New Off Axis Guider

[FLO]

I will take some images using an Esprit refractor, and I think the Skywatcher OAG will drop onto the rear of the flattener if I have read it right.

Yes, you have read it right

I would also need a T-1.25" adapter for use with the Lodestar by the look of it. Might be an idea to have those as a suggested accompanying item?

Done

Here's what it will look like when assembled:

[Tim]

I'll look forward to this arriving then Steve

Thank you.

[Tim]

Thank you FLO, the OAG and Baader accessories arrived today

First impressions are good. I put the new one side by side with my TS "9mm" OAG and compared them directly. I also took some pictures of the two side by side which I will post in due course.

The new guider is much sturdier than the TS model. I was probably one of the first adopters of the 9mm OAG and at the time there was nothing else on the market thin enough to fit correctly with reducers and coma correctors. But there has always been a few issues with it, and it has been with trepidation sometimes that I have hung several £K's worth of camera and wheel on the back of it. The thread on the camera holder is twice as long on the new OAG, and the fitting itself is better designed, having a ring milled into the sides of the clamping parts of the camera holder. Also the stalk for the guidecam is longer, and doesn't allow light to penetrate up the sides of the stalk as far as I can see, a problem that TS OAG users will be familiar with.

One feature common to both OAGs is that the M48 fitting (scope side) doesn't have a shoulder inside, and it is possible to screw attachments in so far that they press against the stalk, and the stalk is locked and cannot be adjusted. Without care the prism could be damaged or scratched by this.

The hole for the light from the prism up to the guidecamera seems a little wider in the new OAG, although I forgot to actually measure that and have now fitted it to my telescope ready for first light.

Here's a direct comparison in figures with the TS model.

• Width (measured to include the shoulder on the M48 fitting)
  
• New OAG = 16.3mm -- TS OAG = 11.3mm
  

(NB In practical terms the female M48 thread on the TS is only a couple of mm, and most attachments will protrude a bit from it. The shortest width I could ever get my TS OAG to in actual use was 13.2mm, so the new guider is roughly 3mm wider than the TS version in use.)

• Weight (Measured like for like without eyepiece or guidecam holder in place)
  
• New OAG = 170grams -- TS OAG = 80grams
  
• Weight of camera holder alone (Separated here from OAG but included in above figure)
  
• New OAG = 14grams -- TS OAG = 8grams.
  

All I need now is some clear skies to test it out properly, but I have to say, I have used several OAG's and this one is looking good so far

Cheers

Tim

[FLO]

Thank you FLO, the OAG and Baader accessories arrived today

You're welcome, sorry it wasn't sooner.

...I have used several OAG's and this one is looking good so far

That's a good start

[steppenwolf]

Not much to add to Tim's excellent first look report other than to confirm that the construction appears to be excellent.

The hole for the light from the prism up to the guidecamera seems a little wider in the new OAG, although I forgot to actually measure that and have now fitted it to my telescope ready for first light.

I am awaiting a small adapter (48mm to 'T') so haven't been able to attach mine to the telescope yet so I am able to measure the light path hole in the turret and it is 5.6mm in diameter.

[Tim]

Well, it works

Using PHD and my lodestar camera, the image on the screen is an image circle that just about touches the sides of the PHD view and is cut off top and bottom, nicely centred to the cameras chip.

Shining my torch all around the fittings showed that indeed no light is leaking into the guidecamera/imaging camera, a big advantage over its more expensive rival.

The prism produces clean round star images, and using the Baader Focusing Eyepiece Holder 11/4"/T2, getting focus was a doddle. You can get it almost perfect by measuring the distance from the front of the prism to the imaging camera chip, and then just replicate that distance from the centre of the prism, up the stalk to the guide camera chip. (There is a Minimum distance, governed by the length of the stalk. I haven't measured it, but estimate it to be around 50mm).

In summary then, this new OAG offers a sturdy, well made option for off axis guiding where width of the guider is a consideration. It addresses the issues encountered with similar guiders, and is priced very attractively.

My only problem now is what to do with the old one, I can only use so many paperweights

[Tim]

I am awaiting a small adapter (48mm to 'T') so haven't been able to attach mine to the telescope yet so I am able to measure the light path hole in the turret and it is 5.6mm in diameter.

For comparison the TS model has an oval tube, 5.25mm x 9mm

Attached is an uncropped picture from an unbinned lodestar to give an idea of the fov offered by the prism and stalk.

[Tim]

Just a couple of pictures, side by side with the TS version, and in situ on my imaging rig.

[Matthew]

Very interesting , I pick up my C11 later so I will be looking to purchase a O.A.G .

[AndyUK]

Hi Tim - Thanks very much for posting your report... Since we had a chat about them a few weeks back, I've been dithering about getting an OAG as I think it really is the only way for me to go for guiding the MN190... I've been a bit distracted recently, but I can see me placing an order for one of these in the not too distant future!

[Helen]

I've got a quick question Tim.... would it be possible for you to measure the minimum distance to the guidecam? if it is 50mm wouldn't that scupper getting the spacing right behind a FR with the standard 55mm focus point? or have I missed something??

