setting up an OAG

[Ivor]

I have recently brought an OAG and I'm struggling to get the Lodestar working and I was looking for some pointers. The optical chain is QHY8L, spacer, OAG, flattener, FLT110.
My workflow to date is as follows:
During dusk I've pointed the scope toward a tree in the distance and adjusted the focuser until the QHY8L was in focus (exposure 40ms).
I positioned the OAG so it wasn't impinging the QHY8L frame, I have the sloped face of the OAG stork pointing towards the open telescope end.
I have various extension tube attached to the OAG stork which give me about 100mm of travel for the Lodestar however this completely failed. Using PHD I set various exposures lengths from 10 – 500ms whilst moving the Lodestar through the travel range, but all I got was a noisy screen.
I removed the lodestar and tried directly on an LED and it is working. Whilst writing this I've thought I should test it in the dark with a LED at the end of the optical path to at least confirm it can see something.
Does anyone have experience of this or other suggestions?

[RobertM]

I've tried this, supposedly simple, way of getting an OAG to focus by looking at terrestrial target but never had any success.

If you have a lodestar it's best to focus your main camera on a star in a dense area of the Milky Way then focus the lodestar with 0.5s exposures.  Loosen the lodestar so it can slide up the prism stick then move it back and forth slowly; of you can't see any stars then you are either too far in or out.  The most tricky part is measuring the approximate position of where it should go.  Could you post a pic of your OAG setup ?

Robert

[The Thing]

1) The flat face of the prism should point to the OTA and the angle to the main CCD! This probably your main issue The angle is the 'mirror' that sends light up the OAG to the guider. Make sure the prism is far enough into the light path to actually pick up some light! (see 1a).
1a) My prism has a height and tilt adjustment. To set this I use my flat field light source and adjust until illumination of the guider is even. The prism protrudes into the DLSR imaging area but I minimise this by having the DLSR rotated so the width of the chip is parallel to the prism if you follow me. Even at other orientations I have never seen the prism on a sub.
2) The distance from the prism to then CCD and the guider chip need to be identical including the focusing movement of the  guider. You probably got a load of T-mount tubes with your OAG to allow you to do this.
2) I focused roughly during the day, the further away the object the better. I have Brightstar/Orion OAG and the focusing adjustment is very crude with thumbscrews and sliding stuff, so I ended up adding the focusing mechanism from an old lens so I have non-rotating helical focuser on the OAG. You can by these focusers for a price but the tend to be physically long. Life is easy now.

So my set up is Baader low profile SCT-T adaptor, short t-tube to get the reducer distance correct, OAG, DSLR / OAG, focuser, QHY5L-II. I keep it as short as possible.

Hope that helps.

[Carole]

I have never used an OAG, but I have used a QHY8L.  This barely shows bright magnitude stars in short looping being a OSC camera, so I am wondering how you can use an OAG with it, surely it's not sensitive enough.

Carole

[MarkS]

I have never used an OAG, but I have used a QHY8L.  This barely shows bright magnitude stars in short looping being a OSC camera, so I am wondering how you can use an OAG with it, surely it's not sensitive enough.

The guide star (if you can find one!) will be brighter through an OAG compared with the typical guide scope because it is collecting light from the full aperture of the main scope rather than the smaller aperture of the typical guide scope.

[Ivor]

Thanks for the replies, I got a chance to do some analysis last night and I've managed to find the root cause of the problem (other than having the prism the wrong way around).

I pointed the camera at M44 with the goal of working out the difference between the focus point of the Lodestar and QHY8L I success managed to focus the QHY8L in the current configuration but couldn't do anything to  get the Lodestar in focus. It then dawned on me the Lodestar focal could be closer than the QHY8L, unfortunately with the WO FF/Reducer IV in place the the focal point for the telescope is 25mm so I had to remove this from the optical path.

I connected the OAG and QHY8L directly to the scope, the FLT110 has a focal range of 0 - 77mm and the Lodestar came into focal at 76mm. This meant the QHY8L had a focal point beyond the maximum capacity of the focuser so I added a couple of spacers and was able to work out the focal point was 127. This means the Lodestar needs to be 51mm closer than QHY8L.




My preferred set-up would be like this with the WO FF in the path




However this isn't now possible as the QHY8L CCD needs to be 50mm behind the FF so it's impossible to have the three things lined up together in their set-up and maintain the spacing between the Lodestar and QHY8L.

The only way I can think of solving this is by finding a way to reduce the required 51mm gap between the Lodestar and the QHY8L, any suggestions how this could be solved?

[RobertM]

Hi Ivor,

Sorry for the delay.

In this example I'm assuming that the 77mm is the distance where cameras reach focus ?

Since the guide camera is stuck up on the mirror tube of the oag you'd need to work out how far from the diagonal mirror the guider focal plane is.  Yes I know that distance is variable and affects this calculation a lot, I think you're going to have to either make an educated guess or find out what the extremes are.  Take that figure off the 77mm back focus of the reducer to give you the distance of the centre of the diagonal mirror to the reducer.

Back focus distance of QHY8L = 20mm

e.g. If the guider focal plane was 40mm above the centre of the mirror and if your oag had an optical path length of 20mm then:

Distance from the oag mirror to reducer = 77 - 40 = 37mm
(f/l of reducer - guide prism path length)

Path length of spacer(s) to oag = 37 - 20/2 = 27mm
(assuming the oag mirror is at half the optical depth of oag)

Path length of rear of oag to QHY8L focal plane = 77 - 27 - 20 = 30mm

Length of spacer(s) to QHY8L = 30 - 20 = 10mm
(this is the residual spacer length to bring the QHY8L to focus from the back of the oag assuming the back focus distance of QHY8L to be 20mm)

Looks like it's doable, assuming that the focus point of the guide camera isn't too much further out.

Note that no account has been taken of any adaptors needed to fix all this together.

One other thing to take into account is that you want the oag pickoff prism positioned so it doesn't form a shadow over the QHY.  I think it's easier to do this if the oag is closer to the main camera and that's also what you see done in practice.

How does that sound ?

Robert

[Ivor]

Thanks Robert, I've been thinking about what you have said and I must have had some double negatives in my calculations, so I've revisited them and based them on the prism. I've had a look at the TS website and it provides a lot of the dimensions but doesn't provide the details for the prism to guider top so I did it the old fashioned way and I'm going with 45mm for now.



The spacing for the kit are as follows:



Lodestar back focus 12.5

WO FF IV back focus = 55 with an adjustable range of 62.5 to 43.5
Based on 55mm the space between the OAG and QHY8L needs to be 17mm
   FF -> QHY8L chip =  18 + 17 + 20 = 55mm (optimum)
This means the distances to the prism are:
   Prism -> QHY8L chip = 4.5 + 17 + 20 = 41.5mm
   Prism -> Lodestar chip = 45 + 12.5 = 47.5mm (focus can't be achieved)

If I adjust FF to be 62.5mm the spacer now needs to be 24.5mm

This means the distances to the prism are:
   Prism -> QHY8L chip = 4.5 + 24.5 + 20 = 49
   Prism -> Lodestar chip = 45 + 12.5 = 47.5 (1.5mm to play with)

Marathon training clashed with the few clear skies we've had prevented me testing straight away, however I did manage over the weekend and alas I still can't get sufficient focus on the Lodestar to guide. There is potential for inaccuracies in my measurements and with little room for error in the calculation I don't think there is an option but to buy a FF with a greater back focus.

[RobertM]

Sorry I got the mistaken impression it was 75mm back focus

I think you're going to have to use all the back focus that reducer will give you and even then it might not work.

As a matter of interest have you tried a separate guide scope ?  Unless there is a lot of movement in that focuser then that would be far less trouble.

Robert

[Ivor]

I currently use a Bresser 70 as my guide scope, I've found the lodestar has to have a scope with focal length greater than 700mm to guide properly, I've tried to use finder/guider scopes but the pixel resolution is too large so doesn't guide. I'm trying to reduce the overall weight so I can put two scopes on the EQ6Pro.

[RobertM]

A finder guider should be fine with the lodestar.  Guide programs like PHD calculate the sub pixel centroid of the star, the only limit is seeing and mount accuracy (assuming the scope assembly is rigid).  You will get more problems with the Bresser because it was never designed to be rigid and that's just about the key element with guiders.  I've used a lodestar with finder guider and binned 2x2 had no trouble guiding 1000mm and 1200mm scopes to great accuracy.  Is it worth revisiting the finder guider, having been there I can help if needed.

Robert

[Ivor]

Sorry for not replying Robert I've been busy with work.

I experimented with a finder/guider last year (http://forum.orpington-astronomy.org.uk/index.php?topic=9578.msg68921#msg68921) and had a good play with this with Mark and it wouldn't guide well.

I've ended up buying a new TS FF which should be delivered at the end of the week.

[RobertM]

No problem Ivor.

Personally speaking, unless you have a lot of flex in your focuser I can't see why a 50mm finder guider and lodestar wouldn't work.  I've use that combo successfully and it should be even more stable on a refractor.

Which TS flattener did you get ?

Robert

[Ivor]

I will admit there could be an additional problem with mount guiding which I've not been able to eliminate (see reply #11 in the other thread), however at the moment I seem to be able to get away with imaging through the Bresser at high declinations.

It's the TS 2" universal Field Flattener for f/5 to f/8 Refractors from Modern Astronomy.