Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Anyone with experience using Pierro Astro's Artificial Star with 9 um beam?

  • Please log in to reply
10 replies to this topic

#1 givememul

givememul

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 22 Dec 2019
  • Loc: Seoul, South Korea

Posted 11 August 2020 - 03:43 AM

Hi, I've recently bought an 8 inch GSO classical cassegrain telescope, and looking to buy collimation tools adequate for this telescope.

 

I first looked at some tools that many people use to collimate their RCs and CCs such as Tak collimation telescope, Hotech ACT and Howie laser collimator with holographic attachment.

Those "standard" tools seem extremely pricey, because I have to pay excessive shipping fee and customs duties, which almost totals up to about 50% of the initial price tag... for example, Hotech ACT cost about $750. uncool...

 

So I looked at more affordable options and came across a thread here claiming that the star collimation is all you need.

 

The final step of the collimation process is star testing anyway, so I thought an artificial star would fit my need.

 

During my research, I've noticed that the adequate distance between the artificial star and the telescope heavily depends on the size of the beam and smaller the artificial star, the shorter the distance can get.

 

So naturally, I was looking for an artificial star with very small beam size, due to the limited space I have around the city center where I live.

No gardens, no spacey parking lots, no park within reasonable distance. I also live in a 27 story apartment building where I could maybe get 7~8 m maximum of clear space somehow..

 

 

Then I came across this interesting piece of equipment, staggering 9 um beam artificial star from Pierro Astro.

https://www.teleskop...n-diameter.html

https://www.pierro-a...-microns_detail

 

Well I think it's not impossible to get a 9 um glass or plastic fiber, but I'm quite worried about the lack of user reviews or any information about it other than the descriptions on TS and the Pierro Astro websites.

If the item hold true to its specs, I could get my 8" CC and the star just 3 m apart!! (according to the table in Pierro-Astro.com)

Though it may as well be impossible to get the telescope in focus.

 

I'm very curious if anyone here have experiences using this and whether it's better than the common 50um artificial stars or not.

 

Thanks in advance.

 

Dae-Woong


  • Ernesto.Nicola and jeremiah2229 like this

#2 luxo II

luxo II

    Vanguard

  • -----
  • Posts: 2,314
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 11 August 2020 - 06:38 AM

According to the TS website, it says: "... for a telescope with 2 00mm aperture you should set up the artificial star at least 66 m away from the telescope..." which tallies with an old rule-of-thumb that an artificial star should be located at least 20 x the focal length of the scope.

 

Whether its worth it is up to you. My artificial star consists of a piece of aluminium foil, with a sharpened needle dropped on it to make several pin holes, and the foil is then stuck over the LED on the back of my iPhone with tape or blu-tack. 

 

Which works fine; by making several holes you'll find some are bigger/smaller than the others so you'll have a range to play with. Why stop at one I do not know. Cost nil, part from 10 minutes of my labour.

 

FWIW 9 microns isn't all that special, it is possible to make smaller holes.


Edited by luxo II, 11 August 2020 - 06:44 AM.


#3 peleuba

peleuba

    Vanguard

  • *****
  • Posts: 2,469
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 11 August 2020 - 09:53 AM

I'm very curious if anyone here have experiences using this and whether it's better than the common 50um artificial stars or not.

 

The size of the artificial star is dependent upon whether you wish to test the scope optically or just collimate.  The 50 micron artificial star will be sufficient to collimate.  If you need it to be smaller, you can install a Barlow lens in front of it.

 

If performing an indoor star test, you will need to generate a point source that is smaller then the resoving power of the telescope in arc seconds.



#4 peleuba

peleuba

    Vanguard

  • *****
  • Posts: 2,469
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 11 August 2020 - 09:54 AM

 

FWIW 9 microns isn't all that special, it is possible to make smaller holes.

 

True.  I routinely use a 5 micron point source on my bench.



#5 JohnnyLingo

JohnnyLingo

    Explorer 1

  • -----
  • Posts: 85
  • Joined: 04 May 2019
  • Loc: Heidelberg, Germany

Posted 11 August 2020 - 04:28 PM

I had the Pierro Astro 9um unit. Very good product.

 

It uses a 9um diameter optical fibre connected to a very bright LED inside its housing. It works with a 9V battery. 

 

It comes with a manual and the proper minimal spacing between the artificial star and the telescope for collimation purposes is even printed on the unit itself (the picture at the TS website should be updated).

 

However, there are a few things to consider.

 

According to the TS website, it says: "... for a telescope with 2 00mm aperture you should set up the artificial star at least 66 m away from the telescope..." which tallies with an old rule-of-thumb that an artificial star should be located at least 20 x the focal length of the scope.

This refers to star tests and not collimation. This means that you have to put it 66m away only if you want to use it for diagnosing optical aberrations, such as spherical aberration (SA). [This point is actually explained right before the '...' in the above quote.]

 

For collimation, you can live with SA caused by the close spacing, since you only have to center diffraction rings.

 

You can easily compute the minimum distance by considering the angular resolution of your telescope, and that the 9um spot has to become unresolvable by your telescope (i.e., become a point source) at the minimum required distance. Mind you, this minimum spacing for collimation obviously increases with increasing telescope aperture, and can turn out to be too long for collimating indoors. For example, I could not collimate my C9.25 indoors.

 

Another thing to consider is that putting the light source at least at the minimal required distance from the scope does not automatically mean that you can bring it into focus. I had this problem with the C9.25. For this reason, I had to put the Pocketstar much further away than the minimum required distance explained above, i.e., in order to simply bring it into focus. I note that one can also solve this latter problem by using extension tubes, so that the thing actually works at the minimum distance. If you do not know the close focus distance of your scope (I didn't), you would just have to try, I guess.


  • Ernesto.Nicola, bugbit and Louisv28 like this

#6 givememul

givememul

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 22 Dec 2019
  • Loc: Seoul, South Korea

Posted 11 August 2020 - 07:20 PM

According to the TS website, it says: "... for a telescope with 2 00mm aperture you should set up the artificial star at least 66 m away from the telescope..." which tallies with an old rule-of-thumb that an artificial star should be located at least 20 x the focal length of the scope.

 

Whether its worth it is up to you. My artificial star consists of a piece of aluminium foil, with a sharpened needle dropped on it to make several pin holes, and the foil is then stuck over the LED on the back of my iPhone with tape or blu-tack. 

 

Which works fine; by making several holes you'll find some are bigger/smaller than the others so you'll have a range to play with. Why stop at one I do not know. Cost nil, part from 10 minutes of my labour.

 

FWIW 9 microns isn't all that special, it is possible to make smaller holes.

Luxo, thanks for the reply.

 

I've read the description on the TS website, and it seems to coincide with the description on other artificial stars they sell, their 22um artificial star, and Geoptik 50um artificial star.

This I took as just a general tip for any kinds of artificial stars.

Also, for collimation purposes, the distance can be much smaller anyway, so I thought I could give it a try.

 

The pinhole method with the LED flash light, I've already tried and couldn't really get it into focus.

This could just have been due to the nature of my scope, not the hole size, but I also wanted to ask if anyone with this 9um artificial stars was able to use it for collimation on shorter distances than other larger diameter ones.

I don't have any clue that I was able to punch a small enough hole, but since I could get it into focus with my 60mm guide scope, I thought the hole size may be a critical variable here.

I will try the flash light method again, though I doubt the distance between the improvised artificial star and the telescope would be still too close..



#7 givememul

givememul

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 22 Dec 2019
  • Loc: Seoul, South Korea

Posted 11 August 2020 - 07:39 PM

The size of the artificial star is dependent upon whether you wish to test the scope optically or just collimate.  The 50 micron artificial star will be sufficient to collimate.  If you need it to be smaller, you can install a Barlow lens in front of it.

 

If performing an indoor star test, you will need to generate a point source that is smaller then the resoving power of the telescope in arc seconds.

 

 

True.  I routinely use a 5 micron point source on my bench.

 

Thanks for the reply, Paul.

 

 

I saw some threads which use barlow lens or an old eyepiece to achieve longer apparent(?) distance on their light source.

I could give it try, but without any experience of star collimation on catadioptric scopes, I thought I would be better off starting out with something commercially available, standardized tools rather than fiddling with the unknown. I also could apply extra lenses in front of the artificial star later.

The punching hole stuff, I doubt I could achieve such precision with my crude mechanical skills :)



#8 luxo II

luxo II

    Vanguard

  • -----
  • Posts: 2,314
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 11 August 2020 - 07:49 PM

OK I don't use a point source for collimation, however. There are three adjustments to play with:

 

- the focuser should be aligned coaxially to the centre of the secondary mirror;

- the primary mirror tilt has to be adjusted so it is also aligned coaxially with the secondary mirror, and lastly

- the secondary mirror then has to be adjusted with a star-test to bring all the diffraction rings concentric with the poisson spot.

 

I'll describe how I collimate my 10" maksutov - this is a Rumak with adjustment to collimate both the primary and secondary mirror and your GSO will be similar. I use this to align the focusser with the secondary, and to align the primary mirror co-axially with the optical axis of the scope. The secondary mirror is not touched - do that last - using a star test (ie align that at night on a real star). I also have adjustments for the focuser tilt, many scopes don't.

 

I use a Howie Glatter Holographic Laser Collimator, with the cross and circle disk - this projects a cross accurately aligned with the axis of the collimator and is deliberately made with a slight taper so it will fit firmly into your focuser with no slop.

