Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Help please! Collimating an RC with a Howie Glatter


Recommended Posts

And I though collimating a Skywatcher 200P was complicated !

I often think that at this level of imaging these scopes are like highly tuned supercars, where the tolerances are so tight and small that unless you spend ages setting them up with specialist equipment you just don't quite squeeze that last bit of horsepower or performance from them.   David had one (he'll kill me for saying this) "tool" to hand that others are sadly lacking, namely Es...  His experience in optics, and his test methods are second to none, as can be shown in this thread when he re-established the focal length as contributing to David's issue.  

Guys I hope you manage to resolve these issues and collimate these precision scopes.

Edited by malc-c
Link to post
Share on other sites
  • Replies 148
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Popular Posts

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 th

Thanks, Graham, I enjoyed reading your post. I thought, like you, I should explain some background.  After I had owned my RC8 for 12 months, I became convinced that something was not right with m

Thanks a lot Vlaiv, that makes a lot of sense! From what I see happening I'd say that the black ones are indeed simply locking things in place (i.e. turning them doesn't really have an impact unl

Posted Images

23 hours ago, lukebl said:

As the originator of this thread, I've only just caught up with the replies and many thanks for the responses.

After reading all this, all I can say is that I doubt anyone will EVER, EVER buy a Ritchey Chretien EVER AGAIN!

Hi Luke,

I was thinking, today, that your topic has been hijacked somewhat into a general discussion of RC scopes. I hope you don't mind that too much. We are not, after all, addressing your original question.

I agree with Andy, once you get the RC collimated properly, it will capture lovely images and the collimation should be held well without further adjustment. Coming back to your original laser questions: I think that if you put a Howie Glatter laser on a well collimated GSO scope of the type we're talkng about, the likelyhood is that the laser will say the scope is out of collimation, due to the (unknown) pointing error of the focuser relative to the primary mirror.

I moved to the 8-inch RC from a 10-inch Skywatcher Quattro and whilst the Quattro was super fast and I got lovely images from it, it was a tricky one to collimate and because of general flexure of the beast, it wouldn't hold collimation to my satisfaction. And I hated the messy star halo due to the various defraction elements in the light path.

I think the RC gives me star colours which are pretty close to truth. They seem cooler than from my refractor (APM 107 Apo) but closer to truth, I think. The symetrical optical path gives lovely round stars once the collimation is sorted. So I suggest you have a read and see what you can do to improve your scope.

Best, David

Edited by davies07
Typos (many)
Link to post
Share on other sites

Hi Luke (and Malcolm!)

I think it would be a shame if the take home message of this thread were "Collimation of RC is a PITA, better forget about it". As a complete newbie, I have actually tremendously benefitted from reading a few pointers here (@david thanks a lot!) and am coming out with the feeling that it's actually easier than I would have thought before. I sat down twice to try to improve things, and each time I was understanding things better and the results have been improving enough that I wont be trying to adjust it anymore.

If this thread motivated me to revisit collimation I hope it will get you to do the same!

 

845140530_collimationsteps.thumb.png.736e4ba42327e6258bdaf7b9d25266d1.png

 

  • Like 1
Link to post
Share on other sites
27 minutes ago, B4silio said:

Hi Luke (and Malcolm!)

I think it would be a shame if the take home message of this thread were "Collimation of RC is a PITA, better forget about it". As a complete newbie, 

 

 

 

 

Hi,

That wasn't my intention.... But reading the OP's posts and reading about the list of various (and expensive) collimation tools that David used (and still wasn't satisfied with the performance he was getting until Es did the equivalent of putting the scope on a dyno rig) compared to who most of us collimate our bog standard Newts  would  possibly be beyond most people's patience.  Hence the reference to super cars, and like super cars, when it's tuned right the performance is stunning judging by the professional (and award winning) quality of David's astro images.

Link to post
Share on other sites
13 minutes ago, malc-c said:

Hi,

That wasn't my intention.... But reading the OP's posts and reading about the list of various (and expensive) collimation tools that David used (and still wasn't satisfied with the performance he was getting until Es did the equivalent of putting the scope on a dyno rig) compared to who most of us collimate our bog standard Newts  would  possibly be beyond most people's patience.  Hence the reference to super cars, and like super cars, when it's tuned right the performance is stunning judging by the professional (and award winning) quality of David's astro images.

