Does my secondary look normal?

[turboscrew]

I measured my secondary to make adjustments easier, but when I took the secondary out, I was a bit astonished about what it looks like. Does this look normal?
- In the front image, you can see that the sides are not very straight, and the right side even less so.
- In the back image you can see that the top of the mirror and the top of the holder are not aligned. You can also see how the right side comes further out from the holder than the right side.
- In the top image you see how weirdly the mirror has been cut. the left side is almost perpendicular to the mirror face, but the right side is not.
- in the side image you can see the front and back of the mirror. If you draw a straight line along the holder surface, you'll notice that the front and back of the mirror come about equally far from the holder. The difference is actually 2 mm. The middle alignment screw is pretty much in the center of the holder.
It's a 75 mm secondary in a 300 mm telescope. I don't think it takes care of the secondary offset.

[Lady Fraktor]

When you align the secondary under the focuser it will be fine.
Did you buy this telescope from a retailer or used?

[turboscrew]

When you align the secondary under the focuser it will be fine.
Did you buy this telescope from a retailer or used?

Used.
And I think I got it collimated. I even drilled a 1.5 mm focuser checking hole on the exact opposite from the center of the focuser hole and covered it with electric isolation tape. The collimating laser hit the hole, so I guess it's more than fine.
I just wonder about the secondary offset...

I think the star test will show me how I did. That is, when the night sky is not totally covered by clouds.

[turboscrew]

I also fixed and greased (with lithium grease) the focuser. The fine focusing knob was not doing anything.

[helicon]

So is the fine focusing function working again? With my Z10 several years back I was unable to use the fine function. Then I realized the thumbscrew controlling the focuser travel was completely screwed in. Only took me a couple of years to find this out!

[turboscrew]

So is the fine focusing function working again? With my Z10 several years back I was unable to use the fine function. Then I realized the thumbscrew controlling the focuser travel was completely screwed in. Only took me a couple of years to find this out!

I don't know. So far it works fine, but I don't know if it becomes loose again. Time will tell if the mistake was to let the outer ring of the axle bearing touch the tightening nut. Now I left a little gap in between them.

[turboscrew]

Nice. The sky was quite clear, so I took a try. Mars looked as bad as always, a bit too bright and there was some atmospheric dispersion too. I found M31 even if I couldn't see it at all with naked eye. Too bright sky.

I also checked the collimation. When a star is in the middle of the field, you put a high magnification eyepiece and defocus, right? And you should see a "boiling disc"? When you defocus more, the disc becomes a perfectly round doughnut, with the main mirror image in the middle - the sky, shadow of the secondary and the spider vanes?

I used 10 mm (120x) eyepiece, because the next is 3.6 mm (333x), and with that it's hard to handle focusing while keeping the star in the middle. With the 3.6 mm eyepiece the star passes the FOV in 48 seconds.
I think I really need an equatorial tracking mount.

[SkyHiker]

Build an Alt/Az controller for your Dob. I am about to make an OnStep controller for the G11S myself, and got parts for 2. The other one will be on the Z12. OnStep also provides for a derotator for AP.

[turboscrew]

When I was sketching a mount, I became painfully aware that my knowledge about mechanical components is lacking too much. I have no idea about the accuracy and rigidity of bearings, pulleys and such. I think I'm going to buy (with a bleeding heart) some factory-made mount.
Then again, I'm still on visual mode. I think it's good to learn finding the targets first.

[SkyHiker]

When I was sketching a mount, I became painfully aware that my knowledge about mechanical components is lacking too much. I have no idea about the accuracy and rigidity of bearings, pulleys and such. I think I'm going to buy (with a bleeding heart) some factory-made mount.
Then again, I'm still on visual mode. I think it's good to learn finding the targets first.

Large scopes for AP are typically catadioptric because they must be compact. Your 12" Dob will too much of a big sail in the wind, which calls for a much stronger mount than a comparable sized catadioptric. For instance you could put a 14" SCT on my G11S but it would struggle with a 12" Newt. Here's a discussion on CN about the same question, https://www.cloudynights.com/topic/6074 ... oto-mount/

Even strong equatorial mounts have flexibility. Your chances with a DIY Alt/Az mount will be better IMHO. If everything works out I will give it a shot with OnStep but I first need to build the OnStep controller for my G11S so it will be a while.

[turboscrew]

When I was sketching a mount, I became painfully aware that my knowledge about mechanical components is lacking too much. I have no idea about the accuracy and rigidity of bearings, pulleys and such. I think I'm going to buy (with a bleeding heart) some factory-made mount.
Then again, I'm still on visual mode. I think it's good to learn finding the targets first.

Large scopes for AP are typically catadioptric because they must be compact. Your 12" Dob will too much of a big sail in the wind, which calls for a much stronger mount than a comparable sized catadioptric. For instance you could put a 14" SCT on my G11S but it would struggle with a 12" Newt. Here's a discussion on CN about the same question, https://www.cloudynights.com/topic/6074 ... oto-mount/

Even strong equatorial mounts have flexibility. Your chances with a DIY Alt/Az mount will be better IMHO. If everything works out I will give it a shot with OnStep but I first need to build the OnStep controller for my G11S so it will be a while.

It looks like I need a tracking mount even for visual. This dobson feels a bit sticky, and very hard to get pointed to somewhere accurately. The target just swings back and forth, and often gets lost altogether. It looks like the teflon pieces are not slippery enough. Also, if I try to use my 3.6 mm eyepiece for collimation star test I need to hurry, because it only takes 48 seconds for the star to pass through the FOV. Centered star thus disappears in 24 seconds.

