collimation - secondary doesnt seem centered

[Guest]

had a bash at collimating my scope earlier. the first time i have done it. i did the secondary first. while looking through i saw the secondary looked too far towards the front of the scope so scooted it back to be central to the EP. then proceeded to the secondary which needed a minor adjustment. what i saw of the secondary in the primary seemed out though ive included a picture to show what i mean 

the black dot is from my collimation cap but i thought it would have to line up with the centre of the mirror. the red dot is where i believe it should be. i could make it line up, i think, if i moved the secondary back towards the front of the scope. but wouldnt that then move the secondary out of alignment with the EP a bit? confused on the correct course of action. any help is much appreciated.

[Knighty2112]

Some faster scopes have there secondary mirror slightly offset. I used to have (actually still do in the loft) a Celstron Astromaster 130eqMD f5 reflector that had an offset secondary mirror. What scope is it that you have? 

[Guest]

Skywatcher Explorer 130. Spherical mirror not parabolic. Should I slide the secondary towards the front of the scope and have it off align from the Ep then?

[Moonshane]

I presume your primary does not have a centre donut ring? If not I would add one as this makes the critical primary collimation easier to get accurate.  The secondary collimation has hardly any effect on views.

Otherwise your secondary collimation is not bad but could do with a tweak probably.  I'd use a Cheshire personally.  makes collimation easy.

[Ricochet]

 The position of the collimation cap reflection is altered by adjustment of the primary mirror. If your telescope has a spherical mirror then getting everything as concentric circles will probably do well enough. Ideally you would have a Cheshire and your mirror would have a doughnut marking the centre of the primary. In this case the method would be: 

• Centre secondary under focuser
• Adjust secondary so that it appears circular and primary mirror clip reflections are evenly spaced
• Tweak secondary so that primary doughnut is directly under Cheshire crosshair
• Adjust primary so that reflection of hole in centre of Cheshire reflective face appears directly under Cheshire crosshairs (and centred within primary doughnut). 

[Guest]

1 hour ago, Moonshane said:

I presume your primary does not have a centre donut ring? If not I would add one as this makes the critical primary collimation easier to get accurate.  The secondary collimation has hardly any effect on views.

Otherwise your secondary collimation is not bad but could do with a tweak probably.  I'd use a Cheshire personally.  makes collimation easy.

no it doesnt. i did hope it would. im looking into how to add a centre spot now. the video im watching says you can use a paper reinforcing ring, like what you use to strengthen the holes to put into ring binders. does the centre spot have any effect on the image? edit: nvm the centre is behind the secondary so should have any real effect

Edited January 27, 2017 by Guest

[Knighty2112]

1 hour ago, Dakaar said:

Skywatcher Explorer 130. Spherical mirror not parabolic. Should I slide the secondary towards the front of the scope and have it off align from the Ep then?

This has similar specs to my old Astromaster, so more than likely the slight offset is required. I did read a long while ago why shorter focul lengths require this slight offset but for the life of me I can't remember exactly now. 

[REG78]

Looks like your primary just needs a few tweaks we bit at a time keep goin back n lookn through your focuser but you will need tht centre spot sortd just to make it easier for yourself hth 

[Knighty2112]

Here the article explaining offset focusers and the need for them in some scopes.

http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

 

Edited January 27, 2017 by Knighty2112

[Guest]

thanks for the replies. managed to get the secondary centred a little more. movement of the secondary helped. i also now understand how the secondary works mechanically as i managed to unscrew it completely  any idea where i can get a fancy centre spot? i read that using a ring binder reinforcer can cause damp/dew to soak into it and run glue down the mirror. i can't find them anywhere on the internet though 

[alanjgreen]

These are good. Point the corners of the shapes at the adjustments screws around the edge of the mirror. Gives a hint to which screw you need to turn

https://www.firstlightoptics.com/catseye-collimation-tools/catseye-centre-spot.html

3/16 for a small scope with short tube

1/4 for longer tubes

more info here

http://www.catseyecollimation.com/template.html

 

Edited January 31, 2017 by alanjgreen

[Guest]

haha ive been looking for days but didnt se anything. even when i was looking on flo at collimation caps i didnt come across anything. i clearly wasnt looking properly  thank you 

[Dave In Vermont]

One of the best guides to collimation is written by a member here - Astro_Baby. Unfortunately her website is out-of-service. Otherwise I'd toss a link - it has an excellent illustration of the offset commonly found in fast-Newtonians (F5, F4, etc.). It's just a flat-out great guide for us owners of Newtonians, and passing out freely too.

As her site is dead, here's a copy from my collection:

 

Astro Baby's Collimation Guide.pdf

 

Please let me know if you have any problem with the above link? Thanks!

Hang onto the pdf. -

Dave

 

[Alan64]

I recently center-spotted the primary mirror of my 100mm f/4 tabletop.  All you need is several sheets of white copy paper, a compass, transparent-tape, a permanent marker and a polyvinyl(or paper) notebook reinforcement.

I made several patterns, and finally decided on one that would show just a sliver of the mirror all the way around...

The pattern must be cut as perfectly as possible.  The sharp compass-point produces the center point, and I enlarged the point's hole only slightly to effectively mark it with a permanent marker...