Hehe I totally understand and I think we're indeed speaking at incredibly different levels of expertise. I think that what you say is even more true for the "cheap" RCs compared to similarly priced scopes, where with enough time spent on them they can outperform the quality of refractors or newts, but at the risk of not getting to that point before abandoning ship. What David showed us is indeed how you get another step forward, but I also think that at that level there are other questions that come into play as well (from guiding, to processing and everything in between).

Link to post
Share on other sites
11 hours ago, B4silio said:

Hi Luke (and Malcolm!)

I think it would be a shame if the take home message of this thread were "Collimation of RC is a PITA, better forget about it". As a complete newbie, I have actually tremendously benefitted from reading a few pointers here (@david thanks a lot!) and am coming out with the feeling that it's actually easier than I would have thought before. I sat down twice to try to improve things, and each time I was understanding things better and the results have been improving enough that I wont be trying to adjust it anymore.

If this thread motivated me to revisit collimation I hope it will get you to do the same!

 

845140530_collimationsteps.thumb.png.736e4ba42327e6258bdaf7b9d25266d1.png

 

I think this is an excellent result. Well done.

Not only have you now centred the star but. looking at your earlier images, the contrast in the nebula has improved.

David

 

 

 

 

DSCF2985_coll_screws_inside_web.jpg

Lock screw.jpg

Mirror_removed.jpg

Edited by davies07
Removed mistaken images
Link to post
Share on other sites

Sorry, guys, I made a complete cods of that last post. I drafted a post last night to add a bit more info and lost all the text due to finger trouble but the images have remained and attached themselves to my next post - the one you see now. 

Link to post
Share on other sites

I was going to follow up on the topic of collimation screws - thus the photos, above. By the way, if anyone is wondering, I record everything I do with photos and notes. That is why I have photos of what I discovered when I dismantled the scope - and I've now done that several times.

Yes, it was my misnomer to call the screws push-pull. the 3mm silver-headed screw is the collimation screw. It is tensioned with a spring - see the first picture in the post above. The white circular pad is the impact surface for the 2.5mm, black locking screw. Note that the collimation screws bear the weight of your focuser and imaging train. So I keep my collimation screws well screwed in to minimise flex. You can see the collimation screw protruding through the backplate in the third picture.

The second picture shows the standard lock screw. This is a cup-ended screw that seems to be a standard fit for all Chinese orgin adjustment screws I've come across. They are wonderful for digging into and marring the metal (usually aluminium) that they bear on to. I've replaced mine with dome ended screws. The effect is to eliminate the indeterminate stop that you feel when tightening the lock screw. That softness is the screw digging into the metalwork.   

Edited by davies07
correction
  • Like 1
Link to post
Share on other sites

Thank you David!

I just had one of those epiphany moments where you tell yourself "So THAT's how that works!". Very clear images and it totally makes sense! Also indeed, it seems like they've made those screws with the exact purpose of digging in through the metal (the black ring next to the spring in your first image seems to be a witness to that :D). So basically the "lock" screws are mainly staying flush with where the silver screw are so that they dont move back, and in the other direction the spring is making it keep its distance.

If I might think aloud here, that means that if you simply push it will act on the springs but still move a bit on the inside. So when you attach your imaging train, it will always push a bit on the bottom springs and lighten the load on the ones above. For the ones above the lock screw should keep the mirror from tilting, but there's nothing (besides the springs) keeping the mirror from getting pushed in at the bottom. So that's why it's better to have them "as screwed in as possible" while still retaining collimation. (Today I Learned!)

Thanks for the images and the explanations!

604461657_screwsgettingsquezed.thumb.png.e664df572e898afaee8480ab06e43516.png

 

  • Like 1
Link to post
Share on other sites
  • 2 weeks later...

Re-reading all the responses to my thread, and looking closely again at my particular scope, I've come to the conclusion that it's simply not possible to collimate it properly for one simple reason: The focuser is attached to the primary and moves with it.

OK, you can align the focuser with the secondary, and then collimate the secondary. But as soon as you try and collimate the primary, the focuser moves with it and becomes misaligned with the secondary. Having a tilt adjuster doesn't help, as it's fixed to the same axis and moves with it. If you then try and realign the focuser with the secondary, then that puts the primary out again.

