Collimation confusion

[Kon]

I recently acquired a 300P flextube Dob. I tried to collimate it but I am in a bit of a pickle. I am following astrobaby's guide and it has been dead easy to do my 8". I use a Cheshire with side tube. The issue I am having is how things should look on an f5 Vs f6 . Using the same guide, I can't get the Cheshire cross hair to align with the primary donut if I want the clips to show (I am using a short version of the Cheshire). If I opt for the cross hair to align then I do not see one of the clips. I confirmed the Cheshire is fine as my long one was aligning with the donut ok after I finished with the short. If I have all three clips showing, the mirror has severe tilt when I remove the Cheshire. The spidervanes are all equal.

The weather is not helping to do a star test but any thoughts where I may be thinking it wrong? I am using a low profile focuser but that shouldn't be a problem (I moved it from my 8" to the 12"). The tube is fully extended as well (second click).

So which one do I trust until tested?

Three pics describing the above situation. Ignore measurements as the phone was not properly aligned.

[Mr Spock]

Have you checked the position of the secondary? I had a 250 Flextube and it needed quite a bit of adjustment. You can't collimate properly until you know that is right. 

[wookie1965]

https://web.archive.org/web/20200223041214/http://www.schlatter.org:80/Dad/Astronomy/collimate.htm

I used this helped immensely.

[Kon]

4 minutes ago, Mr Spock said:

Have you checked the position of the secondary? I had a 250 Flextube and it needed quite a bit of adjustment. You can't collimate properly until you know that is right. 

Do you mean the up/down move towards the primary? Yes, I had to play with the central screw a bit, maybe not perfect as you say, although I am not noticing much difference even if I screw it up all the way up. In all cases, same issue as above. So I should be able to see all clips in the end through the Cheshire?

 

[Mr Spock]

2 minutes ago, Kon said:

So I should be able to see all clips in the end through the Cheshire?

Yes. If you can't see one of them when the scope is collimated it means the secondary is out of position. You should be able to see the clips and have the scope collimated at the same time. However, having said that, slight errors in secondary position when collimated will only affect where the circle of full illumination is - as long as the primary and secondary are in collimation you should see a perfect star test.

I do find Cheshires difficult, which is why I prefer a laser. I did a write up on laser collimation here 

 

[Kon]

6 minutes ago, Mr Spock said:

it means the secondary is out of position.

Thanks for the guide, very useful. The telescope came with a laser so i will give it a try. How do you know with the laser the secondary is out of position (ie what will the signs be)? I get it about the cheshire not being able to see the three clips. 

[Mr Spock]

4 minutes ago, Kon said:

How do you know with the laser the secondary is out of position (ie what will the signs be)?

You won't - it can't do that. Best to use either a collimation cap or a concentre. But once you have the secondary and primary aligned like it is in that guide you'll get a perfect star test which is what you need for fine detail; small positional errors in the secondary won't matter. 

One thing to be aware of is if the secondary is out of position left to right (or top/bottom as you look through the eyepiece), rather than up/down or back/forward. If the spider vanes are equal it could mean the focuser is misaligned. You can correct that with Skywatchers - on the one I had, the focuser had adjustment screws.

[Kon]

2 minutes ago, Mr Spock said:

You won't - it can't do that. Best to use either a collimation cap or a concentre. But once you have the secondary and primary aligned like it is in that guide you'll get a perfect star test which is what you need for fine detail; small positional errors in the secondary won't matter. 

One thing to be aware of is if the secondary is out of position left to right (or top/bottom as you look through the eyepiece), rather than up/down or back/forward. If the spider vanes are equal it could mean the focuser is misaligned. You can correct that with Skywatchers - on the one I had, the focuser had adjustment screws.

Thanks Michael. I will look into the focuser position too as I changed it to the dual one. From your comments it seems to be a secondary  and possibly focuser misalignment. I will have a play again this week and report back.

[wookie1965]

Once the secondary is in place unless you drop or severely bang the tube you will not have to adjust it again, you will only need to tweak the primary. 

[alacant]

1 hour ago, Kon said:

it seems to be a secondary 

Hi

The main difference between f5 and f6 is that for the latter, you'll find the primary donut reflection will collimate almost centrally, with very little offset. You have it set (perhaps by habit?) more as if it were at f5. Primary visibility is not helped by the small secondaries fitted to sw Dobsonians. The fix is easy:

1. Move the secondary toward the open end of the tube a few mm. To do this, loosen the three secondary adjusters and turn the central screw clockwise.
2. With the secondary adjustment screws still loose, rotate the secondary until it appears circular. Relative to your photos, you'll need to rotate the secondary anti-clockwise.