 

1. Lay the scope horizontally on a bench, or you could set up on the mount if that's convenient; you need access to both ends of the scope.

 

2. Cut a piece of translucent paper (tracing paper or backing paper will do fine) and tape this across the front of the scope. You will need this to see where the cross pattern emerges from the OTA.

 

3. Switch on the laser collimator and check you see a holographic cross projected onto anything in front of it. Insert it in the focuser, and wind it in as close as it goes to the back of the primary mirror - it doesn't have to be at the nominal focus position.  

 

Ideally you will see two crosses projected on the paper:

 

- an inner intense cross pattern emerging from the central baffle projected directly onto the paper - bypassing both the mirrors, the central baffle will limit this to being visible only just around the secondary;

 

- a larger cross spanning the full aperture of the scope, this has been reflected off both mirrors.

 

4. You should also be able to see a shadow of the secondary mirror on the paper, or in the case of the GSO CC, you may be able to see the spider vanes. Adjust the focuser tilt so that the projected cross is aligned with the centre of the secondary mirror (or the vanes).

 

5. Now look at the larger cross reflected off both mirrors. Adjust the primary mirror tilt to put this coaxial with the inner cross and the secondary mirror.

 

Lastly, set up the scope on a fine evening, aim the scope at a bright star, centre that in a high-power eyepiece and adjust the secondary alignment to get the diffraction rings concentric.

 

Notes:

 

If the primary is not aligned coaxial with the secondary mirror (step 4) is equivalent to having the secondary displaced laterally by the misalignment seen between the larger and smaller cross patterns. RC or classical cassegrain are very sensitive to the lateral misalignment as the secondary is quite aspheric and that has to be accurately concentric with the light cone for it to correct for spherical aberration properly; if it is decentred the result will be ugly.


Edited by luxo II, 11 August 2020 - 08:13 PM.


#9 givememul

givememul

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 22 Dec 2019
  • Loc: Seoul, South Korea

Posted 11 August 2020 - 07:51 PM

I had the Pierro Astro 9um unit. Very good product.

 

It uses a 9um diameter optical fibre connected to a very bright LED inside its housing. It works with a 9V battery. 

 

It comes with a manual and the proper minimal spacing between the artificial star and the telescope for collimation purposes is even printed on the unit itself (the picture at the TS website should be updated).

 

However, there are a few things to consider.

 

This refers to star tests and not collimation. This means that you have to put it 66m away only if you want to use it for diagnosing optical aberrations, such as spherical aberration (SA). [This point is actually explained right before the '...' in the above quote.]

 

For collimation, you can live with SA caused by the close spacing, since you only have to center diffraction rings.

 

You can easily compute the minimum distance by considering the angular resolution of your telescope, and that the 9um spot has to become unresolvable by your telescope (i.e., become a point source) at the minimum required distance. Mind you, this minimum spacing for collimation obviously increases with increasing telescope aperture, and can turn out to be too long for collimating indoors. For example, I could not collimate my C9.25 indoors.

 

Another thing to consider is that putting the light source at least at the minimal required distance from the scope does not automatically mean that you can bring it into focus. I had this problem with the C9.25. For this reason, I had to put the Pocketstar much further away than the minimum required distance explained above, i.e., in order to simply bring it into focus. I note that one can also solve this latter problem by using extension tubes, so that the thing actually works at the minimum distance. If you do not know the close focus distance of your scope (I didn't), you would just have to try, I guess.

 

Wow, someone with actual user experience!!

Thanks, Johnny.

 

Since you have high opinion regarding the quality of the Pierro Astro artificial star, I think I may as well get one and give it a try. I've just ordered one!

I was hoping for some positive experiences regarding the shortening of distances, but since your C9,25 couldn't achieve focus indoors, I doubt I can with my similar focal length scope.

If mine can't achieve focus, too bad.  I'll need to try other methods. 


  • Ernesto.Nicola likes this

#10 givememul

givememul

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 22 Dec 2019
  • Loc: Seoul, South Korea

Posted 11 August 2020 - 08:03 PM

OK I don't use a point source for collimation, however. There are three adjustments to play with:

 

- the focuser should be aligned coaxially to the centre of the secondary mirror;

- the primary mirror tilt has to be adjusted so it is also aligned coaxially with the secondary mirror, and lastly

- the secondary mirror then has to be adjusted with a star-test to bring all the diffraction rings concentric with the poisson spot.

 

I'll describe how I collimate my 10" maksutov - this is a Rumak with adjustment to collimate both the primary and secondary mirror and your GSO will be similar. I use this to align the focusser with the secondary, and to align the primary mirror co-axially with the optical axis of the scope. The secondary mirror is not touched - do that last - using a star test (ie align that at night on a real star). I also have adjustments for the focuser tilt, many scopes don't.

