Help please! Collimating an RC with a Howie Glatter

[kookoo_gr]

I checked the collimation of my rc and did another ronchi test at my scope. It appears that the optics are ok. The only things that i need to is to replace the 3d printed T2 to canon lens adaptor since the camera is not centered with the lens that's why the images are not centered, i ordered a new adaptor and i will redo the test. Finally i need to take some images and check the FL of the scope since these tests are made under heavy light polluted sky and pix can't find enough stars even at m13.

[Teo_Limassol]

Hi,

I have also the HEQ5-pro and i am about to buy now the RC8" carbon. What is your experience with RC and payload on HEQ5. It is very close to the limits.

[kookoo_gr]

I would advise against it, you will add a lot more weight on the mount with the accesories and you will need a counterweiht to add on the scope in order to offset the primary mirror + camera thus stressing the mount. This is the main reason i got the eq6-r. Even though i modded my mount with the Rowan belt mod and i had really good pictures there were times that it was strainning. For reference at my scope i have the following thing attched to it

 

Atik EFW2 and OAG

Atik 383L+

SX Lodestar

Pegasus Falcon Rotator (new purchase)

Moonlite focuser with stepper motor

focuser extension

A nuc mini pc in order to control the setup along with a pegasus mini power box, pegasus focus cube

Power supply box in order to stabilize and increase the power to 13.5V

Usb hub

An ADM Losmandy saddle and an ADM dual clamp in order to have a 1kg conterweight on the scope

 

As you can see these add much more weight on the scope and the HEQ5 has hard time tracking, even on the EQ6-r i have an extra 5kg counterweight and a conterweight extension bar

[davies07]

I have to agree with KooKoo_gr. I think you'll be pushing it. Even well balanced, you'll have a lot of inertia on the mount and pressure on the quite small worm gears. The motors won't be able to deliver the torque to move the mount smoothly and in the limit will stall and miss steps. 

Once upon a time, I had my RC8 on an EQ6 with a 100 mm refractor. Most of the time it seems to work fine but then I started losing guide stars and could see that the whole field of view was stepped a small amount to the east in RA.  My RA motor had stalled for a step. Removing the refractor fixed the problem. 

[inFINNity Deck]

On 31/03/2020 at 15:33, davies07 said:

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

...

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. 

...

 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

 

Hi David,

yesterday I tried your method on a RC8 of a befriended amateur astronomer. Your method is very easy and works like a charm. I added a few minor improvements to your method that makes the collimation of the primary mirror even more easy and more accurate and have written an article about it on a Dutch forum (of course with acknowledgements to you). Opening it in Chrome should translate it nicely into English: http://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

It also discusses finding the proper mirror-distance using a Neumann photographic ronchi-filter (images included) and shows the final and successful inspection using a holographic Howie Glatter laser (again image included).

Another RC8 fit for service!

Nicolàs

Edited August 27, 2020 by inFINNity Deck

[kookoo_gr]

On 27/08/2020 at 18:16, inFINNity Deck said:

Hi David,

yesterday I tried your method on a RC8 of a befriended amateur astronomer. Your method is very easy and works like a charm. I added a few minor improvements to your method that makes the collimation of the primary mirror even more easy and more accurate and have written an article about it on a Dutch forum (of course with acknowledgements to you). Opening it in Chrome should translate it nicely into English: http://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

It also discusses finding the proper mirror-distance using a Neumann photographic ronchi-filter (images included) and shows the final and successful inspection using a holographic Howie Glatter laser (again image included).

Another RC8 fit for service!

Nicolàs

Hi Nicolas,

Are there any blueprints for the adapter you made for thr collmation?

[inFINNity Deck]

I fear not.

For the visual back adapter I used a 25mm section of 60mm diameter aluminium rod, the forward approximately 12mm of which I reduced to 2", taking care of a very tight fit with the existing visual back adapter (a few hundreds mm play). The central hole is 1.6mm (simply because I was out of 2.0 and 1.5mm drills 🙂). At the rear I widened that to 1.25" so I can attach all kind of stuff to it if necessary. The depth of that 1.25" cavity is the full thickness of the adapter minus 4mm. For the LED-torch I made a separate bush to give it a nice fit into the 1.25" hole. I used a piece of paper to dampen the light.