Repeat 1 and 2 until you can see the missing primary mirror clip. Now you will be able to complete the collimation

Focuser? Be sure to read the common collimation myths and Seronik's no nonsense collimation guide.

Note that the fast and slow photographs in the AstroBaby guide are incorectly labelled. They should be like this (you're aiming for the slow version):

fast:

slow:

Cheers and HTH

Edited February 22 by alacant
corregimos:f5 f6

[Kon]

37 minutes ago, alacant said:

Note that the fast and slow photographs in the AstroBaby guide are incorectly labelled. They should be like this (you're aiming for the slow version):

For my 12" (f5) i should have the offset (fast). The 8" (f6) also has the offset but if I get my 12" done properly it might be a bit more offset. 

 

39 minutes ago, alacant said:

1. Move the secondary toward the open end of the tube a few mm. To do this, loosen the three secondary adjusters and turn the central screw clockwise.
2. With the secondary adjustment screws still loose, rotate the secondary until it appears circular. Relative to your photos, you'll need to rotate the secondary anti-clockwise.

Repeat 1 and 2 until you can see the missing primary mirror clip. Now you will be able to complete the collimation

I think I was doing a more coarse adjustment of the central screw which did not help. I will repeat it a bit more methodically.

[alacant]

25 minutes ago, Kon said:

For my 12" (f5) i should have the offset (fast).

Yes. My bad. I read that the 8" was f5 and the 12" f6. f5 is offset. f6 is much more central.
But the method I gave to adjust the secondary is still correct; the secondary is both too far toward the primary and is rotated incorrectly.

Cheers

Edited February 22 by alacant

[Peter Drew]

It would always be helpful if the position of the focuser is indicated relative to the secondary mirror in the posted images, this would show whether the offset shadow is correctly orientated.      🙂

[bosun21]

I have managed to get my 10" Flextube bang on. It's also a F4.8 and I found that the secondary mirror is the crucial factor with these. I don't know whether it's due to them having an oversize secondary mirror but I used a 2" Concenter which accurately allowed me to position the mirror on all the axes especially for the mirror itself with the pitch, yaw and roll to achieve a perfect circular disc and positioned correctly as pertaining to the focuser draw tube and focuser itself. Now I can get my Hotech laser, Cheshire and collimation cap to all correspond perfectly.

[Kon]

9 hours ago, alacant said:

Move the secondary toward the open end of the tube a few mm. To do this, loosen the three secondary adjusters and turn the central screw clockwise.
2. With the secondary adjustment screws still loose, rotate the secondary until it appears circular. Relative to your photos, you'll need to rotate the secondary anti-clockwise.

Repeat 1 and 2 until you can see the missing primary mirror clip. Now you will be able to complete the collimation

Ok I had another go tonight. I had to bring the secondary away from the primary but I am not there yet. It is better but I can't quite see the third clip, it's creeping there. The problem I have is that I ran out of screwing. There is still a few mm gap left between the holder and secondary but it will not go further. I saw another post with similar issues and they suggested to cut the spring a bit. Is it advisable? The primary of course can't be moved unless I adjust the collimation screws to be a bit less tight, of that makes sense.

Any thoughts?

[PeterStudz]

9 minutes ago, Kon said:

Ok I had another go tonight. I had to bring the secondary away from the primary but I am not there yet. It is better but I can't quite see the third clip, it's creeping there. The problem I have is that I ran out of screwing. There is still a few mm gap left between the holder and secondary but it will not go further. I saw another post with similar issues and they suggested to cut the spring a bit. Is it advisable? The primary of course can't be moved unless I adjust the collimation screws to be a bit less tight, of that makes sense.

Any thoughts?

The post you saw with similar issues where a solution was to “cut the spring a bit” MIGHT have been from me - with my 8” Dob refurbishment.

From what you described it sounds very similar and I certainly ended up doing that, but it was a last resort. Whatever I tried and looked at - I literally spent weeks - I just could not get the secondary quite far enough away from the primary. The spring would be rammed right down tight, no room left for any more movement, but with a few mm gap.

