Altair Astro 6" RC collimation: help needed :)

[rofus]

Hi all,

I finally got my hands on my new Altair Astro RC 6" and my new  full frame Nikon D750!

I updated all my software, tested all connections (BackyardNikon, EQMOD, Astrotortilla, CdC etc.), all working and nice

So I got a first test image to check collimation...and what I feared happened, that is I need to collimate the scope! This first 30 secs image of M13 shows obvious stars problems:

So I decided to do a star test (defocused star Arcturus as Vega was covered at first), and this was the result:

And I said ok...not really collimated, I need to adjust the secondary, and using my camera's liveview maximum power (out and in focus were out the same), I started trying to guess/adjust things a bit, and after it seemed to be better (small movments and recentering the star every time), I took another pic of same M13:

It looked definitely better down right, but worse elsewhere...overall an improvement, center at least is good, but not so much, so I went on tweaking more on the star test but moving on Vega (Arcturus was now too low and covered by some clouds).

After some adjusting on secondary mirror, and with clouds now gathering, I took another example shot (all 30sec iso 6400 to have consistency):

And it looked again that I'm 'moving around' the problem, but never getting it properly right...so I decided to take another pic of the star test on Vega. Overexposed as I forgot to reduce exposition, but in the 'reflections' the secondary is still visibly de-centered, maybe slightly less, maybe in a different direction...I'm not sure:

So before clouds completely covered the sky, I tweaked it a bit more and took this final test shot:

Now the question is....I have a feeling I'm shooting in the dark, center is pretty much ok but I cannot seem to get decent collimation results, considering I want a quite good one. All I have is a normal collimation laser (the red one I cannot afford the more expensive green one), that seems decently collimated, and I got it today to test it. And then when sky is clear I get a star test.

I'm looking for some help for a down to earth approach working with these simple tools? I've read a lot but there are many different ways and opinions, too much talking about theories and useless stuff, while I wonder if someone with a similar scope have found a simple way to collimate, meaning I can spend all the time and patience but cannot afford too expensive tools at the moment.

Anyone with experience can tell me what I'm doing wrong based on these pics and how to improve my procedure to collimate? Possibly also during the day considering that clear nights are rare so I'd rather do it during the day and just verify it later

As I said I know there are various degrees of perfection, I'm not looking to get it perfect right away but at least to know how to achieve the best I can, how am I doing now, how to do it better etc.

Thanks!!

[johnrt]

You can get collimation good enough to image with a simple Cheshire collimation eye piece http://www.firstlightoptics.com/other-collimation-tools/cheshire-collimating-eyepiece.html

I use one on my 6"RC, forget the laser, and just use the Cheshire. They are pretty straightforward to use and cheap too!

[rofus]

Thanks John!

As I can see the Cheshire basically allows me to tweak the secondary mirror without a star test (that I can use just to verify)? Do you have any guide I can follow?

I read everywhere I will probably not need to touch the primary (that I hope!), I also brought the scope to Altair guys few days ago and they checked that is ok and primary is aligned and not loose.

Any help also based on those images I posted (is that bad collimation?) is very appreciated, ordering a Cheshire now!

[johnrt]

Collimation of an RC is a 2 stage process with a Cheshire.

First look through the Cheshire down the OTA tube, you will see lots of rings of differing brightness, they should all be totally concentric, if they are not then the primary mirror is out and will need to be adjusted with the set of screws at the back of the OTA. Once you have all the circles in concentric bands, you will see a small black dot somewhere near the middle of the rings through the Cheshire. This needs to be adjusted to be in the very center of the inner most circle. To do this you adjust the secondary mirror.

Quite a simple process once you know what you are looking for.

Have a read of page 4 in this manual https://www.astronomics.com/documents/astro%20tech/astro-tech%20at8rc%20manual.pdf

[rofus]

Thank you John, you're a boss!

A simple explanation I needed, I ordered the Cheshire eyepiece you linked and it will arrive tomorrow, so tomorrow I'll try doing what you said and what is written as well on that document you linked.

I only have ONE doubt: the document you link says I need first to check secondary mirror, then primary one (with the baffles), but you said the opposite.... what should I check first?