 

Link to post
Share on other sites
28 minutes ago, lukebl said:

Re-reading all the responses to my thread, and looking closely again at my particular scope, I've come to the conclusion that it's simply not possible to collimate it properly for one simple reason: The focuser is attached to the primary and moves with it.

OK, you can align the focuser with the secondary, and then collimate the secondary. But as soon as you try and collimate the primary, the focuser moves with it and becomes misaligned with the secondary. Having a tilt adjuster doesn't help, as it's fixed to the same axis and moves with it. If you then try and realign the focuser with the secondary, then that puts the primary out again.

 

Yes it is possible to collimate your scope 🙂

@Captain Magenta posted this correct procedure earlier and it worked perfectly on my 6” CC......

I took 3 steps:

1. lined up the focuser axis to the primary axis, a la kitchen table collimation,

2. pointed the primary directly at the centre of the secondary (I also used a Glatter)

3. adjusted the secondary to "symmetricize" the reflection of the laser onto the primary, in my case getting the reflected dot back onto the laser source

Edited by dweller25
  • Like 2
Link to post
Share on other sites
5 hours ago, dweller25 said:

Yes it is possible to collimate your scope 🙂

@Captain Magenta posted this correct procedure earlier and it worked perfectly on my 6” CC......

I took 3 steps:...

Many thanks @dweller25 and @Captain MagentaI think I may have resolved the issue by a circuitous route to the same process using all available tools. Here it is simplified:

  • 1. Use Cheshire to align the focuser with the secondary donut by adjusting the focus tilt adjuster. (I use the Cheshire, as I can't see the laser spot on the secondary. This wouldn't be a problem with larger open-tube RCs, where you can directly see the surface of the secondary).
  • 2. Use the Howie Glatter with concentric circle attachment to align the secondary with the primary, so that the laser circles are central and evenly spaced across the primary.
  • 3. Adjust the primary so that the 'tunnel of mirrors' viewed when looking down the OTA is equal when viewing from the left, right, top and bottom (see pic below). Even a slight misalignment will be clearly visible as the 'tunnel' curves upwards, downwards, leftwards or rightwards if misaligned.
  • 4. Repeat stage 1.  Finished.

49794246703_8c67b069c1_b.jpg

I may be completely wrong, but it looks good so far.

 

Edited by lukebl
  • Like 1
Link to post
Share on other sites

That sounds good. The trick I think with systems where both mirrors are adjustable is to try to work out some way of aligning each mirror independently, which you now seem to have done with steps 1 and 2.

What I’ve found is that these indoor methods are a precursor (they get you into the ballpark) to the extremely more sensitive test of an actual star in good seeing, from where you can converge on perfection.

I do have one question: the step 1, can you not see where the laser strikes the secondary by looking at its 1st reflection In the primary? That’s what I do.

Hopefully you’re almost there...

M

 

Link to post
Share on other sites
19 hours ago, Captain Magenta said:

....these indoor methods are a precursor (they get you into the ballpark) to the extremely more sensitive test of an actual star in good seeing, from where you can converge on perfection.

I do have one question: the step 1, can you not see where the laser strikes the secondary by looking at its 1st reflection In the primary? That’s what I do.....

No, unfortunately I can't see the secondary donut when looking down the tube for some reason. It's too dark down there!

On the point regarding a subsequent star test, surely that would simply tell you whether or not you'd collimated it properly, but wouldn't help you regarding the cause? If it proved that the collimation was still out, you wouldn't be able to tell whether it was the focuser, secondary or the primary which was at fault and you're back to square one.

Link to post
Share on other sites
15 hours ago, lukebl said:

On the point regarding a subsequent star test, surely that would simply tell you whether or not you'd collimated it properly, but wouldn't help you regarding the cause? If it proved that the collimation was still out, you wouldn't be able to tell whether it was the focuser, secondary or the primary which was at fault and you're back to square one.

That's true, but you're not back to square one, after the indoor collimation you'll be much closer than square one. Even after all the indoor collimation in the world it's still going to be slightly out, and from there you use the much more sensitive star test to make the next stage of smaller adjustments. And yes you're right, you won't know which part is contributing most to the remaining misalignment. But you should be close enough that whatever you adjust, you can improve it to satisfaction. I'd probably avoid adjusting the focuser, leaving you with the choices of star-test-adjusting either the primary or the secondary, or both.

Depending on your scope type, often that choice is made for you.

