating the secondary wouldn't that change the flatness of the images? or am i wrong about that?

[davies07]

23 hours ago, Melitastro said:

I'm the owner of a GSO RC6 and have just come across this discussion. One point that stands out is that the consensus of opinion for adjusting the secondary mirror to achieve the optimal focal length is by adjusting the centre screw on the secondary support. I'm not convinced this is the best way as the mirror distance can be adjusted by slacking the locking ring (see photo) and screwing the housing in or out as needed.

 

It's much easier than using the centre screw and in theory will not mess up the collimation.  However @davies07's post says that Es Reid used the centre screw adjustment method so I'd really like clarification on this as I must be missing something.  A better view of the secondary mirror housing construction can be found on Han Kleijn's site http://www.hnsky.org/RC_collimation.htm

Tony

 

Hi Tony,

This is interesting. I confess, I have not looked closely at an RC6.

I've been looking throught the photos I took of my RC8 rebuild and here is a picture of my secondary mirror holder, taken in August 2017.

The arrangement looks similar to your RC6 but, of course, the cup holding the secondary mirror is bigger on the RC8. What is also evident is that the movable plate and locking ring appear to be the same on the RC6 and RC8. I'm wondering if the original purpose of the twopart construction is to enable the RC6 and RC8 to be made from common parts, or maybe to enable the secondary mirror to be removed without disturbing the secondary collimation.

I'd be cautious about unscrrewing the locking screw itself. I wonder how much thread there is?

Of other possible interest here is the open page of my notebook with the note that the centre screw needed 9.5 turns to release the centre screw, so there is a lot of thread length on the centre screw to play with.  That 9.5 turns gave a focal length of 1607mm, somewhat short.

David

Edited June 21, 2020 by davies07
My original post was just wrong.

[kookoo_gr]

Yesterday the Ronchie eyepiece arrived but they made an error and sent me the visual one instead of the photographic. While i wait to sort this out i did some tests with the visual eyepiece and here is my experience.

1. The whole process with the visual eyepiece is very tedious and i couldn't make out very well the lines this could be from my inexperience as well as the others i will state below

2. While using it i could clearly see the central obstruction from the secondary mirror and the lines were almost covered by it and i could barely make out if the lines were curved or straight

3. Collimation plays a very big part in this whole process if you change the distance of the the secondary you need to recollimate the scope, the lines after the adjustment were more fat down in my FOV than in the upper part of the FOV. I used my Glatter laser and after each adjustment the collimation was faster.

4. By adjusting the focus you increase or decrease the lines you see in the eyepiece i went for five lines, first you focus on a star and the then you use the Ronchie eyepiece and adjust the focus to see the desired lines

 

Right now the FL of my scope is at 1618 from 1605 but the whole process was exhausting doing it outside in the night and i had to call it off. If i use an artificial star at which distance should i place it in order to have good results?

[Merlin66]

I use the Ronchi for testing...

Using a artificial star indoors - find the longest space you can, suggest at least x10 the focal length..... longer is better.

 

[kookoo_gr]

dang it, i'll need at least 16m my house doesn't have 16m

[Merlin66]

When testing the Meade SCT (10" and 12") I mounted the artificial star (Hubble torch) on a photographic tripod and used the full length of the garden. You don't need complete darkness to test.

 

[kookoo_gr]

Hmmm here's an idea what if i shine the stars on a mirror and aim my telescope on the mirror wouldn't that increase the distance even when indoors?

[Merlin66]

Hmmmm, an extra 3 metres on 10 light years won’t make much difference.....

try  the reflected spot from the sun on a Xmas decoration ball to give a artificial star.....but you still need some distance....

 

[SXBB]

There may be another way. I’ve read of people using two scopes and an artificial star in order to increase the optical distance. If I am correct which I’m not sure of actually, one of the telescopes is focused at infinity and the artificial star is placed in front of the objective of that scope.  The other telescope which is the one that you want to collimate is placed in front of that scope and then focus on the artificial star can be achieved.  I think it’s called a double pass system. I have looked for more detail on this and come up empty but maybe somebody here might be able to fill in the blanks?

Cheers,

Bruce

[Merlin66]

Bruce,

I would have to assume any “issues” (ie aberrations, mis-collimation) in the collimating scope would be transferred to the scope under test.

A usual double pass set-up uses a master reflector to pass the light beam twice through the same optics.

 

[skybadger]

What you are doing is providing a star at infinity from the reference scope  to use as a star for the scope under test. It's sound, but you need a 12" scope to feed a 12" scope. This is the same as using an optical flat to dual-pass test optics with a ronchi.  You need the same large optics to feed the scope under test. A false star at the end of the garden or reflected back to you from the end of the garden is the more practical approach in absence of an optical lab and expensive flats.