minimum distance to test focusing for a 8 inch dob?

[starhiker]

I have received my first dob and finished the assembly. I am very happy with the touch and (material) quality of the StellaLyra 8inch dob. However I am struggling to test the focusing. I am trying to focus through my garden door because it is raining outside. I get very limited views and objects to test in this way because the fence and hedges. The best I can try is an object in a distance about 15 meter away. But I could not get it focuses after turning probably 10 full rounds of the focuser. I am wondering if there is a minimum distance to get it focused for dob with the focal length of 1200mm?

[starhiker]

Ok, forget about the minimum distance issue 😀 15 meter seems to be too close. Also I just realised that I missed to loose a bolt, so that the focus tube didn't move at all when I turn the knob. But figure it out now. May come back with other questions later though.

[CraigT82]

The closer the object you’re trying to view the further out the focuser will be when it is focused. So you just need to figure out the minimum distance to the object that you will be able to focus on with your existing focuser travel. There is a calculation on here somewhere that @vlaiv posted not long ago. 
 

With my old 200p f/6 I could just about focus on a fence which was about 60m away but I had to partially pull out the adapter and eyepiece to get it

[vlaiv]

Yes, this is known as thin lens formula or thin lens equation and goes like this:

1/f = 1/od + 1/id

Where - F is focal length of lens (or telescope in this case)

od - is object distance - in this case how far you want to focus to

id - is image distance - or where focal plane will be with respect to lens.

Say you want to focus to 15 meters away with 1.2m focal length

1/1.2 = 1/15 + 1/x

1/x = 1/1.2 - 1/15 = (12.5 - 1) / 15  = 11.5 / 15

x is therefore 15/11.5 = ~1.30435 meters or 1304mm

You already have 1200mm of focal length when you focus at infinity - this leaves additional 104mm to be "added" to this.

You need to rack your focuser out additional 10.4 cm from infinity focus position (which really means to add 10cm extension tube)

[starhiker]

Thanks @CraigT82 and @vlaiv, especially for the formula. This is very useful. I eventually found an aerial in some further distance to focus yesterday, and used that aerial to align the finder. But it turns out the alignment is still not accurate when I test it at night. I will have another round to adjust the alignment in today's daytime.

[TheThing]

I often find that it is difficult to find something far enough away to align the finder scope with during the daytime.  Instead, I tend to get mine roughly in place and then use Polaris to fine tune it.  At 4,087 trillion kms away, that's just about far enough!  Plus, it doesn't move very far, so you have plenty of time to fettle whilst it is still in your EP's FOV.

[Louis D]

I generally keep a 35mm to 50mm extension tube on hand for helping with close focusing.  I've found it really helps with Maks in particular.  You'd think they can close focus with the whole moving mirror thing, but they can't.

[starhiker]

18 hours ago, TheThing said:

I often find that it is difficult to find something far enough away to align the finder scope with during the daytime.  Instead, I tend to get mine roughly in place and then use Polaris to fine tune it.  At 4,087 trillion kms away, that's just about far enough!  Plus, it doesn't move very far, so you have plenty of time to fettle whilst it is still in your EP's FOV.

Thanks for the suggestion. I might need to practice to align the focuser on a star. On the night I got my dob, that is the only night with a relatively clear sky, I tried with Lyra. It was really struggling to aim the dob to something almost vertically. I swept around for some time and gave up. Then I tried Altair that is lower I can use the line-of-sight rule to aim. But it was so dark that I could not identify if the Altair is exactly in the center through the finder and the eyepiece. I may need to spend more time with it to learn it.

[starhiker]

14 hours ago, Louis D said:

I generally keep a 35mm to 50mm extension tube on hand for helping with close focusing.  I've found it really helps with Maks in particular.  You'd think they can close focus with the whole moving mirror thing, but they can't.

I have an extension tube. It seems that I need to use it for the 30mm eyepiece even aiming to the stars.