Off-Axis Guiding - My Assumptions

[darrellx (Darrell)]

Hi All

I have just come inside after a relatively enjoyable night of viewing. After several weeks of poor viewing weather, the Gold Coast has finally had a night of clear sky.

I have a few points I would like to clarify (in my own mind) about off-axis guiders. Tonight is the first night that I have had a chance to use mine since I purchased it, and I think I have made a few incorrect assumptions about them. So I would like to clear them up.

First, the equipment. I have a Megrez90, and have it on an EQ5 mount. The order of the equipment was: scope - oag - field flattener(vIII) - EOS400. I couldn't put the field flattener before the oag (closer to the scope) because they wouldn't fit together that way. I suspect I would need another adapter.

So, Assumption #1 - the view through the "off-axis" recepticle would be the same as that through the "on-axis" view (I am not sure if they are the right terms, but I am sure you will figure it out). Well it doesn't appear to be. The field of view through the off-axis part seems to be smaller, not just shrunk overall.

Assumption #2 - the views through both recepticles would be exactly of the same part of the sky. They seemed to be different. For example, I centred the view in the EOS on a few significant stars expecting to see the same star in the off-axis view, Sirius, then Acrux, then Alpha Cent. In each of these, the main star had to move entirely out of the field of view on-axis, to get it into view in the off-axis. I did find the two little adjustment screws at the base of the off-axis holder, but the limited adjustment they offer didn't allow enough movement to get the view the same.

Assumption #3 - if the view through on-axis was in focus, then the view through the off-axis would also be in focus. I thought the oag would have been made in such a way that the focal length of each recepticle was the same. Well, the focus was way way off for each view. So I figure that I could use a barlows in the off-axis view to help get it into focus. It didn't work. I used a 3x and yes it did bring them closer, but not enough.

Thats it. I suppose my question is this; were my first assumptions really incorrect and I have now learnt something from the experience or does it appear that I may be mis-using the equipment in some way and I need to get back out there and try again?

The focus issue in #3 is the most perplexing of the three. I would like to hear any suggestions to correct this.

Thanks
Darrell

[gbeal]

Darrell,
while I have very limited time with an OAG, you have experienced the frustrations that seem to drive everyone to a separate guide-scope.
#1, the FOV may not be the same, but it shouldn't matter, as they are for differing purposes anyway. As long as you can find a star in the off axis bit, then all is well.
#2 though is as it should be. The on axis (camera normally) will see an entirely different area of sky, as it should, as the pick off prism for the off axis section is way off to one side. Again not an issue, just find a star.
#3, the focus. Yep, it will test you for sure. Can I suggest you set the gear up during the day, and point the scope/camera towards a distant power pole or similar. Then get both into focus. You may find you need to work it out by looking at it carefully. The distance from pick off prism to chip needs to be the same as pick off prism to guiding eyepiece of guide camera chip, if you get what I mean. It could mean trying the FF/reducer in front of the OAG unit. Trail and error is really the only way, and this is always simpler during the day.
I am sure that there are plenty on this forum that use an OAG, tap into them, they will have experienced all that you are going through at the moment.
Gary

[KenGee (Kenith Gee)]

Darrell, A OAG doesn't need to provide a great image they are only used for imaging? I'm guessing you know that. Auto trackers cope well with OAG stars not being round.
I'll add to what Gary said by saying OAG harder to use then a guide scope but will prodive a better image when it all comes together.

[darrellx (Darrell)]

Guys

Well that makes it a bit easier. I thought the stars would have to be perfect pinpoints.

I suspected a separate guide scope would be easier, but my EQ5 would have trouble handling the weight. I figured the oag might be a suitable alternative until I get an EQ6.

I have played around in the daylight, and while still frustrating, I think I am getting close.

Darrell

[gbeal]

Darrell,
while I guess they don't have to be points, the better they are the better it will be. Daylight is easier huh?
Keep working at it,
Gary

[Merlin66 (Ken)]

What make/ brand of OAG are you using?
The previous comments are pretty sound; to make it a bit easier to orientate and understand field movements, position the guide eyepiece in either dec or RA, that way you can move from the object in the centre of the FOV to the prism with only one movement of the scope.
The pick-off prism doesn't give a wide FOV, but usually ( well, sometimes!) there's a star to use.
Focusing is an issue: I need either a prism diagonal or a 2" extension in the guide train.
There's a neat little "virtual star" program available which allows to to practise guiding indoors
http://sweiller.free.fr/SGE/SGE-star...g-emulator.htm
I've used it on my HEQ5pro + QHY5 with PHD.....
I'm playing with a Vixen clone 50/50 beamsplitter for guiding; you get a 100% field as per the camera, albeit with a slight loss of light.

[darrellx (Darrell)]

I got the Lumicon 2 inch. I figured it would make it easier to get the field flattener in there, being 2 inch. So far, even though I haven't quite got it working yet, I am happy with it. I was expecting there to be more light loss through to the camera (the imaging camera, not the guider). But that doesn't seem to be the case.

I hadn't thought of an extension. I suppose I could use any appropriate tube and experiment with the length.

The other issue that has surprised me a little is just how far out of focus it is when I put the oag in my 8inch newtonian. My cameras come to focus quite easily with no mods. I have a DMK21 and an EOS400. Its no big issue - I didn't buy it with this in mind.

Darrell

[Merlin66 (Ken)]

Darrell,
I've got exactly the same model! Was used on my 18" f4.5 and now is "spare".
I found a 1.25" prism diagonal ( has to be prism to give you the compressed focal length) on the guide tube to be the way to go!
There's absolutely no light loss the the camera as the pick-off prism is well out of the light path to the CCD. The lumicon has quite a large pick-off prism and gives a good coverage.
If I can help with photos etc of my set-up just ask.

[darrellx (Darrell)]

Thanks very much for the info and the offer.

When you say "prism diagonal" do you mean like the diagonal I have for the eyepieces? Doesn't that add a lot of weight to the rear end? If thats the case, at the moment I only have a 2 inch WO dielectric. I suppose I could just get any cheapie 1.25inch (might save on weight).

BTW, arn't you supposed to be heading home, or back to Australia by now??

Darrell

[Merlin66 (Ken)]

Darrell,
A cheap 1.25" prism star diagonal, (the same as the mirror type but with a prism instead of a mirror is the solution.) They weigh next to nix.
I'd love to come home!!!
Looks like the assignment will go through to at least the end of the year.
So I'll just have to get used to all this snow, rain and clouds.
We'll be back to catch up with the family probably in July.

[darrellx (Darrell)]

Hi All

I have the focus issue now sorted.

Thanks Alex, the prism worked as you said. I got a loan from a friend and it came pretty close to being spot on.

I have also removed the lens out of a short 2x barlow, and it works well too. So I have choice - I like that.

Now for the camera, guiding software, and the guideport to all work together.

Darrell