Help please! Collimating an RC with a Howie Glatter

[kookoo_gr]

Today my photographic ronchi arrived and it seems i have no adapter for my cameras. The eyepiece has a female T2 thread and a male M52 thread. So i will need a male T2 to T2 adaptor in order to connect the eyepiece to my camera and a female M52 adaptor to T2 in order to connect to eyepiece to what i want. A head scratcher is do i connect the eyepiece to my scope or to lens in my camera and then to my scope? I include some options below for the connection of the ronchie eyepiece

 

camera -> male T2 to T2 adaptor -> Ronchie eyepiece -> M52 female to T2 adaptor -> telescope

camera -> male T2 to T2 adaptor -> Ronchie eyepiece -> M52 female to camera lens -> telescope

[davies07]

Mine arrive today and I concur, the screw threads are really odd. 

I assume that the 52mm male thread is designed to fit into a 'standard' 52mm female thread on a DSLR camera as if the grating was a filter. That would leave the T2 female filter presented to the telescope side as if it was a astronomy camera such as a ZWO. However, I have no DSLR camera with such a filter size. That leaves the problem of presenting a T2 male thread to the camera side. This is such an adapter from TS Optics

https://www.teleskop-express.de/shop/product_info.php/info/p11819_TS-Optics-Adapter-from-SP52-M52x0-75-filter-thread-to-T2-photo-thread.html

FLO also have such an adapter but it is for a Tak and costs over twice as much.

With the above adapter, the grating will have T2 female on the telescope side and T2 male on the camera side. Now why couldn't it have been made like that!!

David 

Edited July 2, 2020 by davies07
Got my males and females mixed up

[davies07]

5 hours ago, kookoo_gr said:

Today my photographic ronchi arrived and it seems i have no adapter for my cameras. The eyepiece has a female T2 thread and a male M52 thread. So i will need a male T2 to T2 adaptor in order to connect the eyepiece to my camera and a female M52 adaptor to T2 in order to connect to eyepiece to what i want. A head scratcher is do i connect the eyepiece to my scope or to lens in my camera and then to my scope? I include some options below for the connection of the ronchie eyepiece

 

camera -> male T2 to T2 adaptor -> Ronchie eyepiece -> M52 female to T2 adaptor -> telescope

camera -> male T2 to T2 adaptor -> Ronchie eyepiece -> M52 female to camera lens -> telescope

I think you can get away with just one adapter if you turn the grating around.

[kookoo_gr]

I've been looking for adapters and this is what i suggest. First of all i use the camera which can be either my ZWO ASI 120MC camera or my Atik camera that have a female T2 thread. I use either this adaptor https://www.firstlightoptics.com/adapters/zwo_t2-t2.html or this one https://www.firstlightoptics.com/adapters/baader-t2-t2-inverter-ring.html so i can connect the ronchi eyepiece to the camera and then use the https://www.teleskop-express.de/shop/product_info.php/language/en/info/p11819_TS-Optics-Adapter-from-SP52-M52x0-75-filter-thread-to-T2-photo-thread.html in order to connect the whole setup of ronchi eyepiece/camera to a T2 thread extension so i can place it at my scope. Is this setup correct?

 

[SXBB]

These are exactly the same questions that I had when I received mine and I didn’t have the cycles to follow it through. Not only which connectors to use but exactly where the ronchi needs to be placed in order to work. If you look at the instructions I believe it says something to the effect of putting it in front of a camera with a normal lens in order for you to get it to come to focus?
 

Best,

Bruce

[kookoo_gr]

well for once i don't have the instructions for the photographic eyepiece so i'll have to ask for a copy but from the description at the shop it mentions a lens but it is not very clear, i will also ask for this possibility

[davies07]

I’ve been chatting with FLO and we’re all confused. 

Checking on Gerd Neuman’s website, the photographic version is designed to be use d with a DSLR camera complete with a lens on it. Thus the 52mm male thread.  That is designed to fit into a 52mm filter holder on the front of the DSLR lens. If you have a lens with a different diameter you can buy a step up or down ring for a few pounds. 

But here’s the thing. How does it then work? You’ll have a telescope terminated with a T2 male thread that screws into the Ronchi grating and then have a DSLR plus lens attached to that. How do you focus it?

FLO are going to contact Gerd for clarification. So don’t buy anything yet  

D

 

[Merlin66]

D,

I use my Ronchi grating with various cameras and telescopes.