For the secondary mirror attachment I simply measured the inner diameter of the secondary mirror's screw hole, which was about 13mm and 5mm deep. So I made the rear stub of the secondary mirror adapter about 13mm and 6mm high and took care that it has a tight fit (a few hundreds mm play). The front plate is 20mm diameter and 4mm thick and the central hole is 7mm, so wider than the actual hole in the mirror holder. I made the rear stub 6mm high to ensure that I could still remove it (using my fingernails) in case the fit was too tight.

For both adapters it is of course of importance that they are made in one go to ensure all parts are concentric. The holes should not be countersunk at the viewing side to avoid observation errors. To ensure that the 1.6mm hole was in the centre I first made an indentation using a centre spotting drill bit.

For maximum contrast and even illumination I covered the visual back adapter with a white paper sticker on top of which I pasted a wide piece of clear packaging tape (to keep the paper clean in the long run), which I then sanded down to give it a non-reflective finish. The secondary mirror holder attachment I gave a non-reflecting finish by sanding it as well (piece of sandpaper on a flat surface and then moving the attachment in circles over it). I did not paste any paper on the secondary attachment.

HTH

Nicolàs

[kookoo_gr]

Thanks for your reply nicolas. I made a design for 3d printing (in various formats)for sanity check of the dimensions you posted. I think i will have some adapters turned too since colimation is giviing a headache right now and the plastic ring didn't help at all. Maybe i am doing something wrong and i will try to follow your instructions

colimation adapter obj.zip colimation adapter slt.stl colimation adapter svg.svg

[inFINNity Deck]

Main importance for an accurate result are the edges: those should be as sharp as possible. Any bevelling will affect accuracy when light is coming from one side. So even when centring is perfect, the bevelled edge will visually shift the centre away from the light source for the hole, and towards it for the outside. Is that achievable with 3D printing? If not, then the light source that illuminates the secondary mirror holder attachment should come from the direction of the camera (i.e. frontal illumination). As you may have seen the original images I posted of the secondary attachment still had a bevelled edge along the outside, but that has been taken off and I just have uploaded new images.

I have also added a section at the end of the first phase that explains how the focuser tilt-adapter can be adjusted using this same method.

Nicolàs

 

[kookoo_gr]

thanks for the comment, i will use metal and not 3d print them, they are only for sanity check of the dimensions. One idea that popped into mind. With this method we remove the focuser and collimate only the mirrors and at the end we align the focuser with the mirros in order not to have any errors the focuser may introduce. Would it be possible to use the howie glatter collimator in the place of the light source in this method and collimate the mirrors?

[inFINNity Deck]

Whether or not the Howie Glatter can be used instead is a good question to which I have no direct answer, because we did not try. It may work, but all depends on the tolerances. If there is sideways play between the HG and the visual back or focuser it should be eliminated as much as possible in a symmetrical way (i.e. at three points around its circumference). There is also a fair chance that the laser is too bright for the camera.

Nicolàs

[davies07]

On 27/08/2020 at 16:16, inFINNity Deck said:

Hi David,

yesterday I tried your method on a RC8 of a befriended amateur astronomer. Your method is very easy and works like a charm. I added a few minor improvements to your method that makes the collimation of the primary mirror even more easy and more accurate and have written an article about it on a Dutch forum (of course with acknowledgements to you). Opening it in Chrome should translate it nicely into English: http://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

It also discusses finding the proper mirror-distance using a Neumann photographic ronchi-filter (images included) and shows the final and successful inspection using a holographic Howie Glatter laser (again image included).

Another RC8 fit for service!

Nicolàs

Hello Nicolas, 

I have been away for a few days and have just picked up your post. Thank you very much for the mention and for the information. What you have made is exactly what I imagined was needed but never got around to making. i must have a go myself.

David 

[kookoo_gr]

After many days of setbacks i got back on my rc 8 i redid the collimation with a tak collimation scope for the secondary and my howie glatter for the primary and went outside for my tests. I rechecked the distance with the ronchie eyepiece and this the result i have, i believe that the distance is correct now and pic show me at 1634.93 mm with a resolution of 0.681 arcsec/px

 

 

I redid the collimation to the best of my power at the field and gave a shot to collimating my scope with the help of ccdinspector. The end result is not perfect but next time i will try to redo the collimation with the plastic card again since i didn't have any luck the last time. But i saw a big improvement at my guide stars since i use an oag and the stars are more round in the guider's FOV

 

 

 

 

[davies07]

Hi Kookoo,

I think this looks pretty good. The star field looks very nice with just some oval stars in the bottom right and left corners which are confirmed by the FWHM and eccentricity plots and might be a bit of astigmatism creeping in. The CCD Inspector plot shows a similar result with collimation 8" out in the vertical. The eccentricity and FWHM readings towards the centre are just lovely.

