Collimation

[Andy1978]

I just read Astro baby's guide, I understand collimation but not how to collimate.

I loosen the secondary mirror allen key nuts and then just tighten them again, nothing seems to really happen.

So anyway I tried to collimate my SW200p scope and... how's my collimating?

Any help is appreciated.

Thanks

Andy

[Lance Bridwell]

I went on you tube and it was very helpful with collimation

[Rob]

Its a tricky thing to grasp, but once there its easy going forward!. From your image both mirrors are off.

Firstly center your secondary mirror. forget how it looks on the primary (the main mirror) then go for primary

http://garyseronik.com/a-beginners-guide-to-collimation/

 

[Ricochet]

When you collimate the secondary you don't just loosen them all. You loosen one or two of the outer screws and tighten another so that the angle changes. However, if you have loosened all three you can carefully hold the centre stalk holding the secondary and move it by hand, and then tighten all three when it is in the right position. However, it will still need a bit of tweaking after this. 

[Lance Bridwell]

Bob's knobs really help

[Andy1978]

Thank you for your replies

The Gary Seronik guide really clicked with me, I will also be getting some of Bob's knobs too as they seem like a worthwhile upgrade.

I believe I am a lot closer to collimation now, or perhaps I already am done?

[Ricochet]

That looks a lot better. There will still be a bit of improvement possible though. If you look at the shadow of the secondary it is not quite symmetrical in the "vertical" direction. There will always be a bit of an offset towards the primary mirror end. 

[Andy1978]

I'm happy that its collimated enough for a star test tonight.  I used a collimation cap mainly but I also have a cheshire, i find the cheshire is not a snug fit in the focuser so you can just wiggle it around so everything looks aligned, or not.

[Jason D]

Photo in the first post has the signature of what is called rotate/tilt error. The secondary mirror silhouette is oval in shape with the major axis pointing in the 5:00 direction. Fixing it requires a slight clock-wise rotation of the secondary mirror looking down the tube.

Photo in the sixth post has the signature of having the secondary mirror slightly high in the OTA. The mirror silhouette is concentric and you can see a small dark crescent on the secondary mirror in the direction of the OTA opening. Interestingly, the secondary mirror placement in the first photo is better than the second though both placements are not too bad.

With respect to the last post, star test is mainly used to evaluate the axial alignment of the primary mirror. It is not ideal for evaluating the secondary mirror placement.

Jason

[Alan64]

No need for Bob's Knobs; those are for the decorative cowls of Schmidts.  Simply take one of the set-screws from the secondary-hub to your local hardware, have it sized, a metric #4 it is more than likely, and get black-oiled socket-head screws instead, like these on my Newtonian...

Get them as long as you'd like, so you can get a hold on them with your fingers whilst adjusting.  They'll be a lot cheaper than Bob's Knobs.  If you'd like you can place a nylon-washer between the back of the secondary-stalk and the screw-tips, and let the screw-tips dig into the nylon instead of the soft metal of the stalk.  I glued my washer onto the stalk, but it doesn't have to be...

 

[Andy1978]

1 hour ago, Jason D said:

Photo in the first post has the signature of what is called rotate/tilt error. The secondary mirror silhouette is oval in shape with the major axis pointing in the 5:00 direction. Fixing it requires a slight clock-wise rotation of the secondary mirror looking down the tube.

Photo in the sixth post has the signature of having the secondary mirror slightly high in the OTA. The mirror silhouette is concentric and you can see a small dark crescent on the secondary mirror in the direction of the OTA opening. Interestingly, the secondary mirror placement in the first photo is better than the second though both placements are not too bad.

With respect to the last post, star test is mainly used to evaluate the axial alignment of the primary mirror. It is not ideal for evaluating the secondary mirror placement.

Jason

Thank you for your post Jason it is extremely helpful.

 

I will attempt to correct the placement of the secondary mirror height in the OTA, as read in a guide I will measure the distance between the edge of the OTA and the middle of the focuser and then adjust the mirror accordingly, I will also eyeball the mirror silhouette to see when the crescent disappears. 

 

I find loosening the middle screw (and the surrounding 3 allen keys screws) on the secondary mirror to be a tad daunting due to it being so tight, after adjustments are made is it okay to not tighten these screws very much?

 

Many thanks

 

Andy

 

 

[Rob]

Great advice there Alan.. ?

[Andy1978]

3 hours ago, Alan64 said:

No need for Bob's Knobs; those are for the decorative cowls of Schmidts.  Simply take one of the set-screws from the secondary-hub to your local hardware, have it sized, a metric #4 it is more than likely, and get black-oiled socket-head screws instead, like these on my Newtonian...

Get them as long as you'd like, so you can get a hold on them with your fingers whilst adjusting.  They'll be a lot cheaper than Bob's Knobs.  If you'd like you can place a nylon-washer between the back of the secondary-stalk and the screw-tips, and let the screw-tips dig into the nylon instead of the soft metal of the stalk.  I glued my washer onto the stalk, but it doesn't have to be...

 

Thank you, this is great intel!

