Towa 339 modification?

[Dave1]

Hello everyone

So guys I need some help. Like Phil here http://philjay2000.tripod.com/classic/Modernising_the_Towa_339.pdf . I am going to upgrade my Towa 339. I'm going to however use a different focuser to Phil. The focuser I am going to use is a Monorail 2" Refractor focuser from Teleskop service http://www.teleskop-express.de/shop/product_info.php/info/p3946_MONORAIL-2--Refractor-focuser---e-g--for-86mm-flange.html

If you read Phil's article and go to the conclusion section. You will see he said that with 5 inch of extension tubes his focuser draw tube was extended 3/4 of its travel. His focuser has 80mm of travel. Well 3/4 of 80mm is 60mm. Ideally like Phil I would prefer the travel at focus point to be 1/2 way of the focuser draw tube to reduce stress, and to give me the most flexibility both ways. The problem is the focuser I want to use only has 50mm of travel in total. So wouldn't even come to focus! Unless I'm mistaken. 

So I need to extend my draw tube even longer that Phil. So wait for it here's the math. I'm not great with this stuff so, if I make a mistake please point it out to me, thanks.

With extension tube up to 127mm the draw tube on Phil focuser was extended by 3/4 . So 3/4 of 80 mm is 60 mm. So to bring the travel of the focuser draw tube to 1/2 way extended, the OTA extension would need to be extend by 20 mm. Correct? So 127mm + 20mm = 147mm.

So if I had a focuser with only 50mm travel, going off the above for reference. To figure out how far I would need to extend the OTA tube extension to find 1/2 way of 50mm draw tube extension of the focuser. Using the above math. To reach half way of 50mm from 60 mm means I would have to subtract 35mm from 60mm equals 25mm, which is the half way point of 50mm. So off the back of this I would need to extend my OTA by 162mm? Is this correct? Or have I messed up?

Dave

 

 

 

 

[Ricochet]

Unless I am mistaken you have only accounted for the length of the focuser drawtubes and not the length of the fixed portion of the focuser, nor the length of the custom adaptor that the other user had made in order to fit his focuser to the tube.  I don't see a length listed for that part although you could probably estimate it from the pictures.  However, you know the focal length of the telescope so I would think that it would be easier to start from that than trying to calculate based on rough approximations of someone else's focuser travel and at least one unknown length. Unless I am also missing something what you want to do is to take the focal length of your telescope (1200mm?) and subtract the following from that:

1. The length of the existing tube from the centre (?)  of the objective cell to the rear
2. The length of the fixed part of your new focuser
3. The 25mm you want the drawtube to be extended at focus
4. The amount that the 1.25"-2" adaptor protrudes from the drawtube if you will be using a 1.25" diagonal
5. The optical path length of the diagonal you will be using with the telescope.

I think whatever is left should be the length of extension tube that you need to fit between the existing tube and the new focuser.  You may, however, wish to go a bit shorter as it will be easier to add extension tubes to the focuser than to find that you need to shorten the tube at some point in the future.

Or for 65 euros more than the price of the focuser you're looking at you could just buy a new focuser with an F13 telescope attached and not worry about the old Towa.

[Dave1]

Unfortunately I don't have the length of the fixed section of Phil's focuser. The length of the custom adapter is the same length as the standard adapter which came with the telescope. From comparing a photo I saw.

In answer to your list.

1. Can you explain more please I don't really understand?

2. Will have to wait for the new focuser to turn up. 

3. Think I understand.

4. I will be using a 2" diagonal. What difference will this make? Some more explanation would help?

5. The diagonal I will be using is the Baader 2" Carl Zeiss prism. Has a optical path length of 110mm.

If I have asked for more explanation it is so I can understand and then hopefully work it out. Hope you don't mind.

I already have a Skywatcher ( Helios ) 120mm refractor. I bought the Towa as a project scope, as I wanted an F15 classic scope. I prefer using the Towa to be honest. I only have room for two telescopes. So I am going to flock, baffle, and add a new focuser. 

 

[Ricochet]

On 07/08/2017 at 23:44, Dave1 said:

1. Can you explain more please I don't really understand?

At the front of the telescope under the dew shield you have the lens cell which looks like this:

You need to measure the distance from somewhere on the lens cell to the end of the white tube where it attaches to the focuser. I'm not sure where by definition the focal length is defined from, but that is the point you want to measure from. I think with a doublet objective it is from the centre point between the two lenses.

 

On 07/08/2017 at 23:44, Dave1 said:

I will be using a 2" diagonal. What difference will this make? Some more explanation would help?

The part I have labelled as "A" is the 1.25"-2" adaptor. It looks like it is approximately 10mm thick so you need to know whether it will be in place so that you can either include or exclude the thickness of it from your calculations. In my previous post I said to measure the body of the focuser, "C", but forgot the 2" eyepiece clamp, "B". You will also need to subtract this distance from the focal length of the telescope. You might find it easier to just extend the focuser to the 25mm mark and then take just the one measurement "D" rather than measuring parts individually. I have assumed that, as with the previous mod that you linked to, you will be removing the flange from the new focuser and creating a new one that is part of your extension. If you are leaving it attached and fixing your extension to the rear side of the flange you will need to include the length of the flange in your measurements.