Secondary Mirror with 4 straight-vane spider

The Secondary Mirror is supported in an adjustable holder by a 4-vane spider. The vanes are thin but sturdy metal so as to reduce obstruction of the light and image entering the telescope tube.

Some diffraction spikes are noticeable at lower magnifications or with wide-angle eyepieces, although this is to be expected with Newtonian designs, simply because of the placement of the secondary mirror and supporting spider vanes in the path of light. All spiders will cause diffraction. In a normal, straight, four-vaned spider, the diffraction will be in the form of four distinct spikes eminating from any bright object.

Click here for more information on diffraction spikes, the secondary mounting spider, and other types of vanes to help reduce diffraction spikes.

 

Secondary Mirror Collimation

Secondary MirrorAdjustments to the Secondary are by means of three long 2mm Allen (hex) key set screws in the centre hub of the spider, and one central spring-loaded tensioning pivot bolt.

 

Secondary HubThe 3 set screws adjust the tilt of the mirror. Remember that as you adjust any one of the screws, the other two opposite are acting like a hinge.

The tensioning bolt adjusts the distance of the secondary from the primary, and this is used to align the secondary with the focuser tube.

 

 

 

Precautions

Note that correct allignment of the Secondary should be confirmed, before attempting to align the Primary.

Wash your hands before you start! You may need to grasp the mirror support stub while making adjustments, but if you accidentally touch the mirror surface itself you do not want any grease from your hands getting on it.

Try to avoid touching the mirrors entirely because even after washing, your fingers will still make imprints. Note: If you do accidentally make finger marks on the mirrors, leave them alone - DO NOT TOUCH THEM! Trust me, unless you plan to fully and properly clean the mirror, you will make more of a mess by doing half a job, and you run risk of scratching the mirror.

Secondly, ALWAYS POSITION THE TELESCOPE HORIZONTALLY WHEN COLLIMATING THE SECONDARY, because if you accidentally drop the Allen key or screwdriver, or undo the secondary central bolt too far, this will prevent anything dropping down onto the primary mirror which could have disastrous consequences!!

On some occasions I strap the main OTA tube down using a bungee cord, but for work on the secondary I find it better to have the OTA level, and to tighten the Altitude friction knobs to hold the tube steady.

Adjusting the Secondary Collimation

It is not often that you will need to adjust the Secondary, as it should have been aligned at the factory.

The collimation process for the Secondary is described in the instruction manual, but the following notes are intended to provide additional detail.

Better still I highly recommend you view Andy's Shot Glass page on Collimating a Newtonian. This provides a 5 minute multimedia presentation showing exactly how you go about adjusting the telescope. This should help you understand and follow along with my own notes below.

Generally if the Secondary is already fairly well collimated it is not necessary to undo the central bolt, adjustment being achieved simply with the 3 set screws. However if you think the placement of the Secondary in relation to the focuser drawtube could be better, then the following notes will be useful.

Unfortunately for me, during delivery of my scope the screws can't have been tight enough and vibration had loosened them so much that the Secondary mirror was literally "dangling" and "flopping" about! Much further and the whole unit would have fallen out and god only knows how much damage could have been done. So as a matter of urgency I had to learn how to collimate my scope before I had any hope of using it properly - and I did not have a Laser Collimator at the time.

This page assumes use of just the Collimation Cap provided with the scope, but a Laser Collimator makes the process much easier because you can remain at the end of the tube looking down while adjusting.

The Collimation Cap is shown in the picture to the right, at the bottom. It has a small viewing hole in it, and a highly reflective surface on the inside of the cap.

 

The actual collimation of the Secondary is quite fiddly without a laser collimator as you have to do a lot of swapping between being at the front of the scope to change the Allen key into a different set screw to adjust, then at the side looking through the collimation cap.

Here is a view through the focuser without any eyepiece or collimation cap. What you are seeing is the bright light from the roof of our conservatory shining down into the telescope, and being reflected out of the focuser, which is why it looks a little odd.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Secondary ScrewsFirst you need to achieve centralisation of the Secondary mirror to the focuser drawtube (also the distance away from the Primary mirror), using the central bolt to loosen or tighten against the spring. Clockwise will pull the Secondary nearer the tube front opening, while anti-clockwise pushes it nearer the Primary mirror.

If undoing the screw to move it nearer the Primary be careful to grasp the Secondary firmly in case the bolt comes to the end of its travel and disengages from the Secondary. If this happens and you aren't holding it, the spring may come off, the mirror will drop and could get damaged. There is plenty of travel before this happens, but be warned! (You will need to do this if ever you need to completely remove the Secondary for inspection or cleaning. The mirror itself is quite a heavy little fellow - so be prepared). A soft cloth folded and placed underneath the Secondary inside the main telescope tube would help prevent any possible mishaps.

 

 

 

 

Central Alignment to Focuser Drawtube

Aligning the Secondary centrally to the drawtube requires slackening off the 3 set screws a bit (by equal amounts), and once loose the mirror wobbles about on the centre pivot bolt. In fact you can hold the Secondary mirror (by its stub) with your left hand reaching round into the front of the tube, simultaneously looking through the collimation cap, so even though it is wobbling you can test its position, and rotate it laterally, while viewing through the little hole in the colli-cap to get a feel for how much to adjust the centre bolt. At this point you are deciding how much, to the left or right, the mirror needs to move.

After determining how much to adjust, go to the front of the scope again and use both hands to actually make the adjustment. Although the spring is intended to maintain pressure on the mirror stub, it is not particularly strong, and it is better to support the weight of the mirror with one hand pulling gently away from the vanes to keep tension on the bolt, while you tighten or loosen the central bolt with your screwdriver hand. (Ignore the 3 set screws for the time being, which should be loosened off enough not to impede adjustment of the central bolt).