I then took one of my discarded patterns and made a sizeable hole in the center.  Next, I placed a narrow strip of tape across the hole and lightly adhered the top of the reinforcement to the underside of the tape, then laid the pattern over the mirror, centered the reinforcement over the mark and pressed it down, then peeled off the pattern and tape...

I then cleaned the mark away with a cotton swab.  It worked like a charm...

The secondary scene as viewed through a collimation-cap, before and after the center-spotting...

Incidentally, the telescope arrived nigh on perfectly collimated.  It also has a glued-in primary mirror, and is essentially the same kit as the Sky-Watcher Heritage 100P.

In the case of your spherical mirror, I'm thinking that it might be necessary to add some thicknesses of paper on the underside of the pattern near the outside edges so that the reinforcement will just hover in the air above the mark before you press it down.  I may be wrong about that, however, as I do not have that particular telescope. 

Edited January 31, 2017 by Alan64

[Alan64]

14 minutes ago, Dave In Vermont said:

One of the best guides to collimation is written by a member here - Astro_Baby. Unfortunately her website is out-of-service. Otherwise I'd toss a link - it has an excellent illustration of the offset commonly found in fast-Newtonians (F5, F4, etc.). It's just a flat-out great guide for us owners of Newtonians, and passing out freely too.

As her site is dead, here's a copy from my collection:

 

Astro Baby's Collimation Guide.pdf

 

Please let me know if you have any problem with the above link? Thanks!

Hang onto the pdf. -

Dave

 

Dave, is this what you're looking for...

http://www.forumskylive.it/Public/data/serastrof/201281510358_Astro Babys Guide to Collimation.pdf

[Dave In Vermont]

Ah! It works!

Is this link going to stay up & running? Thank you - regardless. It fits the bill.

Dave

[Alan64]

4 minutes ago, Dave In Vermont said:

Ah! It works!

Is this link going to stay up & running? Thank you - regardless. It fits the bill.

Dave

I don't know if 'twill or not.  It's embedded within an Italian website(.it).

I just discovered it recently, within the last month I think.

Edited January 31, 2017 by Alan64

[Dave In Vermont]

Well - Thank you! I'll use it until it explodes, and keep the extant Pdf. on file regardless.

Good find!

Dave

[Alan64]

Yes, I have the file saved, too.

Cheers.

[Guest]

1 hour ago, Alan64 said:

The secondary scene as viewed through a collimation-cap, before and after the center-spotting...

Incidentally, the telescope arrived nigh on perfectly collimated.  It also has a glued-in primary mirror, and is essentially the same kit as the Sky-Watcher Heritage 100P.

In the case of your spherical mirror, I'm thinking that it might be necessary to add some thicknesses of paper on the underside of the pattern near the outside edges so that the reinforcement will just hover in the air above the mark before you press it down.  I may be wrong about that, however, as I do not have that particular telescope. 

am i right in thinking the shape of the secondary holder is down to offset? im still learning about the offset trying to get my head around it

 

1 hour ago, Dave In Vermont said:

One of the best guides to collimation is written by a member here - Astro_Baby. Unfortunately her website is out-of-service. Otherwise I'd toss a link - it has an excellent illustration of the offset commonly found in fast-Newtonians (F5, F4, etc.). It's just a flat-out great guide for us owners of Newtonians, and passing out freely too.

As her site is dead, here's a copy from my collection:

 

Astro Baby's Collimation Guide.pdf

 

Please let me know if you have any problem with the above link? Thanks!

Hang onto the pdf. -

Dave

 

excellent guide. i had heard about astro babys site and it going missing but didnt know it had this little gem on it. saved for re-reading

[Alan64]

1 minute ago, Dakaar said:

am i right in thinking the shape of the secondary holder is down to offset? im still learning about the offset trying to get my head around it

Your Newtonian is a 130mm f/7, therefore the secondary off-set is not going to be that apparent at f/7.  Note the off-set here with my 150mm f/5, on the right...

On the left, an f/4, and even faster, therefore the greater the off-set.  Fortunately, the off-set occurs automatically during a normal collimation procedure, and does not require a separate step to perform.  Once you get the collimation-cap, and a Cheshire too, hopefully...

https://www.firstlightoptics.com/other-collimation-tools/premium-cheshire-collimating-eyepiece.html

...then, you can insert the collimation-cap where an eyepiece normally goes, aim the telescope at a white, blank,  illuminated wall or other, place a small camera over the pinhole of the cap, zoom in if needed, snap a shot of the scene as I have many times, then post it here for us to have a look.  The collimation-cap allows you see the entire optical system inside at a glance. 

Incidentally, a secondary off-set is to ensure that all of the light collected by the primary mirror reaches the eye.  But, again, there will not be much of an off-set with your f/7 Newtonian after a proper collimation with the cap, and the Cheshire...

A Cheshire is useful for for centering the secondary mirror directly under the focusser...

The secondary mirror must also appear perfectly round, as a circle, or very near to it, as shown.

A Cheshire also serves to ensure that the center of the eyepiece is aimed directly at the center-spot of the primary mirror...

I look forward to seeing a nice snapshot of your secondary scene.

Cheers,

[benzomobile]

On 27/01/2017 at 22:24, Knighty2112 said:

Here the article explaining offset focusers and the need for them in some scopes.

http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

 

http://www.skyandtelescope.com/wp-content/uploads/Mirror_offset_L.jpg

Totally wrong analisis! 

Beppe