In the end I took everything to bits (I had to anyway as I was going to flock) and put it back together a few times before I finally took the plunge and cut the spring down a little. That certainly fixed the problem for me and it’s been fine ever since. 

[Kon]

5 minutes ago, PeterStudz said:

The post you saw with similar issues where a solution was to “cut the spring a bit” MIGHT have been from me - with my 8” Dob refurbishment.

From what you described it sounds very similar and I certainly ended up doing that, but it was a last resort. Whatever I tried and looked at - I literally spent weeks - I just could not get the secondary quite far enough away from the primary. The spring would be rammed right down tight, no room left for any more movement, but with a few mm gap.

In the end I took everything to bits (I had to anyway as I was going to flock) and put it back together a few times before I finally took the plunge and cut the spring down a little. That certainly fixed the problem for me and it’s been fine ever since. 

Thanks, I think it was your post. I am having the same issue. One thing I noticed is that the primary screws have pushed it up a bit. The previous owner had only used 6 times so I am not sure what they were doing as collimation is completely off. I will give it another try later tonight as I don't fancy messing up with components unless absolutely necessary.

[PeterStudz]

13 minutes ago, Kon said:

Thanks, I think it was your post. I am having the same issue. One thing I noticed is that the primary screws have pushed it up a bit. The previous owner had only used 6 times so I am not sure what they were doing as collimation is completely off. I will give it another try later tonight as I don't fancy messing up with components unless absolutely necessary.

Yes, I certainly wouldn’t do it unless you can’t find an alternative or other reason! I fiddled and played with everything (even taking the focuser to bits) before I made the decision. 

Out of interest, my Dob was also way off too with the secondary obviously well played with - eg the secondary hex-screws were visibly worn. I got the impression that whoever had it in the past might have had collimation problems. 

Edited February 22 by PeterStudz

[alacant]

52 minutes ago, Kon said:

Is it advisable

Hi

No need to cut the spring. Simply remove it. That should give you the distance you need.

To help you with the secondary, replace the hexagonal grub screw adjusters with M4 hex head bolts; much easier to take up the slack that way.

HTH

 

[Spile]

Ignore the clips until steps 1 and 2 have been completed so a=b=c=d and e=f

 

and the cross hairs of the Cheshire eyepiece and sight tube and and the dark offset secondary reflection (x2) is offset toward the primary mirror.

Edited February 22 by Spile

[Kon]

2 hours ago, alacant said:

Hi

No need to cut the spring. Simply remove it. That should give you the distance you need.

To help you with the secondary, replace the hexagonal grub screw adjusters with M4 hex head bolts; much easier to take up the slack that way.

HTH

 

It did the trick!!. I removed the spring and I managed to get all three clips into view. Very tight space left, not sure why or what the previous owners did. I wonder if the primary is not installed properly or pushed a bit higher? I know for the 300p you need to install the mirror. I might try remove it and reinstall?

Anyhow I am happy now. Star test confirmed excellent collimation although there is room for improvement.

 

Thanks again to everyone who gave excellent suggestions.

Edited February 22 by Kon

[Kon]

18 minutes ago, Spile said:

Ignore the clips until steps 1 and 2 have been completed so a=b=c=d and e=f

 

and the cross hairs of the Cheshire eyepiece and sight tube and and the dark offset secondary reflection (x2) is offset toward the primary mirror.

That was my main issue that I couldn't get these equal and in the end I had to remove the spring from the secondary screw.

[Mr Spock]

Not quite there, but a lot better 👍 You should be good now.

[Kon]

3 minutes ago, Mr Spock said:

Not quite there, but a lot better 👍 You should be good now.

Which part is not ok? I think there might be a bit of tilt.

[bosun21]

46 minutes ago, Kon said:

Which part is not ok? I think there might be a bit of tilt.

Much closer but I definitely wouldn't remove the springs. They are there for a solid reason which is to keep  tension on the secondary mirror holder at all times. This helps the secondary to be held in the correct position and stay there. You can get yourself tied up and chasing your tail with the secondary but never seem to get it just right. The telescope will most definitely be able to achieve collimation with them in place. The fact that you couldn't with them fitted tells me something else is wrong. I just want you to get it bang on for the 12"to enable you and I to draw the views and planetary images from both our 10" & 12" dobsonians. If you can't get it just right i would be more than happy to assist you with it (springs in place) as we are pretty close to each other.