You might need to repeat this viewing, then adjust, view again, adjust, procedure a few times until you are happy that the circular reflection of the Secondary mirror is lined up perfectly central horizontally in the focuser tube, with your view through the hole in the colli-cap.

Adjusting the 3 Set Screws

Once you have the "in-out" (left-right) distance about right with the central bolt, you can then re-tighten each of the 3 set screws by equal amounts. It is quite important to establish a good even (un-tilted) starting position by adjusting them equally until they are just beginning to exert pressure on the stub of the mirror.

Tip: To help ensure the set screws are being adjusted equally, undo all 3 completely until their heads are exactly flush with the surface of the hub. Then do them up again with the Allen key, counting exactly how many turns you do, i.e. insert the Allen key with its bar facing at 12 o'clock, then go through complete turns noting how many times you pass the 12 o'clock position (don't lose count!).

For my scope, after adjusting the "in-out" centre bolt, I found that each set screw required about 10 whole clockwise turns before they were getting close to the metal stub of the secondary, ready for fine tilt adjustment. At this point you then want to tighten each screw one by one in turn, only by one whole turn, or half turns until you can just see contact or feel resistance as they butt up to the mirror stub. I think this took about 2 more turns, so in effect about 12 turns in total.

Using this method you will ensure the Secondary mirror stub is exactly parallel to the main tube, before making the final tilt adjustments to visually collimate the centre mark on the Primary mirror in the centre of the Secondary (easier with a laser collimator).

Making the final fine tilt adjustments is again a bit of an art! As well as adjusting the tilt you need to make sure the lateral rotation of the mirror is correct as you tighten everything.

Looking through the colli-cap and focusing your attention on the circular centre-mark on the primary mirror, you have to carefully consider which direction the secondary mirror needs to tilt, then choose the correct set screw to adjust. Insert the Allen key into the chosen screw, then tighten while looking through the colli-cap (and also noting by how much of a turn).

What I found was that the pressure of my hand while tightening the Allen key tends to flex the whole central hub on the vanes, so you have to gauge how much to tighten, push past that a little bit, and then release the pressure to see how much you have actually affected the alignment. Repeat for the other screws depending on required direction. Be careful not to overtighten, as this could either bend the vanes or stress the set screws. You may need to slacken off a screw, in preference to tightening one of the others.

Finally, tighten the central tensioning bolt just a little more to make sure everything is secure. This will pull the mirror stub hard against the 3 set screws, and ensures a firm contact which should keep the secondary collimated well against light knocks during transportation. Re-check the alignment, and if it could be better, loosen off and tweak again if necessary.

The reason why I recommend doing it this way is so that you get the whole Secondary unit "nice and tight" and this should prevent any loosening or movement in the future. The Secondary should not require aligning again after this, whereas it is common practice to check and collimate the Primary mirror every time you use the telescope.

Sounds tricky, but its just a case of trying it. To collimate my Secondary, I tried the method first one day, then a few days later I wasn't entirely happy because I noticed the 3 set screws were at uneven projections, and although the Secondary and Primary seemed visually collimated, the Secondary mount looked skewed, so I repeated the whole process, spending about half an hour to do it slowly and carefully and refine my technique as described above.

Collimation is nothing to be afraid of. As long as you are careful it is unlikely you will damage anything. I have read many forum messages where people shy away from collimating their telescope. They seem to assume it doesn't matter, or the factory got it right, and they need not do any more.

Collimating all adjustable items is an essential part of getting the best out of the scope, so the sooner you get to grips with it, and practice, the better you will know your scope, and be confident of how to do it. It's just simple mechanics - so master it!

Primary mirror collimation is certainly something you should be happy about doing regularly, and in the dark.

Secondary collimation is more of a once-in-a-while workshop procedure.

 

Removing/Replacing the Secondary Mirror

Occasionally you will need to remove the Secondary Mirror. For example to clean the mirror, or when making modifications to the main telescope tube which require clear access into the tube.

The chrome-plated thumbscrews are simply unscrewed, and once you get near to unscrewing all of them, grasp the secondary firmly so that you can withdraw it safely from the tube without risk of it dropping down onto the primary mirror, or falling out of the tube front, depending on how you have the tube angled.

The more important aspect is replacing the Secondary correctly after you have finished any work.

Have the thumbscrews within easy reach as you return the secondary back into position through the mounting holes in the tube, ensuring the angled face of the mirror is facing the focuser drawtube, and put each thumbscrew on by only one or two turns, so that each one is on securely enough not to fall off.

Next do just one turn on one thumbscrew, then rotate the tube say clockwise by 90° to do up the 2nd thumbscrew by just one turn, rotate clockwise 90° again and do up the 3rd thumbscrew by one turn, then again for the 4th thumbscrew. Continue rotating and tightening each thumbscrew by one turn each time until all of them are good and tight, and are tensioning the spider vane.

When I say "one turn" it does not have to be exactly one turn. This is difficult to tell because there are no markings on the thumbscrews to gauge it by (unless you use a permanent marker to put a dot on each thumbscrew, or a piece of tape; I prefer to just rely on my fingers turning accurately). So instead just make sure your fingers turn each thumbscrew by the same amount each time. The object of this task is to help centralise the mirror, by doing up each screw by measured equal amounts.

This method will ensure that the Secondary Mirror is mounted central within the telescope tube. This is another important aspect of collimation.