My grating has a 1.25” nosepiece which fits to the focuser instead of an eyepiece.

without using a camera, set up the grating and look through it at a star....

you should see an out of focus disk with numerous lines across it, use the telescope focuser, the closer the grating gets to focus, the less lines are seen.

Its usual to work at a focus which gives around five or six lines visible.

The job of the camera plus lens is to replace the eye.

Position the camera/ lens combo behind the grating and focus the camera to give clear image of the lines.

Hope that helps.

 

[kens]

Is it possible to use an astro camera (i.e. no lens) with the eyepiece style of grating? I'm thinking along the lines of an eyepiece projection tube to hold the grating with the camera on the end of the tube. 

[Merlin66]

Hmmm

Don't know, never ever tried it that way....but you do need some means of re-focusing the optical beam emerging from the grating.

Any camera (with lens) will work...

(This example isn't mine but shows a Ronchi test set-up with compact camera)

https://stellafane.org/tm/atm/test/ronchigrams.html

http://www.mmresearch.com/articles/article1/

 

[skybadger]

On 03/07/2020 at 04:23, kens said:

Is it possible to use an astro camera (i.e. no lens) with the eyepiece style of grating? I'm thinking along the lines of an eyepiece projection tube to hold the grating with the camera on the end of the tube. 

In short, no. You need to reimage the grating which needs a lens.. I have one setup with filter step ring to camera lens and t2 on the s ppr side. Works a treat apart from droop due to slop on the lens. Use a fixed lens for better operation. 

Move to focus and then wind in or out a touch to see several lines. 

 

 

[davies07]

I've made some progress and have got my Gerd Neuman grating working on Es Reid's artificial star test bench. This is my camera setup:

One thing to be aware of is that the Ronchi grating needs to be close to the focal plane of your telescope. This is the normal location of the sensor on your imaging camera so the Ronchi test camera has to move further back and you will probably need to use an extension tube.

To sight the telscope on your test star you can remove the camera and look directly into the grating, rack the focuser in and out with the grating close to the focal plane. The light from the star will fill the aperture of the scope and you should see the Ronchi lines across the aperture. They are quite low contrast so you need care to spot them. Here is an image from a refractor under test at Es's.

I now need a clear night to have another go on my RC.

   

[Merlin66]

Hmmm

The Ronchi grating - should show lines and gaps of equal widths....

Check your settings.

[davies07]

On 09/07/2020 at 12:05, Merlin66 said:

Hmmm

The Ronchi grating - should show lines and gaps of equal widths....

Check your settings.

Mmm! Thanks, Merlin66. This is using Es's 25 l/mm Ronchi grating with his artificial star. Es says the grating is very sensitive but give low contrast graphs.

I thought I'd update you all on progress. I got out at the weekend and manged to get these graphs off my own RC8 scope using the Gerd Neuman 10 l/mm grating. I've used the setup with my Fuji X-T2 camera, 35mm lens and a 43mm - 52mm step-up ring as shown in the post above. I also added a 50mm extension tube to bring the Ronchi grating to the focal plane of the telescope.

I started by slewing to a bright star and centring it with my imaging camera. I then replaced the imaging camera with the Ronchi grating but without the camera. Looking into the grating by eye, I could see the lines and was able to adjust the focus to bring the Ronchi grating to focus. I then added the Fuji camera and lens and took this shot:

What you can see here is one grating bar across the centre and we can see some of the imperfections in the mirror surface.

Racking the focuser in towards the telescope, the grating lines appear. You get more bars the further from the focal plane the grating moves. So this is an 'intrafocal' graph.

 

 

 

And a bit further in, we get:

The second picture of each pair is taken with the camera at 90 deg to to the other.  

I think the lines are almost straight and evenly spaced indicating the scope is operating close to its correct focal length and is more or less collimated. There is a slight suspicion of barrel distortion on the first pair of graphs which would indicate slight over-correction - mirrors too far apart.

Racking the focuser to put the grating outside of the focal plane, I get this view:

This is the 'extrafocal' view. Now there is a slight indication of pincushion effect, again indicating slight over-correction.

So I deem my scope to be nearly there. I'll try to get it to a fully corrected state by unscrewing the secondary mirror centre screw out by, say a quarter turn. That should move the focus out by around 5 mm and I'll test it again. 

Just a reminder, I've used a plastic disk with a  hole in it to collimate the scope as I've described earlier in this thread. I've improved the disk by adding a black cross, intersecting at the central hole. This makes it much easier to see where the reflection of the hole is when you adjust the secondary mirror. All I have done here is to adjust the position of the secondary, recollimate the secondary using the plastic disc and then retest; I haven't touched the primary.

