Primary mirror centre spot

[SteveA]

My Quattro has been living outside in my obsy for several years now and on checking the collimation the other day I was surprised at just how filthy the mirror had become. So I'm going to remove it and give it a nice wash. I've not removed this one before and so have never actually checked how well placed the centre spot is, I don't really have any reason to think it might be off but if I end up re-spotting I was considering using maybe a triangular sticker, like those available from "Cats eye". I've no funds for the expensive kit that Catseye produce, but I might just stretch to a template and triangular shaped centre spot. Is this a change worth making, irrespective of whether the spot needs changing? I realise that the non circular spots have a function in these expensive collimation kits, but does it help at all when collimating using more traditional methods.. ie. with a Cheshire etc.

Steve

[big_scot_nanny]

I believe it might provide some benefit if there is a way the triangle can be positioned in such a way that it points to the relevant colimation screws? Especially if you are not able to see collimation changes whilst you are at the back of the scope.

 

I am sure someone else could give a better response, but hope that makes some sense.

[triton1]

Yep if I was to centre spot the mirror with a triangle I'd have the points directed at the collimating screws.

[SteveA]

3 hours ago, big_scot_nanny said:

I believe it might provide some benefit if there is a way the triangle can be positioned in such a way that it points to the relevant colimation screws? Especially if you are not able to see collimation changes whilst you are at the back of the scope.

 

I am sure someone else could give a better response, but hope that makes some sense.

That's a pretty good point...and now you mention it I feel slightly embarrassed that it hadn't occurred to me!  I was mainly thinking that whilst collimating everything is circular and that introducing a non circular spot on the mirror would maybe help the brain a bit. I think I may give it a go...thanks

[tony210]

Hi - this all seems fair but a supplier pointed out that the only way to accurately collimate is with a star test as the optical and geometric centers can be off!- still good to have a geometric center and perhaps a small point but interesting I think- Tony.

[Peter Drew]

You could always rotate the primary mirror if spotted with a triangle, to make sure it lines up accurately with whichever collimation method.   

[SteveA]

Does anyone know how easy it is to remove the existing centre spot? I seem to remember that when I cleaned my previous newtonian mirror, the spot survived the washing process, so clearly its not just going to come off with water. Is there not a danger of damaging the mirror coating if using a solvent?

Steve

[big_scot_nanny]

Aye, perhaps, but I'm not sure it matters as it's in the shadow of the secondary so not playing any role in capturing and focussing photons. 

I suppose you could damage the coating in such a way that it might cause some chain reaction failure on the rest of the mirror, but unlikely.

Crack on, I'd say :-)

last time I did this I picked it off with my fingernail, and replaced with new one (ensuring centered using paper template of course).

[tony210]

I agree - light travels in straight lines and this is the one area of the mirror that isn't used because it is obscured by the secondary - even if the optical centre is off this spot the difference is very small so I wouldn't worry too much - just don't touch the rest of the mirror surface-I had a great 8inch mirror with a small side chip - once painted matt black it was optically irrelevant and the mirror was a cracker and performed brilliantly

[SteveA]

Thanks guys....having now ordered a Catseye triangle and template, I'll hopefully be respotting the mirror this weekend.Martin at FLO pointed me in the direction of a thread on SGL, where Dion suggests using nail varnish remover. I subsequently found on of his Astronomy Shed videos on this exact subject?. I do take your point about the centre of the mirror not being critical, but I still feel nervous about using a solvent, so,I think I will at least attempt the finger nail picking approach to start with and see how that goes before trying a solvent....fingers crossed!

 

[SteveA]

I've subsequently discovered that the removal procedure for the existing centre spot is covered in the Catseye documentation for spotting the mirror with the Catseye triangle etc.

http://www.catseyecollimation.com/Spotting_hotspot.pdf

Steve

[SteveA]

Catseye triangle and template has arrived today from FLO. Very please as I only placed the order yesterday afternoon...so yet again, well done FLO.

I fear a nerve racking weekend ahead :-(

Steve

[N3ptune]

On 8/22/2017 at 08:10, SteveA said:

. So I'm going to remove it and give it a nice wash. I've not removed this one before and so have never actually checked how well placed the centre spot is, I don't really have any reason to think it might be off Steve

If the collimation was good before and the existing sticker is still holding tight, why replace it? It's like going to the garage and replacing a good part on the car..

[big_scot_nanny]

Wow, that was fast - and that's a helpful looking tool - might get one of those for the wee dob after the next mirror cleaning. Let's us know how it goes.

[SteveA]

3 hours ago, N3ptune said:

If the collimation was good before and the existing sticker is still holding tight, why replace it? It's like going to the garage and replacing a good part on the car..

The mirror definitely needs cleaning, so it's coming out of its cell and will obviously need a major recollimation afterwards. This seems like an opportunity to maybe improve the collimation process and changing the donut for a triangle seems like a way of doing this. At the end of the day I'm hoping this makes life easier?

Steve

[N3ptune]

On 8/24/2017 at 17:20, SteveA said:

The mirror definitely needs cleaning, so it's coming out of its cell and will obviously need a major recollimation afterwards. This seems like an opportunity to maybe improve the collimation process and changing the donut for a triangle seems like a way of doing this. At the end of the day I'm hoping this makes life easier?

Steve

Then enjoy the process (:

[iapa]

On 22/08/2017 at 13:10, SteveA said:

My Quattro has been living outside in my obsy for several years now and on checking the collimation the other day I was surprised at just how filthy the mirror had become. So I'm going to remove it and give it a nice wash. I've not removed this one before and so have never actually checked how well placed the centre spot is, I don't really have any reason to think it might be off but if I end up re-spotting I was considering using maybe a triangular sticker, like those available from "Cats eye". I've no funds for the expensive kit that Catseye produce, but I might just stretch to a template and triangular shaped centre spot. Is this a change worth making, irrespective of whether the spot needs changing? I realise that the non circular spots have a function in these expensive collimation kits, but does it help at all when collimating using more traditional methods.. ie. with a Cheshire etc.

Steve

I've just ordered the template and the triangular centre spot - at least the template will let me check that the existing spot is central.

[iapa]

On 24/08/2017 at 22:20, SteveA said:

The mirror definitely needs cleaning, so it's coming out of its cell and will obviously need a major recollimation afterwards. This seems like an opportunity to maybe improve the collimation process and changing the donut for a triangle seems like a way of doing this. At the end of the day I'm hoping this makes life easier?

Steve

I've just been stripping and collimating my Quatro 10" CF - only thing not done was the centre spot.

I started a couple of weeks ago taking small steps - Im a wimp  - in this thread;

 

Hoping you get done faster