I believe a US metric #4 is the equivalent of a UK Imperial gauge size 8 so I think these should be okay https://www.westfieldfasteners.co.uk/Unbrako/Unbrako-Socket-Head-Cap-Allen-Screw-Part-Thread-M4x35-Black.html

My guess from your photo is that you have 35mm length screws!?, please can you confirm this as they look like the perfect length to me.

Do you use your own screws for the primary mirror as well?

Andy

[Jason D]

6 hours ago, Andy1978 said:

Thank you for your post Jason it is extremely helpful.

 

I will attempt to correct the placement of the secondary mirror height in the OTA, as read in a guide I will measure the distance between the edge of the OTA and the middle of the focuser and then adjust the mirror accordingly, I will also eyeball the mirror silhouette to see when the crescent disappears. 

 

I find loosening the middle screw (and the surrounding 3 allen keys screws) on the secondary mirror to be a tad daunting due to it being so tight, after adjustments are made is it okay to not tighten these screws very much?

 

Many thanks

 

Andy

With regards to the crescent in the 2nd photo, the following diagram illustrates what I meant. Once the secondary edge appears concentric with the primary mirror reflection when looking down the focuser axis, the secondary silhouette is expected to appear shifted towards the primary mirror. If you make the secondary silhouette concentric with the primary mirror reflection then the secondary mirror edge will appear shifted towards the OTA opening. You can't have both the secondary silhouette and the secondary edge both concentric with the primary mirror reflection simultaneously.

 

With respect to making fine adjustments to the secondary mirror, consider the following DIY hack that I came up with many years ago only if you are little adventurous: Insert or slide two washers that are cut from a soft plastic container -- preferably with a bumpy surface. Here is the USA, milk jugs make a great material for this hack. It will make secondary mirror fine adjustments much easier since plastic will give you little elasticity to make these fine adjustments.

In general, the secondary mirror placement under the focuser is the least critical collimation alignment. Just do your best and accept the results. Honestly, your secondary mirror placements as shown in the two photos are not too bad. You can leave them as such and enjoy your scope. A slightly misaligned secondary mirror as shown in both of your photos will NOT impact the sharpness of your views. The only impact, which I doubt even the most experienced observers can notice notice, is a minute imbalance of brightness at opposite edges of the FOV when using low power EP.

No need to take measurements as you stated. Relax, collimate, and enjoy your views

Jason

[Andy1978]

Thank you again.

I have read, and re-read, your 2nd post and I now have a better understanding.

That's the 2nd time I have read about insert a type of plastic washer onto the stalk, something for when I am more experienced.

Andy

[Alan64]

8 hours ago, Andy1978 said:

Thank you, this is great intel!

I believe a US metric #4 is the equivalent of a UK Imperial gauge size 8 so I think these should be okay https://www.westfieldfasteners.co.uk/Unbrako/Unbrako-Socket-Head-Cap-Allen-Screw-Part-Thread-M4x35-Black.html

My guess from your photo is that you have 35mm length screws!?, please can you confirm this as they look like the perfect length to me.

Do you use your own screws for the primary mirror as well?

Andy

The ones within my image are 40mm in length.  Simply take a short metric-rule and hold it up against the side of the hub, placing the end of the rule where the screw-tips would touch the back of the secondary-stalk, and determine the length above the hub desired, and enough to ensure that you can get the fingers on them...  

Theoretically, they could stretch out into infinity and still not interfere with the light-path.

My 150mm f/5 Newtonian, along with your Sky-Watcher, were both made by Synta in mainland China.  Mine is branded "Orion", of California, and its primary-cell is fully equipped with metal-springs and thumbscrews...

 

That of the Sky-Watcher utilises fastening-type screws and rubber o-rings, there on the right, and with that of a GSO there on the left...

 

Of all the designs of telescopes, the Newtonian lends itself to all manner of enhancements and improvements, and just as I have done for my own.  You can replace those screws with M5 thumbscrews, and replace the o-rings with metal-springs.  You'll need to study your cell to discover what is possible and what is not in improving its machinations.  These links should be of help...

https://www.cloudynights.com/topic/515629-skywatcher-6-inch-dob-primary-collimation/?p=6839671

http://www.astronomyforum.net/astronomy-beginners-forum/160480-o-rings-behind-primary-mirror-s-w-8-a.html

In addition, research the improvements further, online.  The addition of metal springs may require some doing.  The design of the primary-cell of the Sky-Watcher came out first, many years ago, and it simply isn't up to the standard of other brandings.  That's the problem in a nutshell.

[Alan64]

16 minutes ago, Andy1978 said:

Thank you again.

I have read, and re-read, your 2nd post and I now have a better understanding.

That's the 2nd time I have read about insert a type of plastic washer onto the stalk, something for when I am more experienced.

Andy

Yes, you may want to wait until you're more familiar with the telescope's construction...

[Ledge1962]

Hi all.

I too would like to thank you for you explanation as I have just purchased the xx14g and the initial collimation was so frustrating, the dog was lying on the couch with his paws over his ears. I managed to align it pretty close (with grand daughters help), but still not really sure how I did it. I have ordered Bobs Knobs so will be using the bottle washer trick when I put them in, but does anyone know the correct dimmensions and thickness I should be making these washers for the 14g secondary mirror please?