Black dot while looking at stars

[batemantom]

Tried out my new Heritage 130p last night. I tried looking at some stars while I was waiting for Saturn and Jupiter to come out. Why is there a black dot in the middle of the star? What have I done wrong? Is it out of focus? I taken a quick photo with my smartphone.

Thank you for your help in advance. 👍

I managed to see some great views of Saturn and Jupiter with 4 moons last night. I'm very impressed with this telescope.

Edited July 20, 2020 by batemantom

[rl]

That looks to me like it's out of focus, and out of collimation. At focus a star should be like a bright point of light too small to see the middle, no black dot. At low/ medium magnification it really is just a point.  

As the magnification goes above 100  you might start to see a bright central dot with a few very faint rings due to the wave nature of light (called a diffraction pattern).

Out of focus the black dot should be in the centre; it's due to the secondary mirror blocking light. 

It's worthwhile reading up on collimation. 

[batemantom]

9 minutes ago, rl said:

That looks to me like it's out of focus, and out of collimation. At focus a star should be like a bright point of light too small to see the middle, no black dot. At low/ medium magnification it really is just a point.  

As the magnification goes above 100  you might start to see a bright central dot with a few very faint rings due to the wave nature of light (called a diffraction pattern).

Out of focus the black dot should be in the centre; it's due to the secondary mirror blocking light. 

It's worthwhile reading up on collimation. 

Is that fairly common with brand new telescopes? Saturn and Jupiter looked fine.

[rl]

It's fairly common for Newtonian scopes to arrive with the collimation slightly out. They just don't travel well....part of life with a Newt is learning how to twiddle the knobs and screws to get the best out of it. It's not that difficult after you've done it once...look upon it as the astro equivalent of learning to ride a bicycle!

Before touching any of the screws find out what stars look like on a well-collimated scope and see how yours compare. A bit of reading up front will pay dividends..don't dive in first before you're sure of what you're looking for.  And then make the adjustments in small steps. 

The main mirror actually needs to be very slightly loose or the locking screws will bend the shape. The secondary supports are often a bit flimsy in order to obstruct the minimum amount of light, and to minimize the faint "cross" effect caused by diffraction. 

There are articles on this site of how to do it. 

Collimation is one reason why a lot of people prefer refractors...they are pretty much adjustment-free. But Newts show a lot more per pound of investment once you've learned to get the best out of them . 

You may well find Jupiter and Saturn look even better with the scope collimated..

[batemantom]

13 minutes ago, rl said:

It's fairly common for Newtonian scopes to arrive with the collimation slightly out. They just don't travel well....part of life with a Newt is learning how to twiddle the knobs and screws to get the best out of it. It's not that difficult after you've done it once...look upon it as the astro equivalent of learning to ride a bicycle!

Before touching any of the screws find out what stars look like on a well-collimated scope and see how yours compare. A bit of reading up front will pay dividends..don't dive in first before you're sure of what you're looking for.  And then make the adjustments in small steps. 

The main mirror actually needs to be very slightly loose or the locking screws will bend the shape. The secondary supports are often a bit flimsy in order to obstruct the minimum amount of light, and to minimize the faint "cross" effect caused by diffraction. 

There are articles on this site of how to do it. 

Collimation is one reason why a lot of people prefer refractors...they are pretty much adjustment-free. But Newts show a lot more per pound of investment once you've learned to get the best out of them . 

You may well find Jupiter and Saturn look even better with the scope collimated..

I've just ordered a collimation tool. I'm going to test it out again tonight. Just to see if I was just out of focus. 

[batemantom]

1 minute ago, batemantom said:

I've just ordered a collimation tool. I'm going to test it out again tonight. Just to see if I was just out of focus. 

Also, Saturn and Jupiter looked fantastic! I've also ordered a X2 Barlow. 

This telescope is 10 times better than my old one.

[rl]

I should have added "beware laser collimation tools".......the principle looks great at first sight but unless a lot of geometrical details are correct it's easy to get the wrong answer with them. I've always found a good Cheshire eyepiece to be more reliable.

Too late now!

Is there a local friend or society than can walk you through it?

[batemantom]

4 minutes ago, rl said:

I should have added "beware laser collimation tools".......the principle looks great at first sight but unless a lot of geometrical details are correct it's easy to get the wrong answer with them. I've always found a good Cheshire eyepiece to be more reliable.

Too late now!

Is there a local friend or society than can walk you through it?

I ordered a Cheshire collimation tool! It was half the price of the laser ones.

[batemantom]

6 minutes ago, rl said:

I should have added "beware laser collimation tools".......the principle looks great at first sight but unless a lot of geometrical details are correct it's easy to get the wrong answer with them. I've always found a good Cheshire eyepiece to be more reliable.

Too late now!

Is there a local friend or society than can walk you through it?

Can you recommend an good step to step guides or videos on how to do this? I don't know anyone else who owns a reflecting telescope.

Thank you 😊

[Pixies]

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

https://garyseronik.com/a-beginners-guide-to-collimation/

 

[batemantom]

Thank you, I appreciate everyone's help.