QHY5 - Can't fully focus image

RPW (Rod) · #1 12-09-2009, 11:09 PM

Hi,

I recently purchased a QHY5 that ultimately I want to use as an autoguider. In order to get a feel for it, I connected it into my SW-102 refractor (F=500mm, D=102mm) and my laptop to have a look at the images that it produces. I found that I could not fully focus on an object before I ran out of focus (couldn't wind the focus in any further). Tried looking at Jupiter, Fomalhaut and various faint stars and found the same problem. I tried using a 0.5x focal length reducer screwed into the T-piece adaptor of the QHY5 to increase the fov and it seemd to have the opposite effect, narrowing the field (?!?).

Does anyone have any ideas on what I can do to fix this problem / increase the fov??

Octane (Humayun) · #2 12-09-2009, 11:18 PM

Rod,

Sounds like you need an extension tube. I had the same issue using a Meade DSI for autoguiding.

Bintel, and, other retailers stock them.

Regards,
Humayun

RPW (Rod) · #3 12-09-2009, 11:58 PM

Hi Humayan,

Thanks for the tip! Anything I need to know about extension tubes before getting one?

Rod

Octane (Humayun) · #4 13-09-2009, 12:22 AM

Rod,

They come in various lengths. Most common is 35mm and 50mm, however, you can also get 80mm.

Do you feel that you get almost there? If so, 35mm will be sufficient. But, just to be safe, it may be worthwhile getting a 50mm. Also, they come in two sizes: 1.25" and 2". You'll need to get the appropriate one.

Typically, you don't want to rack the focuser all the way out, and then use a short extension tube. You introduce focuser slop by doing that. It's best to have the focuser as out as little as possible and rely on extension tubes. So, it's typically better to get a longer extension tube than to rack the focuser all the way out and whack in a 35mm.

Hope this helps.

Regards,
Humayun

Merlin66 (Ken) · #5 13-09-2009, 12:40 AM

Rod,
Were you trying to focus the QHY5 through the star diagonal?
The camera should be used without the diagonal, but as mentioned, you'll probably need an extension. I use the 50mm size.

RPW (Rod) · #6 13-09-2009, 12:56 AM

Thanks Humayan.

As I wind in the focus knob, it feels like that I am not too far away from achieving focus. Probably better to be safe and get the bigger tube as you suggested.

Just as a check, my issue is that I run out of focus winding the tube in (not out). So the extension tube is still appropriate (in reading your message, I started to wonder whether you were describing the situation where the focus had been wound all the way out)?

Rod

RPW (Rod) · #7 13-09-2009, 01:19 AM

Hi Merlin,

Missed your message earlier.

I was focusing through the star diagonal as you summised. Will give it a try without it in as per your suggestion.

Many thanks.

Rod

MJ_ (Matt) · #8 13-09-2009, 08:58 AM

i had to use an extension tube with mine

MJ_ (Matt) · #9 13-09-2009, 09:02 AM

Quote:

Originally Posted by Merlin66

Rod,
Were you trying to focus the QHY5 through the star diagonal?
The camera should be used without the diagonal, but as mentioned, you'll probably need an extension. I use the 50mm size.

i tried a diagonal with mine and it seemed fine, where does it say it shouldnt be used and why is this bad?

pmrid (Peter) · #10 13-09-2009, 12:01 PM

Am I right in understanding that you can't wind your focuser IN far enough to get focus? If you take the diagonal off, you should be able to get focus without any issue. Did you try that? Answer that before you go off buying extension tubes.
Peter

[1ponders] (Paul) · #11 13-09-2009, 12:19 PM

Quote:

Originally Posted by MJ_

i tried a diagonal with mine and it seemed fine, where does it say it shouldnt be used and why is this bad?

Depends on the scope. One of my refactors will reach focus with one of my planetary camera with the diagonal in and one of the others wont and I have to use an extension tube.

Octane (Humayun) · #12 13-09-2009, 12:42 PM

Ah, yep. Thanks, Merlin66. I completely forgot to ask if Rod was going through a diagonal.

I think he'll find he'll be OK now without the diagonal. If not, extension tubes.

Regards,
Humayun

Merlin66 (Ken) · #13 13-09-2009, 04:48 PM

On the ED80 type refractors with a diagonal in place, it doesn't give a lot of back focus. Sure, with a webcam (or DMK if you're lucky) it might reach focus, but with other CCD's and DSLR's it's well neigh impossible. The QHY5 needs about 12.5mm back focus...
The answer is to remove the diagonal and use a extension spacer tube.

mithrandir (Andrew) · #14 13-09-2009, 06:16 PM

Quote:

Originally Posted by Octane

Ah, yep. Thanks, Merlin66. I completely forgot to ask if Rod was going through a diagonal.

I think he'll find he'll be OK now without the diagonal. If not, extension tubes.

I can get my guide camera to focus using either a diagonal or an extension tube. The extension gives a better focus, no doubt because there is one less mirror to degrade the image.

multiweb (Marc) · #15 13-09-2009, 08:22 PM

Quote:

Originally Posted by RPW

I recently purchased a QHY5 that ultimately I want to use as an autoguider. In order to get a feel for it, I connected it into my SW-102 refractor (F=500mm, D=102mm)

Just to give you an idea on a FL 600mm/ED80 (similar to your rig) I need a 80mm spacer + 55mm of focuser racked out to get to focus.

RPW (Rod) · #16 13-09-2009, 08:26 PM

Hi Everybody,

Thanks for all of the good advice. I took out the diagonal as suggested and found that now I have insufficient back travel to achieve focus, so I will get an extension tube as suggested.

Take care.
Rod

RPW (Rod) · #17 13-09-2009, 08:30 PM

Hi Marc,

I think that your advice is spot on. When I took the diagonal out, I still had a way to go to get to focus before racking out. I was planning on going for the 80mm tube. You have just confirmed my instinct. Thanks!

Rod

multiweb (Marc) · #18 13-09-2009, 08:35 PM

Quote:

Originally Posted by RPW

Hi Marc,

I think that your advice is spot on. When I took the diagonal out, I still had a way to go to get to focus before racking out. I was planning on going for the 80mm tube. You have just confirmed my instinct. Thanks!

Rod

GSO 2" 80mm extension approx. $35.00