Helen

[steppenwolf]

Helen, the turret table can be adjusted down the turret itself to reduce this distance to at least 36mm. This means that the turret projects upwards into the throat of the guide camera but provided it doesn't touch the sensor it would be fine. I'll get an accurate final measurement for you and what that projection is as soon as I can.

[Helen]

Thanks Steve. No immediate rush, the weather looks awful for a while!!!

Helen

[steppenwolf]

At a 'table' height of 36mm from the centre of the prism, the projection of the turret is 10mm. The fixed height of the turret from the prism centre is 45mm.

The absolute minimum table height is 31mm as the prism would then be at the edge of the OAG opening which means a 15mm projection of the turret above the table.

Once I have the adapter I need, I will test this with a FR requiring the standard 55mm spacing and report back as there are more measurements to be taken into consideration but with 10mm maximum to play with, this is going to be tight!

Sent from my iPhone from somewhere dark .....

[AndyUK]

Thanks Steve - That's really helpful!

[Tim]

The stalk/turret could be shorter than it is.

[steppenwolf]

The stalk/turret could be shorter than it is.

I'm thinking hacksaw ......

[Helen]

...I'm thinking back to my dovetail thread....

Helen

[steppenwolf]

Ok, although I haven't got the 48mm male to female 'T' physically here, I know that it adds an extra 3.5mm to the light path.

As you can see from the diagram below, using this adapter to connect to a William Optics FR IV, FRIII or a Hotech Field Flattener as examples, (all of which have a 55mm spacing requirement), the new OAG will not allow the correct spacing no matter how you 'cook' it as 3.5 + 3.0 + (13.0 / 2) + 45.0 = 58.0mm.

However, the excellent Sky-Watcher range of focal reducers for the ED series already have a 48.0mm male thread on them so they will attach directly to the front of the OAG without any adapter so they will be entirely compatible as will any focal reducer that has an allowance of greater than 55.0mm of backfocus (like Tim's for example). The WO FR IV will also be compatible by adjusting its distance adjustment ring by 3.5mm to compensate.

[Gina]

I'm using this OAG with my MN190 but havent had more than an hour's clear sky to test it as yet. I can say I think it is very well designed and made - the engineering of it is superb. The oval guide camera stalk is a nice clearance fit in both the OAG body and the camera holder. There is a very useful knurled knob to move thestalk and prism in and out to help with finding a guide star. As has been said, the guide star is perfectly round. I found focussing on a star by sliding the guide cam disc up and down the oval light pipe was very much easier than I expected, helped by the smooth surface and good engineering that meant it would slide smoothly and not bind. I was able to use my finger as a sort of Crayford focuser on the side of the oval tube and move the QHY5 up and down the stalk surprisingly accurately.

I don't know who makes this OAG but the quality is reminiscent of Baader Planetarium - I would say it is that good

[steppenwolf]

I don't know who makes this OAG but the quality is reminiscent of Baader Planetarium - I would say it is that good

I agree, it is beautifully and solidly machined and finished - I'm very impressed thus far as I can't see flexure being and issue here.

[AndyUK]

I think the last two posts have decided it for me...

Gina - My primary aim for possibly buying an OAG is for my MN190, so the fact that you've said it seems to work well is good enough for me.... and...

Steve - Although I *may* need yet another adapter, as my reducer has a spacing requirement of 80.5mm, I don't think I'm going to have much of a problem... even the over length stalk may work in my favour, and hopefully it'll improve my balance too . I just need to see if I get any leftover cash from my birthday .

[Tim]

There is a very useful knurled knob to move thestalk and prism in and out to help with finding a guide star....

While you can do this, it is probably better practice to set your prism as close to the edge of the fov as possible without blocking the light to the prism. This way any potential for shadows is reduced and only one set of flats will be required. The OAG has three potential camera positions with a bit of rotational movement, but with a sensitive enough guide camera there really shouldn't be any problem finding a suitable star to guide on just leaving the OAG set up as it is. Sometimes just a bit of nudging of the target a little bit is all that is required to bring a star into view.

I find that once you have a working position, the less you mess about with things, the more time you get on the sky, and the more productive you can be

[Gina]

While you can do this, it is probably better practice to set your prism as close to the edge of the fov as possible without blocking the light to the prism. This way any potential for shadows is reduced and only one set of flats will be required. The OAG has three potential camera positions with a bit of rotational movement, but with a sensitive enough guide camera there really shouldn't be any problem finding a suitable star to guide on just leaving the OAG set up as it is. Sometimes just a bit of nudging of the target a little bit is all that is required to bring a star into view.

I find that once you have a working position, the less you mess about with things, the more time you get on the sky, and the more productive you can be

I certainly agree with keeping things the same as much as possible and not fiddling about. The sensitivity of the guide camera could be an issue but I'll have to see - need some clear night skies first to try things out. Guess I may have to steel myself to spending lots of £££££ on a Lodestar. Still I may have saved myself over half of that by not buying a Moonlite focuser

[dmahon]

Ok, although I haven't got the 48mm male to female 'T' physically here, I know that it adds an extra 3.5mm to the light path.

<snip calculations>

However, the excellent Sky-Watcher range of focal reducers for the ED series already have a 48.0mm male thread on them so they will attach directly to the front of the OAG without any adapter

Doesn't that mean you need a male T to male 48mm adapter to fit most focal reducers to the front of the OAG? This can be had as zero length.