GSO 12 Newtonian -How much the screws of secondary mirror is out ?

[jsmoraes]

I am having problem of astigmatis with my GSO12. I am thinking that the offset of secondary mirror can cause this. Therefore I would like to know how much mm the screws of adjustment  is out on your GSO 12. With this infomation I can check my adjustment here.

I know that there are more accurate method: offset = secondary size / (4 x focalration).

And I am not sure if the offset of secondary mirror can cause astigmatism. 

That information is only a fast check for me.

Thank you, mates.

[Dom1961]

I think I read that astigmatism is caused by over tightening and therefore warping the primary slightly

[jsmoraes]

Thank you, dom.

Yes, that is the main source. The bad optic mirrors, also. I checked the screws of primary. And they are ok. I found in the internet some texts talking about the possibility of secondary mirror cause astigmatism. For me, this is possible if it is due to offset, since it is not easy adjust it after washing the mirror.

I am not sure if it can cause this issue. I would like to do a fast check by comparison with another GSO 12. The postion of screws will tell me if I am too much out of the offset.

[Dave In Vermont]

The offset of the secondary mirror depends on the F-ratio of the primary mirror. A fast Newtonian - say F/5 and less - will appear way off, but be perfectly fine for the instrument to function precisely as it should. If you are concerned about the screws holding the secondary, you can simply back them all off and run a normal collimation of the telescope.

Collimation of a Newtonian reflector is often a confusing process for those new to it. Many, if not most, guides to collimation are murky and poorly written for a variety of reasons. As a result, people often end up thinking something terribly wrong has taken place. So they embark on trying to fix a situation that hasn't happened. All that was needed was to collimate the scope. So, without further ado, here's a link to one of the very best collimation-guide's available. And the author of this is a member here:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Within you will find the offset situation with 'fast' Newtonians is well-covered. And with excellent pictures too!

Do keep us informed, please,

Dave

[Jason D]

To OP, astigmatism can't be caused by secondary offset. The cause could be in the secondary mirror, the primary mirror, or your vision. 

If it is your vision then astigmatism will track your head as you move/rotate it

If it is your primary mirror then astigmatism will track your primary mirror -- rotate it in the cell and retest

Make sure the primary mirror clips are not pressing on the primary mirror edge.

Jason

[Uranium235]

Like above, it may be the primary mirror clips being done up too tight. There needs to be something like a cigarette papers depth between the clip and the mirror, the offset of the secondary is (in my experience) automatically set when you are rounding/centering the secondary under the drawtube.

[ollypenrice]

To OP, astigmatism can't be caused by secondary offset. The cause could be in the secondary mirror, the primary mirror, or your vision. 

If it is your vision then astigmatism will track your head as you move/rotate it

If it is your primary mirror then astigmatism will track your primary mirror -- rotate it in the cell and retest

Make sure the primary mirror clips are not pressing on the primary mirror edge.

Jason

What an excellent point! I had never thought of this.

Olly

[jsmoraes]

Thank you mates for attention and messages. As answer for my issues we have that secondary mirror only cause astigmatism if it is a bad mirror.

About the offset my solution was:

Searching in the internet by photos of secondary mirror on GSO 12" Dobsonian I saw that the screws are more outside than mine.

Therefore I changed de position of my secondary mirror and now my screws are with 7.5 mm outside. Top of screw to surface of secondary mirror cell.

The astigmatism is present, yet. With much less intensity, although.

I will publish the photos for comparison tomorrow.

[jsmoraes]

As it was:

Beta Centaurus - 10 seconds - ISO 3200

As it is:

Alpha Trapezium

[jsmoraes]

These chromatic distortion from atmosphere or grease on filter, coma corrector can be culprit of my ovality seen as astigmatism ?

Raw file not processed (copy screen with PhotoScape) and crop of stacking and converting to 16 bits in photoshop.