Primary mirror defect?

[wolfpaw]

Checking and adjusting the primary mirror clips takes just a few minutes and then you will be sure the mirror is optimised. Coma will happen in a f4.7 'scope with medium and low power EPs. I mostly go 170x upwards for Jupiter and just about everything else and don't notice any coma. A CC must be the way to go but coma doesn't bother me that much.

But how can it not bother you if coma is spread over 50% of the FoV? It makes a total mockery of using even remotely wide-angle eyepieces if half of the view through them is affected by coma.

[Reeny]

The MaxVision eyepieces have very good light transmission

Giving more risk of starbursts, flares, astigmation, and noticing elongation.

My scope was good to go straight out of the box, until I got the 24mm and 16mm MaxVision EP's.

I had no astigmation with my Plossls - but had massive errors with the 24mm due to the glare and light scatter, (less so with the 16mm)

The first thing I did was collimate the scope to fix the problem.

Result = less starbursts from the spider, no noticable Coma, and gone forever was the awful astigmation.

If you don't want to collimate, or check your mirror for pinching then that is fine, it is your decision.

[Karen]

But how can it not bother you if coma is spread over 50% of the FoV? It makes a total mockery of using even remotely wide-angle eyepieces if half of the view through them is affected by coma.

My EP collection was originally bought for my 5 inch f11 maksutov. If I put a 13mm wide angle eyepiece in the 250px and get bad coma I know it is not the fault of the 'scope, it is my fault for not buying very expensive medium power wide angle eyepieces! There is a much cheaper alternative of course, a coma corrector. There is no coma at high power and it is a 10 inch reflector which blows the 5 inch maksutov away with it's performance, even with a bit of coma.

[Moonshane]

everything's a compromise one way or another.

for eyepieces choose any two from three (but never three)

cheap - wide field - top quality

[wolfpaw]

The MaxVision eyepieces have very good light transmission

Giving more risk of starbursts, flares, astigmation, and noticing elongation.

My scope was good to go straight out of the box, until I got the 24mm and 16mm MaxVision EP's.

I had no astigmation with my Plossls - but had massive errors with the 24mm due to the glare and light scatter, (less so with the 16mm)

The first thing I did was collimate the scope to fix the problem.

Result = less starbursts from the spider, no noticable Coma, and gone forever was the awful astigmation.

If you don't want to collimate, or check your mirror for pinching then that is fine, it is your decision.

You wouldn't expect to get much "noticeable coma" in an f.6 telescope anyway. An f4.7 telescope however apparently has 50% coma built into its optics. In other words you can collimate it within an inch of its life and it would still show significant aberration across a significant portion of the field of view.

[wolfpaw]

My EP collection was originally bought for my 5 inch f11 maksutov. If I put a 13mm wide angle eyepiece in the 250px and get bad coma I know it is not the fault of the 'scope, it is my fault for not buying very expensive medium power wide angle eyepieces! There is a much cheaper alternative of course, a coma corrector. There is no coma at high power and it is a 10 inch reflector which blows the 5 inch maksutov away with it's performance, even with a bit of coma.

This isn't correct, is it? Surely what your cheap, poorly-corrected eyepieces were showing was off-axis astigmatism. As I understand it, coma is almost entirely a product of the telescope's optical train and has nothing to do with eyepieces. In fact good quality eyepieces seem to show more coma merely from the fact that the coma isn't being masked by the astigmatism of poor quality eyepieces.

[John]

This isn't correct, is it? Surely what your cheap, poorly-corrected eyepieces were showing was off-axis astigmatism. As I understand it, coma is almost entirely a product of the telescope's optical train and has nothing to do with eyepieces. In fact good quality eyepieces seem to show more coma merely from the fact that the coma isn't being masked by the astigmatism of poor quality eyepieces.

Thats just how I understand it as well.

Coma and astigmatism, their causes and solutions, are the subject of some confusion in many threads on here with the terms becoming almost interchangeable at times, which is incorrect of course.

[wolfpaw]

Finally received a reply from Skywatcher after I sent them a long and detailed email recounting what I had been experiencing with the telescope, which eyepieces I was using, and whether 50% coma was within acceptable parameters.

The reply was:

If the aberration is too severe. You can consult your local distributor about service or replacement. Please use this link to find your local distributor.

Isn't that nice and helpful? It must've taken someone all of 10 seconds to tap that out before pressing 'send'.

I removed the primary mirror this morning to check the mirror clips. They were touching the mirror, some slightly more than others, but didn't seem excessively tight. I loosened them all off so the mirror could just about rotate freely within the cell. I'm going to look at the focuser and secondary mirror this afternoon.

[wolfpaw]

I removed the secondary mirror and checked the focuser which, as far as I can tell, seems pretty square.

But I'm disgusted with the state of the secondary mirror. The primary had some small dust motes on it, as one would expect from a four-month-old telescope, but the secondary is filthy. I wish I'd checked it as soon as it arrived as I cannot believe it has got into this state with limited usage over the last four months or so:

[Moonshane]

depending on where stored and used, open systems like newtonians do get dust on them from time to time and this builds up.it will have no perceptibe effect on the views and can be washed off with care - make sure you read up on this if you decide to do it as it's easy to scratch coatings.

