DIY 10” Dobsonian Mount

[dr_gn]

marksx said:

That's a great build! I'd love a big dob but just don't have the space.

Struggle to find a home for my old 5" flex tube dob

Mines currently living in a corner of what we jokingly refer to as the dining room. Stored with the tube vertical it doesn’t take too much room up.

[cwis]

Well you did ask....



Clear night last night - fairly good seeing but a little windy in the upper atmosphere. I thought I'd have a go at getting pics of the diffraction patterns.

So here we have Polaris Aa and it's dim companion Polaris B - their dimmer third companion is not visible. I think it's in the Airy disk of Polaris Aa.

Above are the best diffraction patterns when you move extra-focus I got - ISO800 with 1/30 second exposures on my Samsung S7.

I think you can just see the Airy disk around Aa - broken because of the wind, and the disturbance in the diffraction patterns on mainly the right hand side are also caused by the wind. Interestingly the wind was from left to right in the images and it looks like it makes the patterns flap like a flag!

To the eyeball they are concentric but I was having difficulty getting my phone in the middle of the light cone from the eyepiece...

This is the best resource I've found on collimation, exit pupil, focus tips (I didn't know there was a right way to focus) etc etc.

https://www.handprint.com/ASTRO/ae3.html

It's also got some nice example generated diffraction patterns. Apparently the little dot in the centre is called the Poisson spot. Who knew?

Edited by cwis on Thursday 11th February 10:28

[dr_gn]

cwis said:

Well you did ask....



Clear night last night - fairly good seeing but a little windy in the upper atmosphere. I thought I'd have a go at getting pics of the diffraction patterns.

So here we have Polaris Aa and it's dim companion Polaris B - their dimmer third companion is not visible. I think it's in the Airy disk of Polaris Aa.

Above are the best diffraction patterns when you move extra-focus I got - ISO800 with 1/30 second exposures on my Samsung S7.

I think you can just see the Airy disk around Aa - broken because of the wind, and the disturbance in the diffraction patterns on mainly the right hand side are also caused by the wind. Interestingly the wind was from left to right in the images and it looks like it makes the patterns flap like a flag!

To the eyeball they are concentric but I was having difficulty getting my phone in the middle of the light cone from the eyepiece...

This is the best resource I've found on collimation, exit pupil, focus tips (I didn't know there was a right way to focus) etc etc.

https://www.handprint.com/ASTRO/ae3.html

It's also got some nice example generated diffraction patterns. Apparently the little dot in the centre is called the Poisson spot. Who knew?

Edited by cwis on Thursday 11th February 10:28

Thanks for that - I’ll digest the article later. Some of the patterns I’m getting look very much like the ones you posted. I’m waiting for another clear night.

[dr_gn]

First proper test tonight - success. Spent an hour with the family looking at nothing more than craters on the Moon. Also figured out how to take pictures through it with the iPhone:





Quick video:

[cwis]

From the crispness of that moon shot I'd say you've pinned down collimation then!

Pretty good seeing last night - glad you took advantage. It's been a while...

[dr_gn]

cwis said:

From the crispness of that moon shot I'd say you've pinned down collimation then!

Pretty good seeing last night - glad you took advantage. It's been a while...

Yep, I did a star test too, and visually filtering out the moving stuff from the constant stuff, I think the optics are good condition, and the rings are pretty much concentric. I doubt I could adjust them any better - might be different, but not better.

Strange how the reaction is the same no matter who looks at the Moon for the first time through a telescope - "Oh wow" - even though everyone has seen thousands of much better photos and videos of it over the years in books or TV or online.

[Ash_]

Those pictures are fantastic!! Well done!

[dr_gn]

Thanks both.

To be fair you can get similar images with a decent camera and lens setup, but I couldn't get the phone aligned properly for the higher magnification eyepieces.

[STR160]

That looks fantastic. I only have a cheap telescope, and the mount for it is terrible. Would love to have a good quality scope and mount.

[dr_gn]

STR160 said:

That looks fantastic. I only have a cheap telescope, and the mount for it is terrible. Would love to have a good quality scope and mount.

Worth sorting the mount out - it makes things so much nicer to use. And the view isn't wobbly.

[dr_gn]

Ramped up the magnification a bit - still hand-holding the iphone next to the eyepiece. The large crater in the first picture is Copernicus, it’s about 58 miles in diameter: