Ironing out a few problems

[Carole]

Problem:
a) Getting accurate focus
b) Noise on PHD

a) I went out last night as the skies were clear(ish) at 7.30, but they clouded over by about 9.00, but I'm getting lots of practice at setting up if nothing else!!!!
I focussed my main scope on Mars and I think I now have accurate focus (see images below), done at 1400 ISO 12 secs on the DSLR.  Can't remember if I could see Mars Live on the screen.  Having now found focus (which is only a hairs breadth away from where I had it when I did my last image), I have locked the focus and am reluctant to "put it away".  Is there any reason why I shouldn't now leave the scope in this position when I put it away?

b) PHD connects OK to the camera and scope now, but there is loads of noise on the screen and very difficult to see anything at all.  I cannot seem to change the settings (Bottom right) as they are greyed out, so how do I clear all this noise in order to be able to get a clear screen.  (This also happened when I did my one and only guiding session recently, it did Guide but kept flashing red accross the bottom bar saying the SNR was low.

Before focus (with focus mask on):


After focus (with focus Mask on):

[mickw]

Looks a bit flat on top

[Mac]

Having now found focus (which is only a hairs breadth away from where I had it when I did my last image)

perfect focus is a very very fine line.

To me it still looks a tad out of focus.
It still looks like there are three slight bumps.

When you get to the focus you have now, turn down then length of the shutter so that the image is not as bright.
You will then get a further image.

as the image is so bright, it might be the fact that the star (mars in this case) is still ever so slightly out of focus.
and that the light is welling over, giving the impression that your three dots have in fact turned in to one.

Mac.

[Ian]

don't think that's in focus yet Carole, but it's hard to tell because the star is saturated. As you get closer to focus shorten the exposure time, you'll see much more.

[MarkS]

Re: focus.
Once you find focus, mark it as accurately as you can to provide a starting point for the next session.  Yes you could lock the drawtube as well but almost certainly you'll still need to refocus next session anyway.

Re: PhD.
Probably you didn't have a bright enough star in the guide camera frame.  When you do find a bright star, the amp gain reduces and the background noise level reduces with it.  I had the same problem when I did my own Mars image - it was difficult to find a brightish guide star nearby - I had to guide on a dim one and I kept on getting low SNR warnings.

Mark

[Carole]

Oh Bum!!

Are you sure this isn't focus, I've blown the image up a bit more:


Carole

[Carole]

I had the same problem when I did my own Mars image - it was difficult to find a brightish guide star nearby - I had to guide on a dim one and I kept on getting low SNR warnings.

Did you get a lot of noise as well Mark?

Carole

[Ian]

Oh Bum!!

Are you sure this isn't focus,

Can't tell because it's saturated. But it just doesn't quite look right...

[Carole]

Roll on the Bahtinov mask then when it comes. I think if you have the central spike CENTRAL then you are sure you have perfect focus.

Carole

[Ian]

you'll *still* need to make sure the star isn't saturated or you won't know where the middle is for the spike to be central to! (as translated from lower Swahili, can someone with a better command of English rewrite that...  ) 

The Hartmann mask you used for that can get you in focus, with the right technique.

[MarkS]

Did you get a lot of noise as well Mark?

Yep - I had lots of noise.

By the way - focus masks will only work well on stars.  Focus on a star first then move to your planet or dep sky object.  But you already knew that

[Carole]

as translated from lower Swahili, can someone with a better command of English rewrite that...

It's OK, I understood.

As you previously said Ian what a PIA!

Carole

[Carole]

Deviating slightly from my thread, but it's relevant!!  Has any-one seen the book they had on sale at Astrofest:
ASTRONOMERS ANONYMOUS
(springer.com)
It's a collection of Typical Astronomy questions and hilarious answers.

Has any-one read it? Is it worth a read?
I bet they could compile another book from the things they read on this forum.  Things like this thread for instance.

Carole

PS.  It's not available yet, so no-one will have read it.
http://www.springer.com/astronomy/practical+astronomy/book/978-1-4419-5816-7

[Mike]

Looks wrong shape to me to be in focus. Only when you stop it down to a very short exposure will you see for real.

[Carole]

Looks like it's back to the drawing board!!!!!

Carole

[Carole]

One last attempt to see if I can make it clearer.  

There are some faint stars in the same image, so I have now gone back to the full frame and blown them up. Had to stretch the levels for these to show up.

Stars and Mars (with focus mask), before focussing.



Stars (and Mars) after what I had hoped was focus (with focus mask):



top left stars zoomed in (with focus mask):  



If these aren't round I give up!!

Carole

[RobertM]

By the way - focus masks will only work well on stars.

Not quite true.  At shorter focal lengths planets are perfectly usable with a Bahtinov mask but there may be a problem with longer focal lengths where the image scale is much larger.  I reckon that if you can see the spikes then you should be ok - try it and see.  With the image scale you have in those images there's unlikely to be any problem.

Robert

[Mike]

How are you taking pictures of stars in the middle of the day ?

[Carole]

 

How are you taking pictures of stars in the middle of the day ?

a) I went out last night as the skies were clear(ish) at 7.30

[RobertM]

If these aren't round I give up!!

Don't forget the scope will need time to reach ambient after going from inside to the cold outside, maybe up to an hour or so.

[Carole]

Don't forget the scope will need time to reach ambient after going from inside to the cold outside, maybe up to an hour or so.

Hi Robert, I keep the scopes in my conservatory which is very cold in the winter especially now as the dog flap has broken off and there's a cold blast coming through it.  It's DIY built not UPVC and heated.  So the telescope is already pretty cold before it goes outside. 
I don't of course keep it out there in the summer when it is the opposite (like an oven) but I will hopefully have the observatory by this summer.

Also it's normally getting on for an hour setting up and alignment etc etc by the time I get to focus.

[doug]

I have a very large drawing board here, if you want one, Carole.

[Carole]

OK.  I've bought myself a Bahtinov mask, and like every-one else I've been waiting a few weeks to try it out.  

I took some images on Monday evening but they are pretty rubbish as I didn't take enough subs due to (target disappearing behind the trees, or clouds), so I won't be posting them unless you guys really want to see them.  Also did not get the targets centred in the frame.  Lots of lessons learned there.

I managed to focus on my last alignment star and it worked.  So I hope this will solve my problems.



Carole

[Mac]

your mask is round the wrong way.

normally the lines are vertical.

Not that it makes any difference.

Mac.

[MarkS]

Sounds like good progress Carole.

Mark

[Ian]

Nice diffraction spikes there Carole...

Hmm, need a "looking over my shoulder" smilie I think.