I'm somehow scared of touching primary mirror collimation (not sure I understand how those two screws work), so I hope I'll not have to, but at least it seems I now know how to check it and how to make a rough collimation and check it later with a star test

I'll report back here as soon as I have my first go with it, and if successful I'll post some test pics similar to my original post so others will see differences/results. If I can sort it out maybe it will be useful for others to be less scared by RC scopes!

[rofus]

Ok...

I got my Cheshire and I spent a good few hours tweaking with it. Initially the secondary was out of the central dot, a quick turn and it was back in. Looking around it seemed to me all about right, did not touch the primary as I don't see the need...this is what I see in the Cheshire eyepiece (as you know is hard to take a pic so I tried with these two to illustrate what I see):

I bring my scope, take a bright star (Artcturus in this case) for a quick star test...defocused OUT (left) and defocused IN (right)...and I can see still that the donut is not centered...the black part in the mirror is still on one side. So the question here is that on the internet there are tons of different guides..someone says on star test to adjust the secondary (but in Cheshire it looks perfect), and someone else to adjust on star test the primary:

I did nothing and decided to quickly turn the scope (roughly aligned as I have signs on the floor for my tripod) to Polaris for a quick 30 secs test without any tracking or guiding (on Polaris trails will be minimal) to see what's situation of collimation:

Considering I quickly focused with Bahtinov mask, it seems anyway clear to me that situation is quite bad on the left in particular...the rest with tracking/guiding and better focusing seems ok.

Question is....am I right? What should I adjust to fix this problem? Is the primary? Secondary? I don't know how to proceed....the Cheshire looks great, but the scope is still unusable...

[rofus]

I got another look in the Cheshire, and tried tooking a better pic to show here. Is it me or the circles around the center dot (the baffles if I understood correctly) are perfectly concentric?

Precisely the outer black disc of the inner portion of discs (the baffles) is not perfectly centered? What about the rest and why my star test shows that not perfect donut? Is it something I can adjust with the Cheshire and primary mirror?

I cannot understand if and how much off is this collimation...the result of my test pic of the post above this is quite bad and make the scope unusable...and I seem unable to understand what is wrong with this collimation

[johnrt]

Looks like the primary mirror is out to me, difficult to tell from the internal images of the OTA, but compare the crescent moon stars to this image I took with my RC when I had poor alignment of the primary mirror.

[rofus]

Looks like the primary mirror is out to me, difficult to tell from the internal images of the OTA, but compare the crescent moon stars to this image I took with my RC when I had poor alignment of the primary mirror.

Hi John,

Indeed your image has some similarities. So I suppose you think is primary from my test image and not from the pics in tube (I agree is difficult to judge from those).

My question is: how do I adjust primary?

1) on the star test (on Light Vortex with a scope like mine is what I read: collimate primary, not secondary, on the star test)

2) with the Cheshire? If so someone says I should do it seeing the concentric circles in rhe center (baffles), someone else that I should do it seeing the primary (where I see my wall) external border to the tube black outer border. What is the good way?

I'm just scared of doing something wrong on primary and make it all much worse

[rofus]

Another important note: on my test picture of Polaris, my camera is orientated exactly horizontal and level, so the aberration I see on stars on the left of the field is not due to 'heaviness' of my camera distorting the focuser axis.

I use one of the extensions before the focuser (the 3" one), then focuser and my camera attached with 2" adapter. Camera weighs about 750 grams so is quite light, with one single tiny usb cable not hanging.

So it seems indeed to me that is a collimation problem. 

[rofus]

Someone is going to borrow me a Glatter with holographic concentric circles extension...so I'll try again and more detailed Cheshire collimation and then later if possible a star test and centering the donut moving just secondary.

And tomorrow let's see if Glatter can make it better or solve the problem.

I just want to say that customer support from Altair Astro till now lives up to their name...absolutely fantastic, patient, detailed and useful support, both in person and by email, explaining me things way over what they 'should' do as a shop.

I live in Norwich just nearby them, makes me proud and happy I got it from them and makes me even more motivated to get a grip on it and use it without being put off by collimation initial struggles

Kudos to Ian and Nick at Altair Astro!

[rofus]

A short update....SUCCESS!

I actually patiently waited for clouds to clear a bit, and followed suggestion of Nick at Altair telling me to keep things simple and just adjust the secondary on a star. Knowing I'll have soon a Glatter I told myself...why not, Altair Astro must be right about their 'own' scopes

Also because he explained that secondary donut cannot be used to precisely set the centre of the mirror, that often is not really that precise and that is only a reference.