If you have an SCT, you can only easily adjust the secondary "in the field". If a SW-style Mak, it's just the primary. If a Rumak-Mak (like mine) or RC, it's either or both, and here you must choose.

Whatever you choose, though, you should be able to improve it from its indoor state.

In my case, I "set" the primary (indoors) by removing the secondary and aiming the laser at the exact centre of the resulting opening. Quite an exercise. I then, much more conveniently, adjusted the secondary (having re-installed it) to reflect the laser back to its source. On subsequent star-testing, I found my diffraction-rings slightly squashed. I decided to adjust only the secondary, and got the rings symmetrical. And the view through the scope was really nice, much better than before, so I was reasonably happy.

However, back inside, I noticed that the secondary was no longer pointing back to the source, and yet the star-test was good. What was going on? The laser is a Glatter which I've tested over 25 metres in my garden. Obviously my tweak of the secondary was a final compromise for the errors remaining in the primary, and elsewhere. My next step will probably be to restore the secondary to its "pointing back to itself" state, and star-test-adjust again this time adjusting the primary only.

Anyway, the point is that, for any telescope, all components will necessarily be "out" to a lesser or greater degree. Your final configuration after a star test will be a compromise with the residual errors in the system more or less compensating for each other.

Hopefully, as you go through this process, you will inevitably be thinking about it and gaining collimation experience, and will then think "what if I try this". As you can see from the various contributors to this thread, there are many ways to skin the cat test and diagnose what's going on.

Sorry this reply a bit longer than intended,

Cheers, Magnus.

PS I use a torch pointed down the tube to look at the secondary's reflection and centre-mark

PPS I have read that RC secondary centre-marks are often not that accurately placed. My Rumak-Mak doesn't even have one!

Link to post
Share on other sites
  • 1 month later...

I am having some trouble the past months in collimating my rc8 and i found this topic and i have some questions about davies07'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 davies07 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

Link to post
Share on other sites

I sold my RC scope years ago now but at the time I did some videos on this and put them on YouTube.  Search for Ritchey Chretien collimation authored by me (Stephen Kirk).  I got into some arguments on Cloudy nights about this method that I withdrew from since I could not be bothered to keep arguing.  It is reliable and works well if you follow it to the letter.  Indeed, all I did was video record a method taught to me by some German astronomers who showed me.  That said, you can achieve perfect collimation with just a Cheshire EP as I also show in those videos. 

I could deliberately mess up my collimation completely and then get it back in a few minutes.  People get scared but all you are doing is adjusting the angles of a pair of mirrors.....  it's not a nuclear bomb...!  YOu are not going to damage anything.

You're getting that effect because your primary mirror is not orthogonal to the secondary and so you need to adjust the primary so that the concentric rings that are projected onto the wall are *complete and equidistant circles* from the central shadow.  Assuming that is that the focuser is orthogonal and the secondary is then adjusted.

 

Link to post
Share on other sites

Thanks for the reply, your videos were the ones that made me invest into a howie glatter laser. But the last year the collimation always seems a bit off, i have no idea what might be wrong since the laser method shows me no variation in my checks and i have no idea what might be wrong, so i am looking into different methods.

Link to post
Share on other sites
59 minutes ago, kookoo_gr said:

Thanks for the reply, your videos were the ones that made me invest into a howie glatter laser. But the last year the collimation always seems a bit off, i have no idea what might be wrong since the laser method shows me no variation in my checks and i have no idea what might be wrong, so i am looking into different methods.

I was asked by a guest to help with collimating one of these scopes as well. We looked at Steve's videos, amongst others, and found them sincere - but they did not solve our problems because there was clearly another variable in play. What it was I do not know, but it defeated all our efforts.

Olly

Link to post
Share on other sites
1 hour ago, ollypenrice said:

I was asked by a guest to help with collimating one of these scopes as well. We looked at Steve's videos, amongst others, and found them sincere - but they did not solve our problems because there was clearly another variable in play. What it was I do not know, but it defeated all our efforts.

Olly

When it doesn't work out it is because there are issues with the secondary mirror and/or the focuser is not square with the optical axis.  These scopes are built to a budget.  You can get a diamond or you can get a dog.

Link to post
Share on other sites
2 hours ago, kookoo_gr said:

Thanks for the reply, your videos were the ones that made me invest into a howie glatter laser. But the last year the collimation always seems a bit off, i have no idea what might be wrong since the laser method shows me no variation in my checks and i have no idea what might be wrong, so i am looking into different methods.