So what is amiss? There seems to be some tilt in the vertical axis. I'm suspecting that this might be due to some slight misalignment of the secondary in the vertical axis. since it is the secondary that governs the distribution of the optical quality over the field of view. I'm wondering, if you used the Tak alignment scope in the focuser, is the focuser itself slightly misaligned with respect to the primary mirror?

I wouldn't touch the primary at this stage. I would be tempted to tweak the secondary very, very slightly in the vertical and retest.

 

David

[inFINNity Deck]

Recently I collimated a RC8 for a friend using the methods described here. The first images showed great improvement, but still show oval stars in the corners like in Kookoo's image, all pointing diagonally in the direction of the centre. Is this normal for a RC8 or does this mean that the mirror distance is not correct yet?

Nicolàs

 

[davies07]

3 hours ago, davies07 said:

 

I wouldn't touch the primary at this stage. I would be tempted to tweak the secondary very, very slightly in the vertical and retest.

 

David

By tweak, I mean just to slightly tighten or relax the tension in the secondary screws, not to turn them. 🙂

[davies07]

2 hours ago, inFINNity Deck said:

Recently I collimated a RC8 for a friend using the methods described here. The first images showed great improvement, but still show oval stars in the corners like in Kookoo's image, all pointing diagonally in the direction of the centre. Is this normal for a RC8 or does this mean that the mirror distance is not correct yet?

Nicolàs

 

Hi Nicolàs,

Are the ovals in the corners equal? Are you using a flattener or reducer?

If the ovals favour one edge of the image, I would suspect the secondary being slightly out as I've suggested to Kookoo.

If the ovals look equally distributed and you have no flattener, then I suspect you need one and that the scope is collimated. If you are using a flattener, I would ask if the separation from the flattener rear surface to the sensor is correct. I'm using the TS flattener and this distance is quite long at 109 mm.

I also use the CCDT67 reducer (TS also sell this). If it is spaced for x .67 reduction, then in my experience that is pushing the optics somewhat. I've spaced mine for x 0.7 reduction and am seeing some slight oval shapes in the corners but I generally lose these when I crop the image.

 

David

[inFINNity Deck]

Hi David,

thanks for the explanation. My friend does not use a flattener, so that could make a difference. The first images have shown that the focal length is about 22mm more than the design value. So I suggested to give it another try, see if we can get closer to the design value without the Ronchi lines becoming curved. When we started, collimation and mirror distance were both quite far off due to a previous attempt. At least the spider diffraction patterns are razor-sharp now! 🙂

Nicolàs

 

[davies07]

1 hour ago, inFINNity Deck said:

Hi David,

thanks for the explanation. My friend does not use a flattener, so that could make a difference. The first images have shown that the focal length is about 22mm more than the design value. So I suggested to give it another try, see if we can get closer to the design value without the Ronchi lines becoming curved. When we started, collimation and mirror distance were both quite far off due to a previous attempt. At least the spider diffraction patterns are razor-sharp now! 🙂

Nicolàs

 

Well, my own focal length turned out to be 1660 mm, not 1624 mm. That is where my Ronchi lines are straight. I guess it is pot luck as to what your optics will give you. I wouldn't try to get back to 1624 mm if the scope is not correct at that setting, you'll get bigger, softer star images.

By the way I use a TS flattner

https://www.teleskop-express.de/shop/product_info.php/info/p4006_TS-Optics-RC-1-0x-Flattener-Bildfeldkorrektor-fuer-Ritchey-Chretien---2--Anschluss.html

But if you already have the 2" refractor flattener then it will do the job equally well. You will need a separation of 109 mm between the back of the flattener and the sensor.

David

Edited September 13, 2020 by davies07
add name

[kookoo_gr]

From my collimation attempts the last months i found the following issues i had not considered at all

1. In order to have my camera centered at my focuser drawtube i used a baader clicklock which is great but with a major issue i found while collimating my scope. The 2'' barrel of the clicklock has a groove for better use with the brass strip the focusers use in order not to scratch any accessories placed in the focuser and it is a few mm away for the main body. When placed in the focuser drawtube and tightened it will always be out of alignment and this will affect the focuser alignment. This happens when i completely insert the clicklock so that it is flush at the drawtube, but if i move the clicklock outside just a tiny bit so that the thumbsrcews with the brass strip align with the groove then i have flexure at the focuser and bad centering.

