Which t ring ?

[freemoyoyo]

Hi, I've just purchased a canon 450d, as I'm going to give Astro photography a try, I have a skywatcher evostar 102, but I don't know which t ring to buy, can anyone suggest please ?

[iapa]

Best I can say is that you will possibly want an extension tube if you are using refractor to help focus. e.g. FLO 80mm 1.25" Focus Extension Tube

Refractos normally will have a right angle adapter to bring the eyepiece to an easier to use angle.

The extension tube helps bring your camera to a focus point.

Our friendly vendor (FLO - clock link above) will be able confirm length if required.

Generally you would be looking for an Canon EOS-T2 adapter: T Rings

For my skywatcher StarTraveler 80 I used the extension above plus the Canon EOS ring from the latter link.

 

 

 

[freemoyoyo]

Thanks for the reply, don't think I need an adapter as it is a direct dslr fitment 2 inch focuser. I thought it was Newtonians that need the extension ?

[iapa]

13 minutes ago, freemoyoyo said:

Thanks for the reply, don't think I need an adapter as it is a direct dslr fitment 2 inch focuser. I thought it was Newtonians that need the extension ?

Wasn't;t aware of that

Best check that it is T2 at the camera end - generally these things are - again, tho', the team at FLO are very good at advising if you don't get a specific response here.

[Cornelius Varley]

38 minutes ago, freemoyoyo said:

Thanks for the reply, don't think I need an adapter as it is a direct dslr fitment 2 inch focuser. I thought it was Newtonians that need the extension ?

Newtonians don't usually require extension tubes, sometimes you need to get the camera closer to secondary to achieve focus. Refractors sometimes do require an extension tube to replace the diagonals length if you can't wind the focuser out far enough.  It may be a case of suck it and see.

[ecuador]

50 minutes ago, iapa said:

Best I can say is that you will possibly want an extension tube if you are using refractor to help focus. e.g. FLO 80mm 1.25" Focus Extension Tube

No, the camera will focus with the focuser more drawn-in than when using an eyepiece. You certainly don't need more extension, especially with a scope designed with a T-thread on the focuser. And if you needed an extension, you'd use a 2" one - a 1.25" extension will add severe vignetting to an APS-C sensor.

A simple EOS T-ring will do, they are very cheap on ebay as well (esp. if you are not in a hurry).

There are no multiple kinds of T-rings. T vs T2 simply means that the latter has screws that can rotate the internal assembly, so most t-rings are actually T2-rings, but the threads are all the same. 

Sometimes people call T-rings the rings with an M48 thread. Since the T-thread by definition is 42mm, those are simply M48 rings, not T/T2 rings.

Edit:

5 minutes ago, Cornelius Varley said:

Newtonians don't usually require extension tubes, sometimes you need to get the camera closer to secondary to achieve focus. Refractors sometimes do require an extension tube to replace the diagonals length if you can't wind the focuser out far enough.  It may be a case of suck it and see.

 

That is a valid point, a scope made to focus with a diagonal which is like a very big extension, might need a smaller extension to replace it for imaging. However, telescopes with a T-thread designed for a prime focus attachment like the OPs will not need an extension.

[freemoyoyo]

Thanks very much for the input, I've seen plenty for a little over a fiver delivered on eBay, so I'll order one.

[iapa]

7 minutes ago, ecuador said:

No, the camera will focus with the focuser more drawn-in than when using an eyepiece. You certainly don't need more extension, especially with a scope designed with a T-thread on the focuser. And if you needed an extension, you'd use a 2" one - a 1.25" extension will add severe vignetting to an APS-C sensor.

 

I've been using that extension tube on my ST80 with Canon 700.... 

 

strike 1

[newbie alert]

A focal reducer will help too

[ecuador]

2 hours ago, iapa said:

I've been using that extension tube on my ST80 with Canon 700.... 

You realize that your ST 80 has a 1.25" focuser, so it already has vignetting, right?

This is how a 2" focuser (like the OP's) flat frame on a crop sensor looks like:

And this is how a 1.25" focuser flat frame looks like:

If you add 1.25" extensions or adaptors to your 2" focuser, you lose the advantage of a nicely illuminated field. Obviously you won't use 2" extensions with a 1.25" focuser...

[Alien 13]

There is a simple way to find out if your focusser has enough outward travel and that is to use an eyepiece without the diagonal and try and focus on a star or moon etc, my own experience with refactors is that you need a two inch extension to get the focus tube at a reasonable distance without drooping.

Alan

[newbie alert]

A camera has a different focus.point to your eye/diagonal..

[Alien 13]

10 minutes ago, newbie alert said:

A camera has a different focus.point to your eye/diagonal..

I would have thought that the focus point is the same regardless if its an eyepiece or camera.

Alan

[ecuador]

2 hours ago, Alien 13 said:

I would have thought that the focus point is the same regardless if its an eyepiece or camera.

Alan

I think the focus point is indeed the same, or about the same. When I said the DSLR needs the focuser more drawn in, I meant that a DSLR sensor is 5.5 cm away from the edge of the T-thread, so from the position you'd have the eyepiece you have to draw the focus tube in an extra 5.5cm (so the sensor is where the eyepiece was) and that is if there is a T-thread on it, otherwise even more if you also need to attach a 1.25" or 2" to T-adapter etc.

[newbie alert]

Ok..put an ep in..and find your focus..then attach your camera...still in focus?