 

 

  

Edited July 15, 2020 by davies07
added the word 'too'

[skybadger]

You need to consider that you might not be able to remove under/over correction just by adjusting the spacing. It looks good to me, as good as it might get. But there's only one way to find out. 

 

[davies07]

8 hours ago, skybadger said:

You need to consider that you might not be able to remove under/over correction just by adjusting the spacing. It looks good to me, as good as it might get. But there's only one way to find out. 

 

Yes, indeed. I've just adjusted my secondary mirror to push the secondary towards the primary by around 0.3 mm. I will retest once the clouds clear and let you see the results. It would be good if the result shows me to be slightly undercorrected this time. At least I would then know that the correct position is between the previous setting and the current one. 

[skybadger]

Your error budget should be about 5mm, so worrying about 0.3mm is a nice place to be... ! I forget where my source for this is, but I calculated it from my own 12" f/10 RC for the same reasons you are doing this now. The other way of doing this ( when not collimating on stars ) is using the dual-pass setup over an oil bath. 

[kookoo_gr]

This is my setup for the Ronchi Eyepiece. I had a friend of mine 3d print a T2 to canon lens adaptor and used my old 18-55mm canon lens on my ASI120MC, i then bought from a local photography equipment shop a cheap M58 to M52 ring (8 euros) and installed the ronchi eyepiece which will go to my scope using a T2 ring. As is the setup focuses at 35mm and i haven't managed any trials because clouds, let's hope tonight is the lucky night

Edited July 18, 2020 by kookoo_gr

[dweller25]

On 31/03/2020 at 14:33, davies07 said:

I thought I should give some results from the rebuild outlined above and after Es had corrected the focal length.

Here is an image of a very nice open cluster, NGC 6633, showing the centre and the corners.

 

 

I used PixInsight to measure the eccentricity of the stars in the image. An eccentricity factor of 0.3 means that the minor axis of the star elipse is 96% of the major axis, i.e. a 4% error. The error rises to about 9% for an eccentricity factor of 0.4.

 

 

And I also used PixInsight to measure the FWHM of the stars (you might recall, I was getting about 7 arcsec previously). This means my HFR readings in Nebulosity is now around 2 arcsec.

 

 

So, yes, I was happy with this result. The results show the optics are symetrical.

Whilst all this was going on, I was still investigating collimation methods that could be used routinely. One thing that puzzled me was the role of the TS tilt-plate. If you attach the tilt-plate to the RC8 as shown on the TS website, then you can no longer adjust the primary mirror because the tilt-plate hides the primary collimation screws. So I phoned TS to ask them. Their response was that the tilt-plate is NOT intended to be used to collimate the telescope, it should only be used AFTER the telescope primary and secondary mirrors have been collimated and then only if you have tilt of the sensor.  Tilt of the sensor would give you oval stars across the field of view and whilst stars might be in focus in the centre, they will tend to be out of focus at opposite edges.

I then discovered this reference:

http://interferometrie.blogspot.com/2013/01/10-rc-gso.html?view=sidebar

This is a review of the construction and collimation of a 10" GSO RC and is well worth a read (English version unless you speak German). Here is a key diagram from it:

 

 

This is a view into the back end of an RC scope and shows how the secondary mirror and spider support vanes should look. The method outlined in the paper is to stand about 1m behind the scope and line your eye up to get this view. Adjust the secondary mirror to centre the secondary centre mark and then adjust the primary mirror to equalise the refections of the spider vanes. Simples.

I had a go at this using my DSLR camera as shown here:

 

 

Using the wifi remote control for the camera on my iPad I could adjust the scope 'live' and take test shots to record what I had. So this is with the secondary mirror mark in focus:

 

 

And this is with the reflection from the primary in focus:

 

This is a good, quick check of the state of collimation and with practice you can do it with the scope on its mount.

So to round this off. This is how I collimate my RC8 today. No lasers, no tilt-plate.

First remove the imaging train and extension tubes. Now measure the internal diameter of the centre hole in the primary mirror and cut a styrene disc, or piece of thin card (postcard), to fit. Now drill a small hole (1mm) in the disc (say with a Dremel).

Now insert the disc into the hole in the primary mirror. t should be a snug fit. I wrapped a single layer of tape around the edge of the disc to give it some cushioning. Don't push it in too far; it will drop into a gap and you'll have to fish it out. The hole in the centre of the card is now marking the centre of the primary mirror.