You must be sure the focuser is absolutely orthogonal to the optical axis or else the method does not work and the rings can never be centred properly.   Since I did those videos several others have replicated the videos and show the same method.   Professional RC scopes are collimated in the same way that I illustrate in the videos.

Link to post
Share on other sites
4 hours ago, kookoo_gr said:

I am having some trouble the past months in collimating my rc8 and i found this topic and i have some questions about davies07'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 davies07 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

Hi,

Should you align the focuser? Good question. Your focuser has a facility for alignment, so I think you should check where it is pointing. Assuming that you have aligned the primary and secondary mirrors using my 'disc with a hole' method, then your primary and secondary mirrors should be aligned. I would now fit the extension tubes and focuser and fit a laser to enable you to adjust the focuser to point at the centre of the secondary mirror. Don't touch the primary or secondary collimation screws. 

However, before that I think you should look at your focal length. Your focal length of 1607mm is close to what mine was (1604mm) when I first started my investigations. At that focal length my scope was very 'over-corrected' - the mirrors were too far apart. Es Reid checked my scope with a Ronchi grating but over the past month I've been helping someone collimate his scope including checking the focal length. You'll need a Ronchi eyepiece; Gerd Neumann makes one. Focus on a bright star and rack the focus in and out; more and more lines appear as you move away from focus. My guess is that as you go inside focus, you will see this pattern:

Ronchi_barrel.png.12c5ac2b4075c793c4cda62c2e9360c0.png

A barrel-shaped Ronchi pattern inside focus = overcorrected scope, too short focal length, mirrors too far apart.

Outside focus, I think you will see this:

 Ronchi_pinched.png.15ce6d17018a609c27f00af1e9a40e66.png

a pinched pattern outside focus, again overcorrected.

As a first guess, let us assume your scope will be properly corrected at the specified focal length of 1624mm. So you are 17mm too short. Look at the chart I gave plotting focal length vs. number of turns of the centre screw of the secondary (page 1 of this thread), you'll see that about 3/4 of a turn of the centre screw should bring you to 1624mm. So unscrew each of the three secondary collimation screws one turn, and then unscrew the centre screw 3/4 turn. This pushes the secondary mirror towards the primary by about 3/4mm.  Now tighten the three collimation screws by an equal amount and check on the star again with the Ronchi eyepiece. The lines should be parallel inside and outside focus. If so, your scope is working fully corrected. If you do not get parallel lines, you'll need to keep adjusting.

The Gerd Neumann eyepiece come with a manual which you can download from the Teleskop Service website.

Note that if the patterns are the other way around - pinched pattern inside focus and barrel pattern outside - you've gone too far and the scope is now under-corrected and the mirrors are too close together. 

(I had to move my secondary by four full turns of the centre screw and adjust the primary mirror up the tube until I got a corrected image. My corrected focal length is 1660mm. I'm not far off not being able to focus without another extension tube.)

I use Pixinsight to tell me the focal length of the scope. Take a single image of a star field, say an open cluster, and use Script/Image analysis/Image solver.

Once you are at the correct focal length you will need to recollimated the scope, hopefully by only adjusting the secondary.

David

 

Link to post
Share on other sites
On 19/03/2020 at 16:50, lukebl said:

Can someone help me here?

Now, I know there are endless threads on the web about collimation, but I can't get my head around this problem. Please bear with me while I explain.

I am now the proud owner of a Howie Glatter collimator, complete with the Concentric Circle attachment. It's a gorgeous piece of equipment. Shiny, solid, heavy and beautiful. One of the best, I'm led to believe. I thought that it would be the solution to collimating my 200mm Ritchey-Chretian.

So, I have gone through the two main stages of collimating it:
1: Align the focuser with the secondary mirror, so that the red dot aligns with the little secondary centre circle
2: Adjust the secondary so that the circular pattern projects evenly over the primary mirror.

I've done these stages and it all looks good as shown below. Nice concentric circles on the primary and when projected onto the wall. So I shouldn't need to collimate the primary.

49676611956_8bf8df5c4d_b.jpg

HOWEVER, when I set it up and aim on a star, it doesn't look right and the out of focus image looks like the view on the left in this image (this isn't my image, but it shows the general effect). It's not camera sag, as I've tried it in all positions.