 

I use a Howie Glatter Holographic Laser Collimator, with the cross and circle disk - this projects a cross accurately aligned with the axis of the collimator.

 

1. Lay the scope horizontally on a bench, or you could set up on the mount if that's convenient; you need access to both ends of the scope.

 

2. Cut a piece of translucent paper (tracing paper or backing paper will do fine) and tape this across the front of the scope. You will need this to see where the cross pattern emerges from the OTA.

 

3. Switch on the laser collimator and check you see a holographic cross projected onto anything in front of it. Insert it in the focuser, and wind it in as close as it goes to the back of the primary mirror - it doesn't have to be at the nominal focus position.  

 

Ideally you will see two crosses projected on the paper:

 

- an inner intense cross pattern emerging from the central baffle projected directly onto the paper - bypassing both the mirrors, the central baffle will limit this to being visible only just around the secondary;

 

- a larger cross spanning the full aperture of the scope, this has been reflected off both mirrors.

 

4. You should also be able to see a shadow of the secondary mirror on the paper, or in the case of the GSO CC, you may be able to see the spider vanes. Adjust the focuser tilt so that the projected cross is aligned with the centre of the secondary mirror (or the vanes).

 

5. Now look at the larger cross reflected off both mirrors. Adjust the primary mirror tilt to put this coaxial with the inner cross and the secondary mirror.

 

Lastly, set up the scope on a fine evening, aim the scope at a bright star, centre that in a high-power eyepiece and adjust the secondary alignment to get the diffraction rings concentric.

 

Thank you Luxo for your input on the collimation using Howie laser.

 

I'll give the artificial star collimation a try first and see if it works.

I may as well try your method if I could get my hands on one.

Many people seem to prefer lasers, above all, Howie Glatter.

I believe there is a good reason for that and I might become one of you guys using this laser eventually.

Though, there seems to be very little stock available since Howie passed away, so I highly doubt I could get one any time soon.



#11 luxo II

luxo II

    Vanguard

  • -----
  • Posts: 2,314
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 11 August 2020 - 09:11 PM

Should still be available - Howie Glatter's widow licensed the on-going manufacture of the collimator, though there may have been a hiatus in production.

 

OptCorp have a few in stock, so does AgenaAstro, skiesUnlimited, siderialtrading, a few more.... the trick is to find the holographic attachments - these are sold separately and ideally you need the one that projects a cross, though the square grid could be used too.

 

BTW I forgot - there is another method to collimate a cassegrain primary mirror with the mechanical axis of the OTA, which uses a point source of light placed at or near the centre of curvature (twice its focal length) of the primary mirror - you don't need an artificial star for this, an ordinary LED like those in a mobile phone will do fine.

 

The alignment method relies on being able to rotate (roll) the whole OTA around its mechanical axis. If you place a point source near the centre of curvature of the primary mirror it forms an image nearby, and when the OTA rotates you will see the image of the point source move. This is possible if the OTA is in rings, or alternatively and made a dingbat which the OTA can sit on, and rotate freely.

 

To do this:

 

1. place the OTA horizontally on a table in its tube rings or on rollers (I bought 4 small rollers like skate wheels and made a small dingbat) so it can rotate freely on the rollers) so it can rotate without any risk of it falling off the table. The support under the OTA (rings or rollers) needs to be firmly secured so it wont move sideways, vertically or tilt.

 

2. holding a small light source (LED on a phone is perfect) place this in front of the OTA with a piece of paper behind to catch the reflected image. Move these around till you find the centre of curvature such that the light source and the image are a few cm apart, Secure these on a support - I used Blutack to stick them to a wall, or you could rig up a tripod.

 

 3. With a marker pen, mark the position of the image on the paper.

 

4. Rotate the OTA 180 degrees. As it rotates you will probably notice the image moves in a circle which means the face of the primary mirror is not exactly perpendicular  to the mechanical axis of the OTA. 

 

5. With the OTA rotated 180 degrees from (3), again mark the location of the image.

 

Now adjust the primary mirror collimation to move the image so that it is midway between the two spots from steps (3) and (5). 

 

6. Rotate the OTA watching the image - if it does not move the primary is exactly perpendicular to the mechanical axis of the OTA. This method is quite sensitive too because with a reflection off the primary, any angular displacement of the image is twice the angular tilt of the mirror with respect to the OTA. If the OTA is reasonably accurately circular and the secondary accurately in the centre, this will be very close to being concentric with the secondary mirror.

 

You can also check the concentricity of the secondary mirror with the OTA as well - position an ordinary laser in front of the scope aimed at the centre of the edge of the secondary mirror, and watch where it strikes the secondary as the OTA is rotated - this will indicate any lateral displacement.


Edited by luxo II, 11 August 2020 - 09:49 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics