Help with botched DIY project

[nfnf_]

Hello,

 

I hope I'm not breaking any rules making a new topic right after registering. I've searched far and wide for a solution. Sadly it's for a problem which is totally my fault. So I came here in hope of finding someone who is smarter than me.

So... I am quite new in this whole stargazing movement. I've bought a nice beginner scope (Skywatcher P150). After about a year I started to notice that the worst part of the scope is most certainly its plastic R&P focuser. Right then 've made a bad decision and bought Skywatcher Quattro 2" Crayford Newtonian Focuser (the one that comes on PDS version of Skywatcher). It took me a while (and a little help from grandfather-in-law) to make a bigger hole with the same center and to mount a new focuser.

 

Now when I do a collimation with my laser collimator, the dot is centered very perfectly. It doesn't matter how deep the focuser tube is, the collimation is always great.  Except I can't focus on anything when I try to look through the eyepiece. I picked an easy target (moon), but I can't focus it. It's just blurry as if there was no focuser at all.

 

So I'm asking for your help. Did I miss something important? Did I just make quite an expensive mistake and destroyed my tube?

I just have no idea where to look for a solution. 

 

Regards,

Joze

 

p.s.

Sorry for my weird english. It's not my native language.

[vlaiv]

Hi and welcome to SGL.

In all likelihood - your focuser is "too short" - which is a good thing as it can be easily fixed by just using extension tube.

You can compare the two by measuring how far away is EP holder located with respect to base.

2" focuser is likely shorter / low profile. It is made that way to allow for DSLR to come to focus (DSLR has 44mm of additional light path inside camera body).

Telescopes with low profile focuser are often visually used like this:

Note 2" extension tube that places eyepiece at needed distance in order to reach the focus.

Try not putting eyepiece all the way in the focuser and seeing if this will let you focus on a distant object. You don't have to wait for night and clear sky - you can try focusing on distant mountain top or tree far away. Just note - closer the object is - more outward focus travel it will need to reach the proper focus.

 

[fozzybear]

Joze,

what size eyepieces are you using take a look at this thread as you will need adaptors

 

[nfnf_]

10 minutes ago, vlaiv said:

Hi and welcome to SGL.

In all likelihood - your focuser is "too short" - which is a good thing as it can be easily fixed by just using extension tube.

You can compare the two by measuring how far away is EP holder located with respect to base.

2" focuser is likely shorter / low profile. It is made that way to allow for DSLR to come to focus (DSLR has 44mm of additional light path inside camera body).

Telescopes with low profile focuser are often visually used like this:

 

Note 2" extension tube that places eyepiece at needed distance in order to reach the focus.

Try not putting eyepiece all the way in the focuser and seeing if this will let you focus on a distant object. You don't have to wait for night and clear sky - you can try focusing on distant mountain top or tree far away. Just note - closer the object is - more outward focus travel it will need to reach the proper focus.

 

Thank you, this was very fast. I have indeed not tried holding EP further away. And the new focuser is clearly shorter as the old one.

I use a 2"->1,25" adapter but that probably doesn't add enough length to reach a focal point. 

Thanks so much. I will test it asap and I will let you know if you saved me.

[nfnf_]

12 minutes ago, fozzybear said:

Joze,

what size eyepieces are you using take a look at this thread as you will need adaptors

Great, thank you for the link. It's getting much clearer now. I didn't measure both focusers, but they did look similar in size so I didn't think about that.  After your answer I checked with measuring tape and clearly they are not the same length.

[nfnf_]

1 hour ago, nfnf_ said:

Thank you, this was very fast. I have indeed not tried holding EP further away. And the new focuser is clearly shorter as the old one.

I use a 2"->1,25" adapter but that probably doesn't add enough length to reach a focal point. 

Thanks so much. I will test it asap and I will let you know if you saved me.

Bad news. I managed to catch a moment before the evening fog rolled in and tried your suggestion while watching the Moon.

If I hold EP further away, I just start to see the cross from secondary mirror. The Moon doesn't get any clearer. 

I also found out that I got one 2“ adapter with the focuser and I tried to use it. It's the same - I just see the shadow of secondary mirror in the EP. It doesn't matter how much I rotate the focuser knob. It just doesn't focus at all.

Is it possible I screwed up something else?

[vlaiv]

It would be helpful to determine if you are lacking inward or outward focuser motion.

You need strong star to do that and low power eyepiece.

If you insert your eyepiece and point a scope to a strong star (you need star to be strong as image will be out of focus and light spread around) - it should present a "doughnut shape" - a bit like this:

but probably fainter.

Now you need to determine direction in which diameter grows and in which it shrinks. Move focuser in/out and see what happens to the size of this doughnut.

If it shrinks when you are moving focuser inward (eyepiece gets closer to tube) and then stops when you can't move focuser inward any more - then you are lacking inward focuser travel.

if it shrinks when you move focuser outward (eyepiece gets away from the tube) and then stops when you can't move focuser outward any more - then you are lacking outward focuser travel.

In case you are lacking outward focuser travel - you can solve it by adding extension tube. Too long extension tube can create problem as well - it can move eyepiece past focus point and with long extension tube - you can suddenly be in situation that you don't have enough inward focuser travel. In this case - use shorter extension tube.

In the case you are lacking inward focuser travel (without using extension tube) - well, you'll need to modify your scope a bit more. You need to shorten the telescope tube itself and move primary mirror closer to secondary mirror.

Look at this video to see how it can be done:

https://www.youtube.com/watch?v=u_ufa_RcQaE

You basically drill new set of holes on the bottom side of the scope and cut away part of tube itself (about 20-30mm). Just be careful to remove / protect mirror parts when doing that (video shows this as well).

[nfnf_]

Hello again,

 

sorry for bumping an old thread. I just don't want to be that guy that doesn't follow up with the solution to the problem.

I did the thing that vlaiv suggested and basically found out that my focuser is much longer than the old one. That means that I had to cut the tube short. Which I did, but it took me a while because I don't have enough space/tools to do it at home. I had to shorten the tube for about 2cm (0.8in) with the help of youtube video linked above. It's not a simple thing to do if you are more of an IT guy and less of a handy man, but I had some help.

The new focuser looks a bit silly on this tiny tube (see attachement), but it makes focusing a million time easier.

 

So thank you everyone for all your advice and help. I love this kind of communities.

Regards,

Jože