[AlexB67]

I just washed my SW 10 inch primary mirror for the first time over the weekend. Very easy to remove and straight forward to clean I found.

 

Just follow this tutorial

 

 

 

Mine is getting on 5 - 6 months now  I would not normally have done it, it still looked clean enough overall bar a bit of dust, but for some reason what appeared to be like a centimetre circle of sticky goo on it made me want to do it. I suspect it may have happened recently when a few rain drops fell on it during one session.  Overall it did look not as bad as in your picture ( I store mine inside the house) There again, pictures sometimes can make it look worse due to lighting. 

I would say what you have now is perfectly fine still for use, but while you have it out you can give it go. Rinsing it under warm water and post rinsing with distilled made it look pristine again in my case, that spot of goo just washed off easily thank goodness.

[wolfpaw]

depending on where stored and used, open systems like newtonians do get dust on them from time to time and this builds up.it will have no perceptibe effect on the views and can be washed off with care - make sure you read up on this if you decide to do it as it's easy to scratch coatings.

I'm surprised the secondary has gathered around 100x more dust than the primary given that it's the primary that faces upwards toward the end of the tube.

[wolfpaw]

The focusing bar/knobs aren't square with the rest of the focuser. Is this normal in a SW telescope?

[Karen]

Just had a look at my 250px and the focuser wheels are not lopsided. But if you look at your focuser the tongue at the bottom has a bigger gap on the right side than the left as if the focuser barrel needs rotating anticlockwise a bit.

Is this a 250px? I thought they were all metallic black with white fittings.

[Karen]

Correction. The solid tubes are white

[wolfpaw]

Just had a look at my 250px and the focuser wheels are not lopsided. But if you look at your focuser the tongue at the bottom has a bigger gap on the right side than the left as if the focuser barrel needs rotating anticlockwise a bit.

Is this a 250px? I thought they were all metallic black with white fittings.

I think the Skywatcher collapsible Dobs are black with white fittings. The SW Skyliner solid tube Dobs are white with black fittings.

I'll have a fiddle with it and see if I can get the bar centralised. I just wanted to know first if it was a normal off-set.

[wolfpaw]

I just washed my SW 10 inch primary mirror for the first time over the weekend. Very easy to remove and straight forward to clean I found.

Just follow this tutorial

Mine is getting on 5 - 6 months now  I would not normally have done it, it still looked clean enough overall bar a bit of dust, but for some reason what appeared to be like a centimetre circle of sticky goo on it made me want to do it. I suspect it may have happened recently when a few rain drops fell on it during one session.  Overall it did look not as bad as in your picture ( I store mine inside the house) There again, pictures sometimes can make it look worse due to lighting. 

I would say what you have now is perfectly fine still for use, but while you have it out you can give it go. Rinsing it under warm water and post rinsing with distilled made it look pristine again in my case, that spot of goo just washed off easily thank goodness.

I was just surprised it was so dirty, and the photos I posted show the mirror after I'd cleaned it with a blower brush. The rest of the stuff was stuck onto the surface. I've only used it a few times when the dew has been heavy and given how the secondary points down the tube towards the primary I'm amazed that it has got into such a state. The telescope is always stored inside the house and, after use,  left overnight with the OTA tilted downwards, the dust cap off and covered in a sheet.

Having taken out of the primary and the secondary I would say that putting it all back together is really very easy and nothing for anyone to be too worried about. The telescope is now completely re-collimated.

[Veil]

I've had a few outings with my new 10" flextube. With the standard 25mm eye piece i do see some abberation, though not as much as you appear to.

With my Hyperions though - 8mm and 21mm - i barely notice any coma.

Admittedly, my novice eyes are untrained. Perhaps i will notice more over time but at the moment I'm not detecting the problem to any great extent.

[wolfpaw]

I've had a few outings with my new 10" flextube. With the standard 25mm eye piece i do see some abberation, though not as much as you appear to.

With my Hyperions though - 8mm and 21mm - i barely notice any coma.

Admittedly, my novice eyes are untrained. Perhaps i will notice more over time but at the moment I'm not detecting the problem to any great extent.

I suspect my novice eyes are even more untrained. I only had a 60mm refractor before getting the 250PX. Needless to say, checking the primary mirror clips and the secondary made zero difference to what I was seeing. I'm now in communication with the retailer.

[Dreadz]

Did you check the donut on the primary was dead centre? I've been constantly trying to perfect collimation on my 250px, spending more time on each element to try to make sure i've got it right but each time i do, my viewing experience tells me something is still not quite right. I'm left with the conclusion that the only thing left for me to check is the position of the donut on the primary. Have seen several reports from people on here saying that the donut was 2-3mm off centre so have ordered a template and hotspot to check and replace if necessary

[Uplooker]

I would suggest that the doughnut being 2-3mm off centre would have no effect on collimation, provided that after using a laser or Cheshire, you finish off with a star test to get it spot on - no pun intended ;-)

Ian

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