So I then re-adjust the secondary collimation so that the out-of-focus star looks like the version on the right.

49676934332_36494e3ed3_z.jpg

Although the focus and collimation look good by eye, when I re-insert the Howie Glatter I get the projection show below. Which suggest that the collimation is out, when my eye says that it's not.

49676833192_d03cd6b331_c.jpg

So what is going on here? Is the Howie Glatter faulty? I thought they were the best. Or what else is going on?

You've missed a stage or two...

1 align the focuser to point directly at the secondary (use the pencil beam for this)

2 align the secondary so the pencil beam is reflected directly back at the focuser. (There will be a set of diffraction rings seen around the Howie which help with this alignment)

3 change the filter to the concentric ring pattern. It should produce a set of reflected rings on the primary mirror.

4 make sure your OTA is level and as orthogonal to a screen/wall as possible. You will see the ring pattern projected on the screen/wall

5 adjust the primary mirror so the shadow of the secondary housing is concentric with the innermost visible bright ring.

6 star test and tweak the primary only to produce a circular doughnut shape of a slightly defocused star (Polaris not the best as it's a double).

Once you've aligned the focuser and secondary to each other leave them alone.  All further adjustments are on the primary mirror only.

The Howie filters should be positioned approximately at the focal plane of scope when set to infinity for the exit ring pattern to be parallel.

Whilst the focuser/secondary separation isn't critical for their alignment (steps 1&2), their separation is critical for regarding the design focal length of the scope.

Edited by fwm891
text added
Link to post
Share on other sites
20 minutes ago, davies07 said:

Hi,

Should you align the focuser? Good question. Your focuser has a facility for alignment, so I think you should check where it is pointing. Assuming that you have aligned the primary and secondary mirrors using my 'disc with a hole' method, then your primary and secondary mirrors should be aligned. I would now fit the extension tubes and focuser and fit a laser to enable you to adjust the focuser to point at the centre of the secondary mirror. Don't touch the primary or secondary collimation screws. 

However, before that I think you should look at your focal length. Your focal length of 1607mm is close to what mine was (1604mm) when I first started my investigations. At that focal length my scope was very 'over-corrected' - the mirrors were too far apart. Es Reid checked my scope with a Ronchi grating but over the past month I've been helping someone collimate his scope including checking the focal length. You'll need a Ronchi eyepiece; Gerd Neumann makes one. Focus on a bright star and rack the focus in and out; more and more lines appear as you move away from focus. My guess is that as you go inside focus, you will see this pattern:

Ronchi_barrel.png.12c5ac2b4075c793c4cda62c2e9360c0.png

A barrel-shaped Ronchi pattern inside focus = overcorrected scope, too short focal length, mirrors too far apart.

Outside focus, I think you will see this:

 Ronchi_pinched.png.15ce6d17018a609c27f00af1e9a40e66.png

a pinched pattern outside focus, again overcorrected.

As a first guess, let us assume your scope will be properly corrected at the specified focal length of 1624mm. So you are 17mm too short. Look at the chart I gave plotting focal length vs. number of turns of the centre screw of the secondary (page 1 of this thread), you'll see that about 3/4 of a turn of the centre screw should bring you to 1624mm. So unscrew each of the three secondary collimation screws one turn, and then unscrew the centre screw 3/4 turn. This pushes the secondary mirror towards the primary by about 3/4mm.  Now tighten the three collimation screws by an equal amount and check on the star again with the Ronchi eyepiece. The lines should be parallel inside and outside focus. If so, your scope is working fully corrected. If you do not get parallel lines, you'll need to keep adjusting.

The Gerd Neumann eyepiece come with a manual which you can download from the Teleskop Service website.

Note that if the patterns are the other way around - pinched pattern inside focus and barrel pattern outside - you've gone too far and the scope is now under-corrected and the mirrors are too close together. 

(I had to move my secondary by four full turns of the centre screw and adjust the primary mirror up the tube until I got a corrected image. My corrected focal length is 1660mm. I'm not far off not being able to focus without another extension tube.)

I use Pixinsight to tell me the focal length of the scope. Take a single image of a star field, say an open cluster, and use Script/Image analysis/Image solver.

Once you are at the correct focal length you will need to recollimated the scope, hopefully by only adjusting the secondary.

David

 

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

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.