 

2. The same thing applies for the Tak collimator which has a huge groove at the 2'' barrel that will not allow for flush placement of the collimator on the focuser. If i align the groove with the brass strip of the focuser then the collimator is not centered and tight in the drawtube.

When using the tak collimator first i had to align the focuser with my HG laser then use the tak collimator for the secondary mirror in which i aligned the groove of the 2'' barrel with the brass strip of the drawtube. Finally i used my HG laser for primary collimation and the end result was a good collimation. This issue with the grooves was the main reason i stopped using the tak collimator since every time i used it i had different results, now i know what the issue is i will use an adapter for a flush 2'' barell and recheck the collimation using some 3d printed parts for use with a webcam and maybe i will purchase this adapter https://rafcamera.com/adapter-m90x1f-to-m36-4x1f

 

The issue with my elongated star is that the secondary needs a little adjustment which i intend to do the following days on my vacation. The end result of this discussion is that for a reliable collimation of the scope is to first align the optics without the focuser and at the end align the focuser with the secondary mirror

 

 

[davies07]

On 14/09/2020 at 15:24, kookoo_gr said:

 

 

The issue with my elongated star is that the secondary needs a little adjustment which i intend to do the following days on my vacation. The end result of this discussion is that for a reliable collimation of the scope is to first align the optics without the focuser and at the end align the focuser with the secondary mirror

 

 

Hi Kookoo.

Your findings have confirmed to me the value of the plastic disc method of collimating an RC scope. All of the methods involving lasers, the Tak scope or a Cheshire, define a reference line in space that you try to align the mirrors to. Such a reference line needs to start at a correct position and point in the correct direction. It is extremely difficult to get both of these aspects correct, with the level of accuracy of design and manufacture of the components we are dealing with. 

The plastic disc defines only a reference point which is the centre of the primary mirror. You then align this position reference with others, such as the centre of the secondary circle, by eye. 

I think you are so nearly there with collimating your scope. I think you should make a disc and use it to align the secondary. I bet you a glass of ouzo that you will find that the secondary needs just a tweak in the vertical direction.  

 

David

[kookoo_gr]

Don't worry Dave i will try your method, i have made a plastic disc and i will try it now that i am on vacation, the only problem is we will have bad weather until Saturday. Also i am making plans of turning a metal collimator like Nicolàs has at his site and will screw at the M90 ring i mentioned, so that i will eliminate any errors from loose elements and be able to use the tak collimator or the HG laser

[kookoo_gr]

I redid the collimation combining Davie's and Nicolas' methods and here are the results. First of all i had to spend over half an hour alligning the camera with the scope in order to collimate the primary mirror. Even though i collimated the primary mirror, i had to make adjustments with a star in order to fine tune it.

The secondary was also a bit tricky since i had issues aligning the camera with the hole, so i did the collimation as best as i could and hoped for the best. As for the focuser i checked and adjusted its alignment after the primary collimation was done

When i did my star tests i had bad seeing and some wind so i am not sure about the quality of the images but ccdinspector and pix show good results even though there is some small aberation at the lower right corner which might be due to bad sky conditions and/or maybe being slighlty off focus.

 

 

 

[inFINNity Deck]

Hi Kookoo,

aligning the camera with the scope is indeed far from easy when using a standard tripod, it takes me as will quite some effort. If I look at your image I can see the camera s properly aiming at the OTA, but the dot in the visual back is not properly centred. The refection of the camera lens is even worse, that has a clear offset to the upper left, I have marked that with a yellow dashed circle in a crop of your image:

The spider-arms at the lower left and upper right also clearly show an offset sideways. The other two show an offset in longitudinal direction.

Today I had another go at the RC8 I saw a while ago, trying to get the inter-mirror distance slightly better and checking the focuser-tilt. A screen-dump was taken with a light source (artificial star) in the centre of the focuser with the MoonLite focuser fully out. As you can see the lens, light source and secondary attachment are all centred (although the secondary attachment could be slightly better).

Last week I built a new alt/azi table for the alignment. The previous one was of wood and a bit wobbly. The new one is of aluminium bar (30x40mm) and aluminium strip (200x20mm) and makes aligning a fair bit easier:

Something similar is required for the camera, so that will be the next project. The box with the two knobs in front is an artificial star made by the scope's owner after my design.

Nicolàs

 

[kookoo_gr]

Could those issues you mentioned be caused from not having the camera properly aligned? I believe i took this image before the primary collimation

Edited October 4, 2020 by kookoo_gr