[SkyHiker]

When I was sketching a mount, I became painfully aware that my knowledge about mechanical components is lacking too much. I have no idea about the accuracy and rigidity of bearings, pulleys and such. I think I'm going to buy (with a bleeding heart) some factory-made mount.
Then again, I'm still on visual mode. I think it's good to learn finding the targets first.

Large scopes for AP are typically catadioptric because they must be compact. Your 12" Dob will too much of a big sail in the wind, which calls for a much stronger mount than a comparable sized catadioptric. For instance you could put a 14" SCT on my G11S but it would struggle with a 12" Newt. Here's a discussion on CN about the same question, https://www.cloudynights.com/topic/6074 ... oto-mount/

Even strong equatorial mounts have flexibility. Your chances with a DIY Alt/Az mount will be better IMHO. If everything works out I will give it a shot with OnStep but I first need to build the OnStep controller for my G11S so it will be a while.

It looks like I need a tracking mount even for visual. This dobson feels a bit sticky, and very hard to get pointed to somewhere accurately. The target just swings back and forth, and often gets lost altogether. It looks like the teflon pieces are not slippery enough. Also, if I try to use my 3.6 mm eyepiece for collimation star test I need to hurry, because it only takes 48 seconds for the star to pass through the FOV. Centered star thus disappears in 24 seconds.

See if you can make a Lazy Susan mount for Az and maybe try roller skate bearings for Alt. My Z12 has smooth bearings, zero Teflon. The purists despise them but I love it, and it would be a requirement for any attempt at AP (that is not on an EQ platform).

[turboscrew]

I think I don't have the time. Nor frustration tolerance left. I've spent most of the day just to try to find a way to connect my little camera to 1.25" eyepieces. The camera weights less than half a pound, and there's everything upto female T2. No one seems to sell adapters from 1.25" eyepiece filter thread to T2 male.I think I found a combination and made the order. It took about 6 hours to find the stuff. Digiscoping seems to be almost illegal, except when you use a cell phone.

And those parts - Lazy Susans and bearings are not for sale in the hardware stores here. You need to order each and every one of them somewhere around Finland, and maybe even outside Finland. Then there are the axles. I still haven't learned to weld properly, and the tube only has supporting wheels each side. No axles anywhere.
And I've already burned almost 2500 eur.

About AP, I've started doubting if it's really worth the costs. At least for now.

[turboscrew]

Aha. It looks like in the next couple of nights, the sky will be clear, but turbulent...
Doesn't look good for collimation check...

[SkyHiker]

I think I don't have the time. Nor frustration tolerance left. I've spent most of the day just to try to find a way to connect my little camera to 1.25" eyepieces. The camera weights less than half a pound, and there's everything upto female T2. No one seems to sell adapters from 1.25" eyepiece filter thread to T2 male.I think I found a combination and made the order. It took about 6 hours to find the stuff. Digiscoping seems to be almost illegal, except when you use a cell phone.

And those parts - Lazy Susans and bearings are not for sale in the hardware stores here. You need to order each and every one of them somewhere around Finland, and maybe even outside Finland. Then there are the axles. I still haven't learned to weld properly, and the tube only has supporting wheels each side. No axles anywhere.
And I've already burned almost 2500 eur.

About AP, I've started doubting if it's really worth the costs. At least for now.

Looking at your VX12 Dob mount, it is not easy to modify that metal mount. Best to build a special goto mount from plywood like for instance https://photos.google.com/share/AF1QipN ... RtQ1pMcXN3.

Money is not the issue but DIY skills are. Check out the OnStep section for Dobsonians at https://onstep.groups.io/g/main/wiki/4414 . Several of the designs there require 3D printed parts, which I am not a fan of, but it's a fun section to see some Dob-mod ingenuity. I intend to use a simple friction drive, I have some ideas. Of course my Z12 has a lazy Susan built in, which makes it easy.

[turboscrew]

Well, time is an issue too, as well as a place to work. And my summer vacation ended about a month ago.
And I still need to build a new floor to my garden shed before the winter.

[turboscrew]

I knew it wasn't a good night, but tried still to check the focus. I thought that polaris wouldn't escape my FOV so fast, and is quite high all the time.
Well, the "boiling" ring was round, except for some flaring and occasional waves. Due to these turbulences, it's really hard to say anything more specific. I have to wait for better nights. Well, outside focus was more calm than inside focus. That's probably, because I wasn't using coma corrector?
This time I used the 3.6 mm eyepiece (333x).

[turboscrew]

Tried to check the collimation again. Still quitre some turbulence, but with small out-focus I managed to see some interference rings berween "boiling". And they looked round and equally spaced around the secondary mirror shadow.
The in-focus was still too messy to see any interference rings. I think my first thorough collimation went well.
I'm going to have another peek before going to bed - I think the turbulences should be calming a bit.

[turboscrew]

Yep. This time I also saw arbitrary fragments of the interference rings every now and then also in-focus, and the out-focus rings were a bit more visible. Managed to find the ring nebula, but lost it while changing from 10 mm to 3.6 mm.
Work day tomorrow, so ended there. The main task was to check the collimation, and I think I can say, that it's good.

[SkyHiker]

Good! Did you end up collimating with the laser?