Extension Tubes

iceman (Mike) · #1 16-12-2005, 09:16 AM

Hi.

For my webcam imaging, I use a 5x powermate with my ToUcam.

I pull the ToUcam out to the very edge of the powermate to give me a slight increase in focal length, and therefore bigger image scale.

To get an even bigger image scale with my short focal length newt, I want to use an extension tube between the ToUcam and the Powermate to give me more focal length.

What type of extension tubes are there?
Where do you get them from?
How much are they?
Is there a cheap alternate DIY version available - like, PVC pipe or something?

Thanks

[1ponders] (Paul) · #2 16-12-2005, 11:05 AM

Will you still be able to bring the image to focus Mike?

I guess it depends on how long the extension tube is and how much focus travel you have.

Maybe someone has an old elcheapo cruddy barlow they are not using and you can remove the lenses from it and use that.

Robby · #3 16-12-2005, 11:13 AM

I would have thought the more you extend the ToUCam the further in you would need to rack the focus, so nothing gained. Isn't the focus point always the same distance from the mirror with a given optical train (eg mirror, barlow etc..)? I have used extention tubes before but only when I could'nt acheive focus (eg with 3.3 reducer on my SCT). The extention tube simply extended the optical path so that I could acheive focus.

I could be totally wrong here here though, and stand to be corrected......
Cheers

iceman (Mike) · #4 16-12-2005, 11:26 AM

It depends on the barlow. For most barlows, you're right.

But the 5x powermate is designed so that extension tubes can be used to increase magnification.

See this article on the TV site.

Robby · #5 16-12-2005, 11:41 AM

Wow, I didn't know that... I guess it makes sense. By putting the Toucam further out from the powermate, you'll need to rack the focus in (I guess). This action will cause the powermate to change the effective focal length and hence magnification...
Well you learn something new everyday.
Cheers

slice of heaven · #6 16-12-2005, 11:54 AM

http://www.atmsite.org/contrib/Carlin/barlow/
Have you tried pvc Mike
I think pvc electical conduit is measured by outside diameter, just need to find the right size
John (asimov) has used extension tubes

[1ponders] (Paul) · #7 16-12-2005, 12:01 PM

If you find that dyi is a pita then try this idea Mike. When I get clear nights

. A variable tele-extender with the appropriate "T" thread adapter.

And if you want reeeallly long FL then you can pop in a 20mm or 15mm eyepiece and use it for EPP. I'll be bringing mine to Lotstock so you can try it out IYW

bird (Anthony Wesley) · #8 16-12-2005, 12:03 PM

Hi Guys, I bought a stack of extension tubes of different lengths when I purchased my filter wheel, from Tru-Technologies in the UK.

I'm sure you can find something cheaper here in Australia, mine all came with T-threads on them which was just what I needed but might not be for everybody.

regards, Bird

Robert_T · #9 16-12-2005, 12:10 PM

Mike - I haven't tried this, but I would imagine you could get a could extension tube by simply removing the lens from a dirt cheap barlow - in fact I might try this when I get home...

asimov (John) · #10 16-12-2005, 01:44 PM

Made my own. Took a bit of nutting out because I did'nt want to buy one. It consists of the draw tube (.965") out of my 4.5" reflector & Mick Pinners .965"-1.25" EP adapter that he gave me.

It was just plain luck that it's all interference fit & everything fits together nicely.

And yes Robert_T, that would be one way.

bytor666 · #11 16-12-2005, 03:32 PM

I use an extension tube when I want the image to be magnified more when I use my 2" Barlow. I bought one from an astronomy store very cheap!!!
Here is my 10mm Pentax XW with the barlow in the first picture, and then with the extension tube in between for more magnification, then just the Pentax 10mm XW with the 2" adapter on it.
I also have a 2" adapter on the pentax as well which enables me to use 2" filters on it ,because it is threaded.
If you wanted to make an extension tube , you can just take the lens off of a barlow.
10mm Pentax XW by itself= roughly 145x
10mm Pentax XW with barlow=300x
10mm Pentax XW with barlow & extension tube =400x
-----------------------------
Mark
12" F/5 reflector
2" Extension tube
30mm 1 rpd
21mm Vintage TeleVue plossl
10mm Pentax XW
2" GSO Barlow
Sony Dig 4.1 Mp Cam

#12 17-12-2005, 01:02 PM

Correct me if I am wrong here, but cant the same be achieved by putting the powermate before the diagonal instead of after it. I seem to remember reading about this somewhere recently.

rumples riot · #13 17-12-2005, 01:19 PM

I looked at the TV 2x extension tube and think that this would suit your purposes Mike. Your image will be twice the size. However as you get bigger with image scale the more the image will break up with gain going higher and the more seeing places an important role. I use the 5x on the SCT's I have and Mars is still the size of a 50 cent piece. At that size you need to have excellent seeing. Quite frankly I don't think this is a workable scale for me.

I am sure that yu will find one to suit your needs.

Paul

bird (Anthony Wesley) · #14 17-12-2005, 01:54 PM

Quote:

Originally Posted by Exfso

Correct me if I am wrong here, but cant the same be achieved by putting the powermate before the diagonal instead of after it. I seem to remember reading about this somewhere recently.

Not everyone has a diagonal...

Bird

Dave47tuc (David) · #15 17-12-2005, 09:13 PM

Hi,

Talking of extension tubes. I need one for collimating my Maksutov.
Anyone know where I can get one about 150 mm long?
To do a very accurate collimation on the Mak, one does not use a diagonal to start with. The focus is very limited so I need an extension tube. It must be able to hold a eyepiece at one end only.
Must be very well made! Mick Pinner!!

Robert_T · #16 17-12-2005, 09:43 PM

Hi all, I tried the thought I had about using a barlow as an extension tube tonight - simply unscrewed the barlow lens and voila instant extension tube!

I've got a shorty 2x and a 3x orion barlow that without the lenses give about 30mm and 75mm extension respectively. I used the extra 30mm with Mars tonight and will post later.

cheers

asimov (John) · #17 17-12-2005, 10:03 PM

Good one Rob. I had no doubt at all that would work! (elementary hey!?)

In my case I'm using the only barlow I have...(WANTED: 1 poxy achro 2X barlow....must have scratches all over the elements & useless!)

rumples riot · #18 17-12-2005, 10:09 PM

Ahh Never thought of that. Good idea Rob. Will use the powermate 5x end to make an extension for the 2.5x. Hopefully that will work also.

Paul

Dave47tuc (David) · #19 26-12-2005, 01:03 PM

I was able to solve my extension tube problems with the help from Michael from Bintel Sydney(thankyou)

I brought a cheapo 3x barlow and with it a 2" extension tube. See photo's.

As you can see it gives me about 150mm extension tube which I need to get inside and outside foucus for collimating.

So the 3x Barlow is long, take the lens out, bingo extension tube.