Old 23-09-2018, 03:28 PM
Legin (Nigel)
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Legin is offline
Join Date: Jul 2008
Location: Sydney
Posts: 76
Focuser 9.25" Edge HD


Ok so I have taken a bit of a plunge and obtained a 9.25" Edge telescope. I currently have it on an AZ NEQ6 GT mount. I use a few cameras but mostly use either the ASI 174MC or the trusty ASI120MM.

This is pretty good scope for visual work and I rarely have any trouble focusing. At least it looks good visually.

I am trying to take some images of the planets and Moon. At prim focus I can almost get focus right. I zoom in with Sharpcap to 200% and focus the best I can then go back to 100%. Then I cut the size down to something like 900x900 pixels (can not remember the exact number). Then I take a short video usually 30-50 seconds or so.

The problem I have is that when I try to get closer in using any kind of barlow I find it near impossible to obtain perfect focus. I usually use a 3x barlow. If the seeing is really good I wish to go for a 4 x barlow and vice versa bad seeing I will use 2x if any.

At higher magnification I usually do a similar thing I zoom in to something like 150-200% and try to focus as best I can. This is difficult to do as it is effectively bringing the magnification way higher. But If I get the best I can at this % I figure it should be perfect at 100%.

Then I repeat the process cropping down the image size and capturing a short avi of 30-50 seconds.

The focus never seems to be quite right.

I have a few questions. As it is the scope will focus by moving the primary mirror. This leads to a significant image movement. This really makes it hard to focus at high power as the image often jumps out of frame and I need to move the scope.

As such I am thinking of obtaining an external focuser. What do you recommend. I am happy to pay a bit extra to get a good focuser as I believe it will be well worth it. Messing around with bad focusers is frustrating to say the least.

On another note I currently have two 5kg counter weights on the scop but even at the most distance from the mount still seems a bit heavy on the scope side. Could I put another 5kg counterweight on this mount. I am a bit worried that I am approaching the max weight. It seems within the specs as the mount can handle up to 22kg or so without any problems and I think the scope is around 13-14kg. Would the counterweight cause problems. I think it is bordering on the slightly heavy side but it should be ok.

Any help you can provide will be appreciated...
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Old 23-09-2018, 03:52 PM
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Merlin66 (Ken)
Spectroscopy Wizard

Merlin66 is offline
Join Date: Oct 2005
Location: St Leonards, Vic
Posts: 7,859
I think you're pushing the limits...
Rule of thumb: optimum imaging focal ratio = x5 the pixel size, in micron.
For the ASI 174 (5.86micron) this gives around f29, close to a x3 Barlow on the C925.

Running the mirror through the full focus travel (30 turns or so) re-spreads the grease between the mirror boss and the baffle and usually reduces the slop.
Even if you do fit an external focuser you still have the potential of mirror movement unless you use the fitted mirror locks.

I use a C11 (no mirror locks) and regularly work at f10 for spectroscopy. I can easily focus - using a JMI focus motor on a 20 micron slit with little focusing slop.
I have 5 x 5Kg counterweights on my NEQ6pro - no issues, no drama.
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