 

 

Now go the front of the scope and remove the secondary mirror - mark the screw with a paint spot and count the number of turns. You will also need to unscrew the shade tube - do it carefully and leave it sitting on the bottom of the tube. Place a lamp behind the scope.

Look into the centre hole of the secondary mirror support and you should see the light of the lamp shining through the hole in the card.

 

 

Now centre your eye to keep the bright spot centred and check if the spider vanes coincide with their reflections in the primary mirror. In the picture, above , you will see that the spider reflections are just alongside the spider itself. Now gently adjust the primary mirror to bring the spider and its reflection into coincidence, whilst keeping the bright spot in the centre of the support hole. Again, you can set up a camera to help. This adjustment isn't absolutely crucial because you will do a final check of the adjustment of the primary using a star on the sky.

Now put the lamp at the front of the scope, shining onto the mark in the centre of the card. You can't see the card if you have the shade tube in place which is why we need to remove it. Now go to the back of the scope and look throught the hole in the centre of the card. You should see a reflection of the hole in the card in the secondary mirror. This is a bit tricky, you need a lot of light and a good eye. 

 

Now adjust the secondary mirror so that the reflection of the hole in the card is centred in the secondary ring mark. Take your time. This is a crucial adjustment. This adjustment centres the secondary and governs the symmetry of star images over your field of view.

When you're done, replace the shade tube and secondary mirror. 

When the clouds clear, check the collimation on a star in the centre of the field of view. Defocus the star only slightly so you can just see the central hole, a bit like this:

 

 

 

Don't use a hugely out of focus image, it is not sensitive enough for collimation.

This is the display from 'Fine Focus' mode in Nebulosity. The star is slightly out of focus 'out'. I discovered that the bright edge was due to a hot plume from my dew heater.

 

 And that's it. You can collimate your RC using just a piece of card, some understanding and patience.  

 

Clear skies and stay safe,

 

David

 

 

 

 

Excellent work - thanks for sharing

[kookoo_gr]

Here is my attempt with my equipment, what do you think?

[kookoo_gr]

This is the last image for this night, it seems that the scope is corrected but i may be wrong so i need second opinion. I also did a visual check and the lines were curved only when i had three lines but as per the instructions most telescopes will not pass the three lines test. (sorry for the image not being centered but the 3d adaptor must have some play and i am not sure if that affects the test.

[skybadger]

You need to brighten these up a bit but they look good, straight (without measurement)  and no waver.

[davies07]

I agree. I think the scope is essentially correct. Do I detect a bit of tapering top to bottom; this could be a bit of coma. I would check on a slightly out of focus star image. 

 

For the record, I adjusted my secondary mirror inwards by around 0.3 mm. Here is the resultant Ronchi image inside focus:

 

  

 

 

So the tendency to barrel distortion (over correction) has gone and the lines look straight. The extrafocal image does not look so good:

 

There is a suggestion of tapering which I interpret as a bit of coma. Note that this image is closer to the focus point but I can't move the focuser any further out. I shall check this when I get on to a star - the sky clouded as soon as I took that last image. 

For the moment, I'm calling this done. I'll now use the scope and see how the images look. I did manage a single frame of M13 but with guiding errors due to cloud. It did enable me to check the scope focal length in Pixinsight and it came out as 1658 mm which is very long for this design of scope (should be 1624 mm). I suspect I have a rather edge of spec set of mirrors.    

The coma I'm seeing on the last image could be due to the very extended image train. I'm using all the extension tubes and the focuser is almost at the end of its travel, out, and I'm sure there must be a bit of flexing which might vary with where in the sky I'm pointing. 

By the way, I've edited my images in Photoshop to bright the white point down to closer to the image data. This has made the images brighter.

 

Edited July 19, 2020 by davies07
Added the last paragraph.

[kookoo_gr]

How do i correct the coma? Collimation? The scope does need some slight recollimation and from my first test with the ronchi the lines will be converging if the collimation is off. Here are two videos i captured maybe this is seeing?

Edited July 20, 2020 by kookoo_gr

[skybadger]

 I don't think that is coma. I may be wrong but since you are working at the focal plane the divergence of lines indicates either astigmatism or , since you have a long train of extenders, more likely it's droop, which will show the same. Coma appears as curvature of the spot image at the edge of field in the direction radially away from centre.

I need to do something similar (again) to mine for comparison post it's refiguring.