106 replies to this topic

[patrickt]

Hello All !

                  I wasnt sure if this belonged to the Beginners forum ( this is my first time to collimate a newt), but decided on this forum, since it is an old 100mm Vixen Super Halley newt.  Earlier on, i took out the primary mirror for a much needed cleaning.

 

                  I put a webcam in the focuser. So initially, a portion of the secondary mirror was not even visible. I tried the three little alignment screws behind the mirror, but it didnt hardly do anything to move the mirror-cell unit.

 

                 So i decided to fiddle withe the three external screws (in the pics), that lengthen or shorten each spider vane. I loosened two of them, then shortened the other. This did help to move the secondary mirror and make a portion of the secondary holder visible as well, but still way off center ! It seems no amount of further fiddling with the external screws can move the secondary to the center of the focuser tub.

 

               I'm not sure fiddling with the three external screws was the correct approach ? In fact you can see in the pics how loose two of the external screws are....  What do you guys think i'm doing wrong ?  Thank you,

Patrick

[patrickt]

Hello All !

[patrickt]

Here's a pic of the secondary for good measure

 

[deSitter]

Those too-large holes for the spider vanes are characteristic for this class of scope. You have to find a way to prevent the spider vanes from wandering in the holes before you start collimating it. I put nuts on the inside of the vanes so I could get a good grip from both sides on the tube metal. This worked perfectly and it never came out of collimation. So remove the spider and take it to the hardware store and find some hex nuts that fit the ends of the spider vanes. Remember to paint them flat black once they are in place.

 

-drl

[deSitter]

Or, you could put make a plastic sleeve for the shaft of the knurled nut that fills up the hole. I never understood why the holes are made too large this way. Could make the sleeve from heat-shrink tubing (used in electrical work).

 

-drl

[Woodsx]

The secondary holding vanes are used to center the secondary mirror in the middle of tube.  

You need to push the secondary forward.  That's done by loosing the center screw, (which will let the mirror rotate if loosened to much at one time), and push it forward using the outside screws of the secondary mount.

 

Here's a couple of good links, that really helped me out.

 

https://www.astro-ba...nian-reflector/

 

https://garyseronik....to-collimation/

 

Hope this helps.

[Spile]

Once you have sorted your spider, try my guide https://astro.catshi...limation-guide/

[patrickt]

DRL -

           Thanks for the suggestions.  Given the relative stiffness of using locknuts in the ota, i would try this first. I would only use the plastic sleeves if i'm unable to get properly sized nuts.

 

           So the large holes are normal huh !  You would think that Vixen would have come up with a more elegant (practical) way to put attention to details like these...

 

WOODSX -

                    I, for some reason, never picked up on loosening the center screw (just enough) to allow the mirror to move when turning the three little screws, so thanks for mentioning that. I will certainly check out those links. Thanks for responding, and for the links.

 

SPILE - 

              Thanks for your response. I will most definitely check out your guide.

[mpsteidle]

DRL -

           Thanks for the suggestions.  Given the relative stiffness of using locknuts in the ota, i would try this first. I would only use the plastic sleeves if i'm unable to get properly sized nuts.

 

           So the large holes are normal huh !  You would think that Vixen would have come up with a more elegant (practical) way to put attention to details like these...

 

WOODSX -

                    I, for some reason, never picked up on loosening the center screw (just enough) to allow the mirror to move when turning the three little screws, so thanks for mentioning that. I will certainly check out those links. Thanks for responding, and for the links.

 

SPILE - 

              Thanks for your response. I will most definitely check out your guide.

IMPORTANT NOTE!

If you are going to be messing with the center screw that holds the mirror, make sure to keep the scope HORIZONTAL!

[deSitter]

DRL -

           Thanks for the suggestions.  Given the relative stiffness of using locknuts in the ota, i would try this first. I would only use the plastic sleeves if i'm unable to get properly sized nuts.

 

           So the large holes are normal huh !  You would think that Vixen would have come up with a more elegant (practical) way to put attention to details like these...

