OAG and DMK on refractor?

wysiwyg (Mark) · #1 13-03-2009, 03:09 PM

Hi Guys,

I know this is a fairly specific question, but have any of you had experience using an OAG (Off-Axis Guider) with a DMK on a refractor. Could be any scope I guess.
My main concern is; is the DMK sensitive enough to pick up guide stars from a percenatge of the light cone?

Your comments are appreciated.

Main, reason for looking into one, is it would remove any flexure, reduce weight or free up the ED80 to do some widefield at the same time.

Particularly looking at this model:
http://www.apm-telescopes.de/info.php?id=37397
as it only takes up 9mm of back focus. If its placed between the dslr and the reducer it would remain within tolerance. Other brands take up more back focus and would not be as suitable to my setup.

Cheers
Mark

darrellx (Darrell) · #2 13-03-2009, 03:59 PM

Mark

This is exactly what I have done, and for the same reason. I have a Megrez90 and a DMK21.

To answer your question - yes, it is sensitive enough to pick up guide stars.
I got the 2inch Lumicon from MyAstroShop -
http://www.myastroshop.com.au/produc...asp?id=MAS-115.
I haven't had it that long, but from what I have tied so far, it is working great.

Darrell

wysiwyg (Mark) · #3 13-03-2009, 04:22 PM

Quote:

Originally Posted by darrellx

Mark

This is exactly what I have done, and for the same reason. I have a Megrez90 and a DMK21.

To answer your question - yes, it is sensitive enough to pick up guide stars.
I got the 2inch Lumicon from MyAstroShop -
http://www.myastroshop.com.au/produc...asp?id=MAS-115.
I haven't had it that long, but from what I have tied so far, it is working great.

Darrell

Thats exactly what I wanted to hear Darrell

Have you ever had problems finding a guide star, either in light polluted skies or dark skies?
Also, what exposure time is your DMK set to, 1 or 2 seconds, or more?

Cheers
Mark

darrellx (Darrell) · #4 13-03-2009, 08:43 PM

Mark

First of all, I have only had this gear for a month of so, and the weather has been poor. As a result, I have only had a few opportunities to use it. But it does seem to work fine.

I live on the Gold Coast and do most of my viewing from the front yard. Light pollution is a real problem.

Depending on what I aim at, finding a guide star has not been a problem, BUT, I have only had the time to aim at some significant groups. I tried M42. Worked well. I had 4 or 5 suitable stars available to guide on.

I tried NGC5128. I had 3 suitable stars to guide on.

I tried NGC3372. I had 6 or so stars.

So I would say that I have had no problems finding a guide star.
Exposure time? No more than 2 seconds. Generally. I did go up to 4 seconds at one stage to see what happened.

My only concern I have had so far is focus. I initially found it a bit difficult to get the both cameras in focus. But I have found that PhD doesn't really require the guide camera to be in perfect focus. As long as the guide stars are "reasonably circular blobs" it guides okay.

Hope this helps.
Darrell

AlexN · #5 13-03-2009, 10:45 PM

I used a Lumicon 2" Easy Guider with my Megrez 102, guiding with my DBK31 (quite a bit less sensitive than the DMK21) with good results... It definitely sorted out flexture problems I was having with separate guide scope setups.. and made overall setup time a lot lower, easier to balance, less weight on the mount with more accurate tracking as a result..

For focusing, as Darrell said, it can be tricky.. but what I did, was focus the imaging camera up, then by adding a 2.5cm extension tube (barlow without the lens) the guide camera was VERY close to focus, I then pulled the guide camera out 1mm at a time untill best focus was achieved, once it looked good I scribed a line on the cameras nose piece with a sharp knife, so that next time I could simply slot the camera in and tighten the set screws when the scribed line was at the edge of the extension tube's barrel.. This made overall focusing and setup time shorter again...

Best of luck with it, and remember, in the future you can always move to a more sensitive guide camera... The DMK will work well, and things can only get better from there....

Alex.

gbeal · #6 14-03-2009, 07:05 AM

While not disagreeing with anyone here, the unit that Mark referred to (at APM) looks like the guide-cam screws directly on, and this will give a bit of grief with respective focusing of the two cameras. Yes it is possible to screw onto this an extension tube, or helical focuser, but as the picture stands there is little or no focus adjustment.
Gary

wysiwyg (Mark) · #7 14-03-2009, 10:50 AM

Thanks Alex, Gary.

From what I have read the DMK would screw onto the prisms shaft using a C - T mount adapter.

Just underneath the disk is a grub screw, which you undo and move the DMK up and down the shaft to adjust focus.

I may be wrong but that is how I understood the explanation on their website.

Mark