Possibly broken telescope; I don’t know how to fix it

[Rigatoni]

hi there! It’s my first time ever posting here. A little while ago I started to look for my first telescope, and after a lot of research, I decided a dobsonian was the best recommendation for complete noobs. Unfortunately for me, name brand telescopes are very expensive in my country, so I ended buying a used dobsonian, with optics made by well known Brazilian ATM (Sebastião Santiago Filho). 

However when I received this telescope I noticed half of the view through the eyepiece was completely black, possibly obstructed. After investigating, I believe the secondary mirror found its way inside the focalizer, and the “obstruction” is the reflection of the inside of the focalizer tube. I’ve been in contact with the person that sold it to me, but he doesn’t seem to know how to solve the problem. 

Please excuse if the terms I used aren’t very clear, I’m freely translating from Portuguese.

Here’s some pictures I took of the inside of the telescope: https://imgur.com/a/o545Cmm

Does anyone have any idea how to fix this problem? I’ve tried messing with the angle of the secondary mirror, but it doesn’t seem to do much.

[knobby]

Looking at the second picture, it looks like the focuser tube has been wound in way too far. Does it move away from the secondary mirror when focusing ?

Olhando para a segunda foto, parece que o tubo focalizador foi enrolado demais. Ele se afasta do espelho secundário ao focar?

Edited December 27, 2020 by knobby

[Rigatoni]

4 minutes ago, knobby said:

Looking at the second picture, it looks like the focuser tube has been wound in way too far. Does it move away from the secondary mirror when focusing ?

Olhando para a segunda foto, parece que o tubo focalizador foi enrolado demais. Ele se afasta do espelho secundário ao focar?

No, it’s completely fixed. Unrelated, but thank you for adding a Portuguese translation, that’s very sweet hahah

[knobby]

The focuser tube should move in and out to achieve focus, are you saying it is stuck ?

[knobby]

Can you show a picture of the focuser

[John]

The focus tube should be well away from the secondary mirror for the scope to work. The focus tube in the picture seems to be actually pressing against the secondary mirror pushing it out of alignment.

The view down the telescope, towards the main mirror should look something like this with the focuser tube well away from the secondary mirror:

[Rigatoni]

I think I might have used the wrong term in English. The way that this particular telescope is built (I later found out it was a self build by the person I bought from) has the secondary mirror touch a tube that leads directly to the eyepiece. I’m pretty sure the mirror tilted enough to go inside this tube. In Portuguese we would call it a “focalizador”. I’m not entirely sure in English, though. But it doesn’t seem like it’s supposed to move

edit: looking at the other answers, yeah my telescope looks nothing like that lol. It’s a lot narrower, so there’s no space between the mirror and this tube that I described

Edited December 27, 2020 by Rigatoni

[John]

With a newtonian scope, the main and secondary mirrors are fixed in position and the focuser / focus tube moves the eyepiece in and out to achieve focus. At no time does the end of the focus tube come anywhere near the secondary mirror.

Here is another design using a single vane to hold the secondary mirror:

 

Edited December 27, 2020 by John

[Rigatoni]

Well, it seems like my only option is to try to move the focus tube up. Thankfully, since everything is homemade, I think it shouldn’t be very hard to “customize” it.
 

By the way, is there an answer button anywhere in this forum? I can’t seem to find another way to reply other than adding another comment

[John]

Adding another comment is the way to reply. That is the way that the thread on the subject develops.

If you could post some photos of the whole scope as well as the details of how the mirrors are held and the eyepiece is held, then I'm sure you will get suggestions on what to do to make it a workable scope.

 

[Rigatoni]

Thanks! I took some more photos of the telescope through the top, as well as some other angles: 

https://imgur.com/a/Sstk3kP

this honestly feels like I got a little bit scammed. The website I bought it from did not mention anywhere that the telescope was homemade. 

[John]

For the scope to work, the focuser tube needs to be moved back away from the secondary mirror to about where I have drawn the red line on the photo below and the focuser tube needs to be able to move smoothly back and forward along the green arrow that I have added in with the eyepiece on the outer end of the tube.

There are probably a number of other adjustments needed but that is the main problem that I see at the moment.

I have never seen a focuser mechanism of that design so it is obviously home made.

[malc-c]

It looks as if the focuser on this DIY scope is made from a compression pipe fitting.  If you turn the top "nut" it should release the cream coloured tube for you to pull the tube back as John suggests.  In order for a Newtonian to work that tube needs to be clear of the optical path.

Looking at the condition of the scope I personally feel you have taken on quite a challenge 

[Peter Drew]

It looks to me as though the light coloured focus tube is a sliding fit.  The three bearings outside the main telescope tube are probably offset such that when the outer collar is rotated it will move the focus tube in or out.  If the telescope was posted, the focus tube was probably pushed right in by the packaging.     🙂

[NGC 1502]

What heroic folks we have on SGL.....well done to those trying to help......👍

[Rigatoni]

Thanks guys! I’m gonna try to move the focus tube out of the way of the secondary mirror. Slightly angry that this wasn’t exactly what I bought online, but if it doesn’t make me an expert on telescopes, nothing will lol 

Edited December 27, 2020 by Rigatoni

[knobby]

Positive thoughts 👍🏻 as you say, learning about scopes can be as much fun as using them 😊

[Rigatoni]

So I followed you guy’s suggestions and it worked! The mirror is completely out of the focalizer! Can’t wait to use it tonight finally! Thank you so much to everyone that helped!

[knobby]

Very happy for you 👍🏻

[malc-c]

Glad you managed to get the draw tube away from the mirror.  It may be worth doing some basic collimation checking to make sure the optics have not been pushed out too much before giving the scope first light.  

If you want to post up some clearer pictures of the secondary now the tube is out of the way, plus an image of the primary it may help us advise you further should there be any issues

[Peter_D]

@Rigatoni 

If this is an ATM scope, the tube could be from a 'classic' manufacturer like Edmund Scientific etc (judging from the relatively thick vanes, it looks an older model to me). As well as here, it might be an idea to post some photos on the US astro forum 'Cloudy Nights'. There is a 'Classics' section - maybe someone will recognise the tube and give you advice on a more modern focuser. 

Classic scopes can give excellent images so fingers crossed for you.

Best of luck!

Edited December 27, 2020 by Peter_D

[reezeh]

I am going to mention that you might need to collimate that telescope too. That is adjusting the mirrors so that they are all centred and aligned. But that is something you will be doing fairly often with that optical design anyway. An inexpensive simple device called a "Cheshire eyepiece" will make it a lot easier. So if you find that your views are bad, that would be the most likely problem. Just look for collimating newtonian reflector / telescope for how-to.

There is a thread on here and other forums on how to do it

Edit: I think it is possible that the tube was pushed in to hold the secondary mirror in place (if the tube is larger than the secondary mirror)  for shipping - not that it is a good idea. But if you push the tube back out the focuser and you disturb the secondary, you have not broken the telescope unless you chip the mirror. All that will need to be done is collimating the telescope. Only thing that you or anyone needs to worry about when collimating is not to drop anything down the telescope tube onto the mirror. Like all things, first time or two will be the worst but it will get easier with practice. 

Edited December 27, 2020 by reezeh
Additional

[JOC]

My thought on looking at the photo was wondering if the focusser tube had been put in upside down?

[JOC]

NB.  The OP might find it useful to have Astrobaby's definitive work on collimation to hand http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/