t-mount adaptor and ring trouble shoot?

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  • Member since
    May, 2004
t-mount adaptor and ring trouble shoot?
Posted by alancorey1979 on Thursday, June 28, 2012 9:58 PM

Hello everyone... whew, I haven't been on here in awhile. Guess I've been busy with school and what not. Anyway, I have a problem... 

I bought a hotech sca t-adaptor and a t-ring for my canon 600D for a attempt at prime focus astrophotography. I am using a 5.5 in newtonian reflector and I am certain i am attaching the adaptor to my focuser rack correctly. However, I cannot achieve a focus in my camera. I can get close objects like trees but as for getting the moon.... a no go. I'm guessing I don't have a short enough focal length to allow a crisp focus, but I can't find any information on the hotech website to give me a definitive conclusion on the possible cause of this issue.

has anybody heard of this problem? Anybody with suggestions?

thanks 

Alan C.

" Astronomy compels the soul to look upwards and leads us from this world to another." -Plato, -source: The Republic Book VII. 529

Alan C.

President of Central Wyoming Astronomical Society

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  • Member since
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  • From: Texas
Posted by chipdatajeffB on Thursday, June 28, 2012 10:57 PM

It is fairly common for a DSLR not to reach focus at the prime-focus location of a newtonian reflector.

Possible fixes include a low-profile focuser (replacing the focuser), moving the primary mirror forward in the telescope tube (not rocket science, but not for those who are tool-challenged), and using a relay lens.

A relay lens is any optical lens that extends the scope's focal point outward through the focuser. One common way to achieve this is with a technique called "eyepiece projection" where you attach the camera to an eyepiece which is in the focuser. There a several adapters on the market that provide this capability using interchangeable eyepieces.

Some of these adapters have T-threads at the outer end, so you can screw the adapter directly into your T-adapter.

Some of these adapters alternatively have a camera filter thread machined into the outer end and with these you use your camera's normal lens and thread the adapter with the eyepiece inside it directly to the camera lens (you must ensure you have a camera lens threaded for the appropriate size, or that you step-up or step-down adapters to fit. With this sort of adapter, you're doing "afocal" photography.

The universe is not only stranger than we imagine, it's stranger than we CAN imagine. --- JBS Haldane

Come visit me at Comanche Springs Astronomy Campus (we're on Google Maps) in Texas.

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  • Member since
    May, 2004
Posted by alancorey1979 on Friday, June 29, 2012 11:13 PM

thanks for the advice. I have a 10 inch dob that is very easy to move the mirror up and down the tube. I htink I will try it on that.

" Astronomy compels the soul to look upwards and leads us from this world to another." -Plato, -source: The Republic Book VII. 529

Alan C.

President of Central Wyoming Astronomical Society

find us on facebookWink

Moderator
  • Member since
    July, 2002
  • From: Texas
Posted by chipdatajeffB on Friday, June 29, 2012 11:30 PM

One way to estimate how much to move it is to notice where the field lens of your eyepieces are in the focuser when they're focused. That is where the focal plane of your camera needs to be. Compare that to the nearest point the camera can actually reach, and the difference is about how far you need to move the mirror.

Since you can rather easily move the camera outward, you might fudge a bit and add a half inch or so just to be sure you don't have to move it twice.

Another method I've seen used is to remove the mirror cell from the tube and then stand the tube on its end over a book or other flat object that raises the mirror inside the tube. With the mirror on that "rest", see if your camera can reach focus. If you have a textured ceiling, it should show up as focused when you have it right. Then you can use the thickness of the book (or more than one book if needed) to gauge how far forward to move the mirror.

The universe is not only stranger than we imagine, it's stranger than we CAN imagine. --- JBS Haldane

Come visit me at Comanche Springs Astronomy Campus (we're on Google Maps) in Texas.

www.3rf.org

  • Member since
    August, 2008
Posted by BrianC on Wednesday, November 28, 2012 12:21 PM

Ah yes...prime focus photgraphy...I have to move the mirror in my up abput 2 inches to get in range where I can focus the camera..a rule of thumb is to measure the depth of the camera body and move the mirror up that much...I was fortunate that my Orion Starshooter was thin enough to still fit within the focusing range of the mirror..if you dont want to paly around with prime focus you can get an adapter for eyepiece projection that uses the same t-ring.

Director of 'Orion Backyard Observatory' a blog for small aperture observers.
  • Member since
    November, 2009
Posted by Poppa Chris on Wednesday, November 28, 2012 1:36 PM

I vote for eyepiece projection also.  It's the easier, most cost effective way to go.  Orion has two models that will screw into your T-ring.  One of them has an adjustable length which varies allows you to vary the magnification of the eyepiece/camera combination.

I don't know why it has never been done, but a variable length T-adapter would solve a lot of these back focus issues.  Typically a standard T-adaptor is approximately 5" long.  A variable could easily be manufactured that would allow setting it up anywhere from say 2"-6".  A two inch reduction in your camera's sensor to main mirror placement would probably do the trick in your setup.  Some other guy may need three, yet another only one.

Think maybe I could patent this thing?  Wink

---Poppa Chris---

Denham Springs, Louisiana USA

"Second star to the right - Then straight on until morning!" - Peter Pan

Celestron CPC1100GPS (XLT) - 279mm aperature, 2800mm Focal length. (f10) Celestron Ultima LX (70deg AFOV) Eyepieces 32mm thru 5mm, Canon EOS Rebel T2i DSLR, Backyard EOS imaging software, Orion Star Shoot Planetary Imager IV, Celestron Skymaster 15x70 binoculars

 

  • Member since
    February, 2013
Posted by Skyler on Tuesday, February 12, 2013 2:48 PM

I have same problem with my telescope.

it didnt help to move the mirror, i could only move it backwords.

I was in touch with meade.de yesterday, and gave them some snapshot, and they said that T-adapter i use are to long to get focus.

They also recommand an 2" lock ring on telescope.

They also said they have an adittional ring for new models that make it possible to connect the EOS camera direct on telescops focus without t-adapter.

they said they would sell it for about €15 each, and i was told that they should check if i could get both for €27 so they would not be in risk to be stoped in customs with extra taxes and fees, and should mail me back today, but i never heard anything from them.

And i dont know what rings they was talking about, only that they was 2" and the additonal ring could connect direct on the cam.

Anyone know what rings they was talking about ? i have a canon EOS 60DA cam and an bresser messier 203/900 with 1.25" oculars but i can use biger ocular if lock/connection ring on telescope is 2" to

  • Member since
    January, 2013
Posted by EddieD on Thursday, January 30, 2014 12:58 AM

I've come across the same problem as I've just started with prime focus. my t-adapter is 4" long but I discovered that I can remove the thread from the tube and connect it directly to the 2" focuser. I also tried to use a barlow with the 4" adapter and it worked well with lunar shots. I've tried to use other lenses inside the tube but I haven't been able to get anything other than pitch black images, even when I removed the 4" tube from the t-adapter. 

On a side note, I'm using a 4.5" reflector with a Sony NEX 5 camera. It's not quite a DSLR in that it is mirrorless, but it uses a huge sensor like a DSLR. I've got plenty of great wide field shots with the camera alone, especially when I went to Mauna Kea in Hawaii (which was the most wonderful place I've ever been).

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