 

WOODSX -

                    I, for some reason, never picked up on loosening the center screw (just enough) to allow the mirror to move when turning the three little screws, so thanks for mentioning that. I will certainly check out those links. Thanks for responding, and for the links.

 

SPILE - 

              Thanks for your response. I will most definitely check out your guide.

Yes, I've seen several iterations of those 4.5" reflectors with the tri-spiders and they all seem to have large holes and those knurled nuts that don't fit the holes. Mine had slots! In my case it was possible to shove the vanes as far back as they would go before collimating, so that if it came out, I just had to shove them back again. But that was not the best solution. It worked. 

 

-drl

[patrickt]

MPSTEIDLE -

                          Yes, i'll keep that in mind, thanks !

 

DRL -

            On closer inspection, i discovered i had the same slots as yours. It looks like it does give extra fore and aft movement for the secondary. 

 

 

 

 

Edited by patrickt, 17 May 2022 - 04:24 AM.

[deSitter]

Screenshot_20220517-172146_Messenger.jpgMPSTEIDLE -

                          Yes, i'll keep that in mind, thanks !

 

DRL -

            On closer inspection, i discovered i had the same slots as yours. It looks like it does give extra fore and aft movement for the secondary. 

Yup and it's one of the few cases where fundamental design errors are in a scope. But you can use the same strategy of shoving the spider vanes as far back as they will go before collimating. Note that if you overtighten those knurled nuts, it will distort the tube and that is also bad.

 

-drl

[patrickt]

    Okay guys, so i'm still working on centering my secondary mirror with the focuser, but still no dice.

 

    Given that the three nuts for the adjusting vanes actually go through slots, i pushed them all the way towards the direction of the primary mirror. It did move the secondary a bit closer to the center of the focuser tube, but still far away... 

 

      I also tried to adjust the length of each secondary holding vane to measure the same (there may be a 1mm or 2mm difference in length between them).    So after having done both these procedures, the secondary is still way far from the center.

 

       I then loosened the 3 tilt screws, and started turning the  central screw a few turns in both directions, but still could not see any real movement fore or aft. Is this normal ?  How much fore and aft movement should i expect when turning the central screw ? (actually, its gotten to the point when i can rotate the mirror). 

 

       Or should i keep turning (with my hand holding on to the secondary cell) ?  By the way,  which direction should i be turning the central screw to get the secondary moving towards the direction of the primary mirror ?

 

      Here is the best i could do after pushing the nuts fully in the slot,  towards the primary mirror. What am i not doing right ?? Your suggestions are always appreciated All.

Patrick

 

 

 

 

 

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Edited by patrickt, 18 May 2022 - 07:22 AM.

[deSitter]

You need to loosen the center post-screw a lot more - use your hand to get a rough check of when it approaches the right position under the focuser, then take up the slack by tightening the other 3 screws. It is a tedious, repetitive operation. But it only need be done once. I would disassemble the entire spider and examine it outside the tube to get a physical idea of how the screws operate. But, in a sentence, you use the center post screw to set the longitudinal position of the 2ndary under the focuser (loosen = toward primary, tighten = away), the the other 3 to control the tilt and lateral position under the focuser.

 

-drl

[patrickt]

Fantastic DRL ! 

             Your right of course? the best way to do this is to disassemble the spider. Its just that i am avoiding having to unscrew the vanes and begin measuring all over again.  That said, it is certainly a big help to know the correct direction to turn the center screw, thank you !   But if worst comes to worst, i will do just that (disassemble the spider).

 

        So while loosening the center screw, is it a good idea to use my other hand to put some light pressure (going towards the primary) on the secondary, just to help it along ? Or do i use the other hand just to support the secondary, in case i accidentally unscrew it off... Thanks so much.

Patrick

[deSitter]

Fantastic DRL ! 

             Your right of course? the best way to do this is to disassemble the spider. Its just that i am avoiding having to unscrew the vanes and begin measuring all over again.  That said, it is certainly a big help to know the correct direction to turn the center screw, thank you !   But if worst comes to worst, i will do just that (disassemble the spider).

 

        So while loosening the center screw, is it a good idea to use my other hand to put some light pressure (going towards the primary) on the secondary, just to help it along ? Or do i use the other hand just to support the secondary, in case i accidentally unscrew it off... Thanks so much.

Patrick

I had the same issues with my 4.5" (have, that one's coming home soon) - you only have to do it once. To me the worst part was the tiny components, not like on my 10" where I can use proper tools. You have that to look forward to

 

-drl

[SteveG]

Fantastic DRL ! 

             Your right of course? the best way to do this is to disassemble the spider. Its just that i am avoiding having to unscrew the vanes and begin measuring all over again.  That said, it is certainly a big help to know the correct direction to turn the center screw, thank you !   But if worst comes to worst, i will do just that (disassemble the spider).

 

        So while loosening the center screw, is it a good idea to use my other hand to put some light pressure (going towards the primary) on the secondary, just to help it along ? Or do i use the other hand just to support the secondary, in case i accidentally unscrew it off... Thanks so much.

Patrick

You hold it to prevent it falling off in case you run out of threads. Turn the center screw counter clockwise. Once it’s in the proper position, tighten the 3 tilt screws until it is firmly in place.

 

Can you post a pic of the open end of the tube looking at the complete secondary assembly?

[patrickt]

Ok i got it, thanks !  A couple of pics

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[patrickt]

     So i finally got the hang of adjusting the secondary mirror screws, or thought i did...

 

     And this is what i did (thanks DRL !)

 

1) I loosened the 3 secondary Tilt screws, which enabled me to turn the Central screw counterclockwise, enough to get the secondary mirror to move towards the primary mirror end, and thus get it centered under the focuser tube (as seen from left to right in my pic). I also rotated the secondary to achieve the best (or closest) circular image i could get.

 

2) I then worked the 3 Tilt screws to further get the secondary as circular as possible.

 

So i've pretty much gotten the secondary centered on the horizontal axis (in the pic). But i am not clear how to center it in the vertical axis (it is lower, as seen in the pic).   Any  advice/tips on doing this ?

 

Sorry about the bad contrast in the pic, i only pointed it near a bright lamp. Thanks again !

 

      

 

 

 

 

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[KBHornblower]

I would do whatever is necessary with the spider assembly to center the mirror in the tube in the up and down direction as seen in #19.  If that does not center it properly as seen in the focuser, I would try tilting the focuser with shims.  I cannot be more specific in detail without examining the scope in person.

[Mike Spooner]

Once you get the spider secure so it doesn’t move you can use thin washers under the appropriate focuser screws to tilt it to finesse the centering of the secondary. There is nothing sacred about the angle of the secondary especially for visual use.

 
Edit: looks like I’m a couple of minutes late to the party! LOL

 

Mike Spooner

Edited by Mike Spooner, 19 May 2022 - 10:48 AM.

[Vic Menard]

   ...So i've pretty much gotten the secondary centered on the horizontal axis (in the pic). But i am not clear how to center it in the vertical axis (it is lower, as seen in the pic).   Any  advice/tips on doing this ?

Generally speaking, errors in the "vertical axis" are combined tilt/rotation errors. But you need to begin with two important steps. First, all images you post here need to be taken through a centering tool--a simple collimation cap will do. Second, you need to be able to see the primary mirror reflection. Your white paper is OK against the inside wall of the tube assembly opposite the focuser, but it shouldn't obscure the primary mirror.

 

At f/10, you'll want to make everything in the focuser look concentric--the bottom edge of the focuser, the actual edge of the secondary mirror, the reflection of the primary mirror, the reflection of the secondary mirror, and the reflection of the underside of the focuser. To make this happen the spider vanes should be the same distance from the front edge of the tube assembly and the same length from the tube wall to the spider hub. The secondary mirror should be centered in the tube  assembly and the focuser should be reasonably "square" to the tube assembly. Given these starting parameters, once you've set the correct length of the secondary mirror center mounting screw (which appears to be pretty close), the three tilt adjustment screws should be more or less "level". 

 

The next logical steps are rotating the secondary mirror to center the primary mirror reflection in the secondary mirror, and then adjusting the secondary mirror tilt to tune the centering. Then post a picture (through a centering tool!) and we'll sort out the rest.

[patrickt]

KB HORNBLOWER -

                                     Thanks for responding !

 

     When you say to do whatever is necessary with the spider assembly, to center the mirror in the tube in the up-down direction in my # 19 pic, did you mean it would be ok to adjust the vanes to the point where they would not be of equal lengths ? 

 

MIKE SPOONER -

                                You said " there is nothing sacred about the angle of the secondary, especially for visual use". 

 

       So are you implying that not getting the so called "ideal" angle is acceptable for visual use (I tend to do planetary stuff, but minimally dabble in planetary imaging ) ?  And thanks for responding !

 

 

VIC MENARD -

                             Lots of great information ! Thank you !     I did use a diy collimation cap. I only have a cellphone and a standard webcam, but unfortunately (due to their design) placing them right by the collimation cap hole gives parallax error views.   

 

             I realize the importance of the collimation cap, and did compare visual views through it and with a webcam inserted in the focuser tube, and they both easily showed the same "vertical" position discrepancy in my #19 picture, hence my having posted this pic. On your advice, i will try though to find a way to get a proper image through the collimation hole.

 

    I understood that placing the white paper to block the primary mirror was a good idea, to better see the secondary mirror position under the focuser...??

 

    I did measure the spider vanes distance from the tube hub, but not from the front edge of the tube assembly ( which i will now check on your say).

 

     I will let you all know how it goes after i start off with Vic Menard's recommendations.Thank you All !

Patrick

[Vic Menard]

1.) I did use a diy collimation cap. I only have a cellphone and a standard webcam, but unfortunately (due to their design) placing them right by the collimation cap hole gives parallax error views.  I...did compare visual views through it and with a webcam inserted in the focuser tube, and they both easily showed the same "vertical" position discrepancy in my #19 picture, hence my having posted this pic. On your advice, i will try though to find a way to get a proper image through the collimation hole.

 

2.) I understood that placing the white paper to block the primary mirror was a good idea, to better see the secondary mirror position under the focuser...??

 

3.) I did measure the spider vanes distance from the tube hub, but not from the front edge of the tube assembly ( which i will now check on your say).

1.) If your webcam shows the same alignment as your collimation cap, then they both can be used as a "centering" tool. It's important that the view is from a point centered above the focuser (focal or apex makes no difference at this point as long as you can see the bottom edge of the focuser, the actual edge of the secondary mirror, and the reflected edge of the primary). 

 

2.) While blocking the reflections in the primary mirror can minimize the "noise" of the other reflections while you're trying to sort out the secondary mirror placement, it's important to still be able to see the reflected edge of the primary mirror (one of the three circles noted above). If you could easily fashion a 4-inch white paper disk that you could fit on top of the primary mirror (with a center spot would help!), that would eliminate the other reflections and still show the edge of the primary. Of course, with a long one-piece OTA, that's impractical, so I just suggest leaving the primary mirror unblocked.

 

3.) Checking the spider from the front edge of the tube should make the spider more-or-less perpendicular to the tube axis. These preliminary "mechanical" adjustments are based on many "assumptions", which will be sorted out as the secondary mirror placement is adjusted (fore and aft, rotation, and tilt).

[KBHornblower]

KB HORNBLOWER -

                                     Thanks for responding !

 

     When you say to do whatever is necessary with the spider assembly, to center the mirror in the tube in the up-down direction in my # 19 pic, did you mean it would be ok to adjust the vanes to the point where they would not be of equal lengths ? 

 

I said to do whatever it takes, and that I cannot be more specific without examining the parts in person.  I cannot tell how accurately the mirror is positioned relative to the hub.  If it is off a bit with no means of correcting it, the by all means I would settle for unequal vanes to get the mirror where it needs to be in the tube.  I would want the optical axis nearly enough centered in the tube to keep the front edge of the tube from encroaching on the incoming light.