So my previously not centred donut on a star test was centred, with a few movements following tips here about where to tighten (http://deepspaceplace.com/gso8rccollimate.php). Below my outward-focus donut

and the inward-focus one:

Pretty much decent to my eyes, very satisfied of the result I so refocused with my Bahtinov and took a 30sec unguided of Vega (where I did also the star test after doing it also on Arcturus to be sure they are in different positions):

The pic I took makes me very happy. There's obvious coma at the edges and borders, BUT I'm using a FULL FRAME Nikon d750 (24 megapixels) so I'm expecting this kind of field distortion and vignetting around the edges, and I'm not using ANY FOCAL REDUCER, so it's really in a way the worst case for the small 6 RC...but I have to say it's just fantastic, with DSLR I always had this problem also on widefield refractor, I'm fine with it because with 24 megapixes I can crop borders without problems

So....I decided to test as well my guiding (Starshoot Miniguider on EQ5 all with PHD/EQMod/Astrotortilla/Backyard Nikon), and I took this 5 minutes guided of M57 (even though forgot to center it with Astrotortilla...too excited to test it!):

Yes was not the ideal night, clouds and what not, but my guiding works perfectly and the focal length of the 6 RC works beautifully with my mount and guiding

Because tomorrow I should put my hands on a Glatter laser, I decided to verify again with a Cheshire what I did with star test:

And with my surprises, like others, I found that on Cheshire not only I'm not precisely at the center of the donut (like Altair Astro told me is normal), but now I also see primary not really centered.

Tomorrow I'll try with the Glatter....but anyone nows how is possible that imaging I get that good field and in a Cheshire the primary seem to be so off? Any suggestion/throught?

All in all a good positive night progressing towards getting a grip over collimation process for me....and fell in love with this little fantastic scope, I can see amazing pics coming from it

And now let's see what the Glatter will tell me and maybe will make it even better!

[rofus]

Just a quick note about what I wrote, that could be confusing, too bad my English

Actually I called 'donut' the black HOLE in the out of focus star, that's actually a hole and not a donut (it's the shadow of the secondary mirror in front of the path light to the primary). So when I say centring the 'donut' in the star test image (like I posted), I meant that shadow. Another useful tip I discovered from AA guys about my star test: in my latest one you can see there's a hint of a white spot in the centre of the black hole. Apparently that diffraction becomes more visible as you get near perfect collimation...I will make it better, but I'm happy in one night I went from an out of collimation scope to a near perfect collimated one

Last but not least, last pics showing the Cheshire check after collimating with star test, there I refer to the 'donut' correctly, where I mean the sign on secondary mirror 'centre'. As Nick told me correctly, that donut on the secondary is good for a first general collimation, but is not really useable for precise collimation, in particular when like me I have a huge/sensitive full frame DSLR/sensor and no focal reducer/ff. Indeed two times I collimated on star correctly, and Cheshire was out, so clearly not enough for precise visual imaging (visually with eyepiece the Cheshire is enough to my eyes).

Later I'll give a try to the Glatter...getting my grip on this collimation, indeed looks haunting at start but then is not...and this little scope is awesome. If I think it costs less than my StarTravel150 and it has so so much more quality to it, it's basically a steal. And at 1370mm still good with my Orion Miniguider Finderscope and light enough to be comfortable on my EQ5 at all weird position (big advantage of short tube).

Below is a crop 1:1 of the M57 test pic of 5 mins, unprocessed, I took last night before big clouds gathered again...I'm VERY satisfied considering is just a test and the demanding imaging chip I have.

I can see it can be even better with more precise secondary and maybe small tilt on primary, laser and CCDinspector analysis will tell me more

My sincere gratitude goes again to Altair Astro guys (Ian and Nick), tons of emails and support at store to help me getting over frustration for collimation...and indeed trusting their suggestion I got it spot on ignoring all the rest of the things on the net that risk to confuse at start 

[rofus]

So a final post to summarise it all up, because I received quite a few messages and it seems many would like to buy one of these RC but everyone is (rightly) scared about the collimation. What everyone agrees with (and me too), is that there is NO WAY to properly collimate your scope if not with a star test: actually it might well be the ONLY thing you need

My personal experience is with a 6RC (so quite small) and no focus tilter and no field flattener, and a full frame DSLR, so is a quite demanding and delicate imaging system (very high resolution, very large chip).

The useable FOV with this configuration is about 70%, so I can ignore the 30% borders actually.

What I learnt in the process is:

1) Cheshire is just useless unless your scope was so out of collimation that you need a VERY APPROXIMATE collimation, or your chip/resolution is small enough to not notice finest errors. This is because the spot on secondary is not really (almost never) central to the real optical axis, so for example in my case centring the Cheshire dot on the center or the secondary spot makes the scope out of collimation (as first posts show). So just ignore it, it will not be precise enough.

2) You can use a Glatter, and I actually have one, it can give you a more precise collimation during the day, but again YOU NEED a star test done with your full imaging equipment, and done through it, so you can also ignore laser. No way around it, you collimate with real stars and through your imaging system all attached, and it's actually very fast to do. The only way I used Glatter laser is to check how much my camera/focuser/primary (they're all together attached to the same plate) was aligned: put scope face down, make the Glatter slide into the focuser without tightening anything just leaving gravity to make it sit perfectly square in the focuser, with focuser already tighten up like I would during photo session. After checking the laser is perfectly collimated on a v-block (it was), procedure is simple: using the 1mm extension of the laser, I would just rotate it in the focuser and notice if it stays in the same point on the secondary while I rotate it, and if the return beam goes on (or close around) the origin beam. If it describes small circles, smaller than the secondary mirror spot and possibly within it, then is right.

3) In 90% of the cases you can IGNORE THE PRIMARY on these RC scopes. Most of the times out of the manufacturer the primary is fine, to check it is easy during a star test: if the defocused star (ignore for now the shadow) is overall round and not oval on most of the FOV, then your primary is fine, and most of the times it is. If you want you can follow all the guides about the Glatter, and if so you might think the following screenshot means collimation is completely out (projections of holographic circles on my collimated scope):

Well that's not true at all, my primarysecondary are collimated! My star test produces this on every star/position, actually even better in nights with better seeing:

In nights with better seeing I can perfectly see the central dot diffraction at all defocused ranges (in and out), and the circles are perfectly aligned. And that means scope is very much collimated. More on this on next point.

4) If you have a configuration similar to mine, without focuser tilt, basically your camera is attached to your focuser, the focuser to the back plate of the scope, and the primary is attached to the same plate, so basically is a single line you're moving (unless your equipment is too heavy and there's flex). In my case the Glatter concentric circles showed me simply that my primary is SLIGHTLY tilted, still producing round defocused stars. It just means that has a minimal tilt pointing not exactly at the center of the secondary, basically is not 100% aligned within the tube (not sure how to better explain this..), as the following picture shows (notice that I can use about 70% of my FOV, and the bigger vignetting on the right -lower in particular- then left side):

Now you might be tempted of moving the primary to recenter it, but it's not worth it because anyway you can't use 100% of the field, so even adjusting it you don't gain any more useable FOV, because the edge to be ignored would be anyway bigger so you're only moving the center of the FOV by a few pixels. Just leave it as it is because moving primary is a much more complicated affair, more time consuming and might lead to worse results. In my case was useless to change that, better start imaging finally!

Following CCD inspector shows what I mean...the useable FOV is up to the green (not including it), so no need to recenter it all:

When I said at start to ignore Cheshire because longer and not precise (you need anyway a star test, just skip directly to it!), you can see below why. Looking at my scope in a Cheshire you would think is way out of collimation, BUT THAT'S NOT TRUE! Primary and secondary are collimated (that's what you need), you just have a center moved by a few mm in the tube, but with a RC without a completely flat field most of the times moving it would not change anything, as I said moving the primary to adjust this would be much more risky and lengthy process without gaining anything. So your primary/secondary collimation you see it only with star test and then real test pics, not through Cheshire.

Could I make my collimation 'technically' more perfect? YES - Would that change anything in my images? NO

5) Given all this, TO COLLIMATE MY SCOPE WAS ENOUGH TO MOVE SECONDARY ON A STAR TEST. Basically doing so I'm tilting the secondary until it's collimated with the primary (that is already square with my camera). Take a good star, center it, then make sure the secondary shadow (the black circle) is perfectly centred in the defocused star. Every time recenter the star after movements. And do it all through the live view of your imaging system. ignore any other suggestion. Once you've done it, choose another star in a completely different position in the sky, and verify is ok, and repeat the process. If second star is much different than first star, it means you have flexing somewhere. What helped me greatly was rotating the liveview (or my head!) to see that all was centred also by a different point of view, or use any of the Windows applications that draw a transparent window you can over impose to the live view a series of concentring circles: http://sweiller.free.fr/collimation.html - Basically once all is centred and there's a decent seeing, you'll start seeing a central white dot diffraction in the shadow...once you see it always (in and out focus), and with at least two stars in different positions of the sky, you're done! It takes 30 minutes, can be done during gaps between clouds, is really easy.

6) Take test shots after focusing precisely with a Bahtinov mask with your image equipment on, and verify that 60/70% of the FOV is good, and you'll see it is and you're collimated with a setup similar to mine.

The basic lesson I suppose was that collimation is quite easy, you just can ignore most of the too much technical useless details, once you get a nearly perfect collimation as I showed here (in my case), let's say 95% there, that 5% will not make ANY difference in your final picture, apart from the placebo effect on your judging and countless days and hours you could use instead imaging

I ended up sending back the Cheshire, keeping the Glatter only because might be useful to check later in time and also when I'll buy a bigger version of this scope or a field flattener, and indeed I recommend it even if I only used to verify camera/focuser/primary squaring (rotating it face down and checking the laser doesn't draw any circle, or very small one). Then just wait for a clear night, a couple of bright stars, star test, in short time you're good to go and just moving secondary (if needed).

Again basic procedure is: most of the times the primary mirror (and so also the focuser/camera) will be ok or just slightly tilted, not a problem leave it there, just verify it is good with a star test and a nice round defocused star (ignore shadow position). If you can get a collimated laser, do the rotation test to verify that camera/focuser/primary are square. Done this you know your camera/focus/primary are aligned and ok, and the primary is not too much out (round defocused star), so you just center the secondary shadow in star test moving the secondary, and basically aligning it's tilt with the primary one, done, very simple.

I hope this makes sense...like other times before, I just think sometimes there's too much confusing talking, and a more down to earth approach is the best one, like Nick from Altair Astro rightly suggested from the start and I got it right once I followed his suggestion (the one I gave here), ignoring all the rest of overcomplications (for my own case/setup). I had already a few nights of good images, very happy!

[prusling]

So a final post to summarise it all up, because I received quite a few messages and it seems many would like to buy one of these RC but everyone is (rightly) scared about the collimation. What everyone agrees with (and me too), is that there is NO WAY to properly collimate your scope if not with a star test: actually it might well be the ONLY thing you need

My personal experience is with a 6RC (so quite small) and no focus tilter and no field flattener, and a full frame DSLR, so is a quite demanding and delicate imaging system (very high resolution, very large chip).

The useable FOV with this configuration is about 70%, so I can ignore the 30% borders actually.

What I learnt in the process is:

1) Cheshire is just useless unless your scope was so out of collimation that you need a VERY APPROXIMATE collimation, or your chip/resolution is small enough to not notice finest errors. This is because the spot on secondary is not really (almost never) central to the real optical axis, so for example in my case centring the Cheshire dot on the center or the secondary spot makes the scope out of collimation (as first posts show). So just ignore it, it will not be precise enough.

2) You can use a Glatter, and I actually have one, it can give you a more precise collimation during the day, but again YOU NEED a star test done with your full imaging equipment, and done through it, so you can also ignore laser. No way around it, you collimate with real stars and through your imaging system all attached, and it's actually very fast to do. The only way I used Glatter laser is to check how much my camera/focuser/primary (they're all together attached to the same plate) was aligned: put scope face down, make the Glatter slide into the focuser without tightening anything just leaving gravity to make it sit perfectly square in the focuser, with focuser already tighten up like I would during photo session. After checking the laser is perfectly collimated on a v-block (it was), procedure is simple: using the 1mm extension of the laser, I would just rotate it in the focuser and notice if it stays in the same point on the secondary while I rotate it, and if the return beam goes on (or close around) the origin beam. If it describes small circles, smaller than the secondary mirror spot and possibly within it, then is right.

3) In 90% of the cases you can IGNORE THE PRIMARY on these RC scopes. Most of the times out of the manufacturer the primary is fine, to check it is easy during a star test: if the defocused star (ignore for now the shadow) is overall round and not oval on most of the FOV, then your primary is fine, and most of the times it is. If you want you can follow all the guides about the Glatter, and if so you might think the following screenshot means collimation is completely out (projections of holographic circles on my collimated scope):

Well that's not true at all, my primarysecondary are collimated! My star test produces this on every star/position, actually even better in nights with better seeing:

In nights with better seeing I can perfectly see the central dot diffraction at all defocused ranges (in and out), and the circles are perfectly aligned. And that means scope is very much collimated. More on this on next point.

4) If you have a configuration similar to mine, without focuser tilt, basically your camera is attached to your focuser, the focuser to the back plate of the scope, and the primary is attached to the same plate, so basically is a single line you're moving (unless your equipment is too heavy and there's flex). In my case the Glatter concentric circles showed me simply that my primary is SLIGHTLY tilted, still producing round defocused stars. It just means that has a minimal tilt pointing not exactly at the center of the secondary, basically is not 100% aligned within the tube (not sure how to better explain this..), as the following picture shows (notice that I can use about 70% of my FOV, and the bigger vignetting on the right -lower in particular- then left side):

Now you might be tempted of moving the primary to recenter it, but it's not worth it because anyway you can't use 100% of the field, so even adjusting it you don't gain any more useable FOV, because the edge to be ignored would be anyway bigger so you're only moving the center of the FOV by a few pixels. Just leave it as it is because moving primary is a much more complicated affair, more time consuming and might lead to worse results. In my case was useless to change that, better start imaging finally!

Following CCD inspector shows what I mean...the useable FOV is up to the green (not including it), so no need to recenter it all:

When I said at start to ignore Cheshire because longer and not precise (you need anyway a star test, just skip directly to it!), you can see below why. Looking at my scope in a Cheshire you would think is way out of collimation, BUT THAT'S NOT TRUE! Primary and secondary are collimated (that's what you need), you just have a center moved by a few mm in the tube, but with a RC without a completely flat field most of the times moving it would not change anything, as I said moving the primary to adjust this would be much more risky and lengthy process without gaining anything. So your primary/secondary collimation you see it only with star test and then real test pics, not through Cheshire.

Could I make my collimation 'technically' more perfect? YES - Would that change anything in my images? NO

5) Given all this, TO COLLIMATE MY SCOPE WAS ENOUGH TO MOVE SECONDARY ON A STAR TEST. Basically doing so I'm tilting the secondary until it's collimated with the primary (that is already square with my camera). Take a good star, center it, then make sure the secondary shadow (the black circle) is perfectly centred in the defocused star. Every time recenter the star after movements. And do it all through the live view of your imaging system. ignore any other suggestion. Once you've done it, choose another star in a completely different position in the sky, and verify is ok, and repeat the process. If second star is much different than first star, it means you have flexing somewhere. What helped me greatly was rotating the liveview (or my head!) to see that all was centred also by a different point of view, or use any of the Windows applications that draw a transparent window you can over impose to the live view a series of concentring circles: http://sweiller.free.fr/collimation.html - Basically once all is centred and there's a decent seeing, you'll start seeing a central white dot diffraction in the shadow...once you see it always (in and out focus), and with at least two stars in different positions of the sky, you're done! It takes 30 minutes, can be done during gaps between clouds, is really easy.

6) Take test shots after focusing precisely with a Bahtinov mask with your image equipment on, and verify that 60/70% of the FOV is good, and you'll see it is and you're collimated with a setup similar to mine.

The basic lesson I suppose was that collimation is quite easy, you just can ignore most of the too much technical useless details, once you get a nearly perfect collimation as I showed here (in my case), let's say 95% there, that 5% will not make ANY difference in your final picture, apart from the placebo effect on your judging and countless days and hours you could use instead imaging

I ended up sending back the Cheshire, keeping the Glatter only because might be useful to check later in time and also when I'll buy a bigger version of this scope or a field flattener, and indeed I recommend it even if I only used to verify camera/focuser/primary squaring (rotating it face down and checking the laser doesn't draw any circle, or very small one). Then just wait for a clear night, a couple of bright stars, star test, in short time you're good to go and just moving secondary (if needed).

Again basic procedure is: most of the times the primary mirror (and so also the focuser/camera) will be ok or just slightly tilted, not a problem leave it there, just verify it is good with a star test and a nice round defocused star (ignore shadow position). If you can get a collimated laser, do the rotation test to verify that camera/focuser/primary are square. Done this you know your camera/focus/primary are aligned and ok, and the primary is not too much out (round defocused star), so you just center the secondary shadow in star test moving the secondary, and basically aligning it's tilt with the primary one, done, very simple.

I hope this makes sense...like other times before, I just think sometimes there's too much confusing talking, and a more down to earth approach is the best one, like Nick from Altair Astro rightly suggested from the start and I got it right once I followed his suggestion (the one I gave here), ignoring all the rest of overcomplications (for my own case/setup). I had already a few nights of good images, very happy!

As the new owner of a pre-owned 6" AA RC many thanks for all the information.

I have so far just used a Cheshire and a brief real star test and it is looking pretty good to my eyes (no imaging done yet).

I'll do a more extensive star test when the skies clear - would an artificial star be a good alternative approach?

Sent from my C6833 using Tapatalk

[rofus]

As the new owner of a pre-owned 6" AA RC many thanks for all the information.

I have so far just used a Cheshire and a brief real star test and it is looking pretty good to my eyes (no imaging done yet).

I'll do a more extensive star test when the skies clear - would an artificial star be a good alternative approach?

Sent from my C6833 using Tapatalk

I looked as well into the artificial star option, but to build one on your own is not guarantee to get really good shaped or light enough, and buying one is expensive (better investing in a Glatter. Also based on the 6rc it should be 17.7 (ish) away from your scope. and well lit and stable. Also consider that is anyway an alternative...so I would really do a proper star test, if it looks good then be sure to focus sharp with a Bahtinov mask (they should sell them with every scope I don't even know how is possible to live without that), and then take a pic of at least 30s (possibly couple of minutes) and check carefully 1:1 crop.

You should get center pin sharp and then edges elongated, but all 'pointing' toward the center of FOV, then you know you're right. Sometimes star test seems ok and then test pic shows it's not, to center precisely the shadow means really precisely..I would say if seeing is decent or half decent, until you don't see the central white diffraction dot then you're not collimated good enough for imaging.

But I think it will be quite straight forward once you know that one or two sessions might be dedicated to that

[rofus]

As a last note on this topic, for future reference for others.

Chasing another problem I got after collimation (star trails in some positions of the sky although guiding was good), in the end turned up to be another case of primary mirror moving in its cell!

Following this http://deepspaceplace.com/gso8rcpointing.php and this very useful topic with images (confirmation of previous doc) http://www.cloudynights.com/topic/486239-at6rc-mirror-flopyes-mirror-flop/ , I disassembled the scope and the same exact thing happened: shaking the cell, lightly, the mirror was moving and rocking inside!

I followed that procedure unscrewing baffle, loosening small locking screws, tightened up a bit the o ring till mirror was firm, all back to full scope mounted. Collimation was only slightly out, but I did it again suspecting that the mirror could also influenced my collimation problems.

And I NOW SOLVED ALL PROBLEMS. Collimation just fine adjusting secondary makes much more visible and constant results (no surprise there's no mirror moving!), perfect tracking also with a small 9x50 finderguider. Below CCD Inspector of my full frame cropped down to APS-C (non full frame) DSLR sensor...for my camera anyway a 4000+ pixels wide picture...basically an almost completely flat field without using any other accessory or focuser, just with collimation after fixing the primary mirror:

So my suggestion is to know that basically ALL those RC scopes have primary a bit loose, probably for transportation, and this needs to be addressed for any precision work (long exposures DSO) to be done on this scope. It looks spooky with is VERY simple, so is another step I recommend.

Something else I suggest is to be sure that, after collimating primary, you tighten up (not too much but firmly) the locking screws of the primary (that is the bigger silver one), to be sure primary doesn't move anyway (with the whole cell in this case!). For reference, in the end for collimation I suggest using Glatter inside, and then this for star test..it worked for me reliably time after time: http://deepspaceplace.com/gso8rccollimate.php

That's it, it takes some time but the quality of this scope is unbelievable, you can get a perfectly flat field and 100% correct colours for ridiculous price..and a lot of work, good learning.

Waiting now for the field flattener to use even more field on my full frame at scope native focal length, and then also waiting for the CCDT67 reducer to have it down to f/6 900mm when I want a faster and wider field!