Off Axis Guiding - About to start

[Aidan]

Just about to start my journey down the OAG route.   I've bought a ZWO OAG - Its got good reviews and only adds 16.5mm to the optical train.   It will fit in nicely with the spacers i already have with only a .7mm difference, which i think i have a delrin .7 if i want to me spot on!

Image train will be   SXVR 25MC - OAG - Spacers - Flattner - Scope

Guide cam will be my QHY5L-II mono.     Any hints/tips that might help me along or trip me up when i get this setup to test?   What are they like to get in focus for use in PHD2? 

I'm busy reading lots - but it can sometimes get overwhelming due to many factors

I'd like to hear your thoughts.

Rgds
Aidan

[ianaiken]

2 things you need to know no matter what you are reading. Focus and prism position. That's it to get these things to work. PHD2 settings and that can come later and easy calculated.

If you can get out when the moon is out this will help with focus as you can use that. Essentially you want to make sure the guide cam is in focus the same time as your main imager. So first get your main scope into focus and then leave that, then adjust the spacing on the OAG with the guide cam until that is then in focus. Usually the cam can lift up and down in its chamber, I think with this OAG the 1.25" cup holder will allow some movement of the cam. Very fine movement is needed once you get around focus. It's good to bounce it up and down in the chamber back and forth over the focal point to get a feel of where focus is.

To get near focus in the first place you need to measure and set spacing as close as possible, so the distance from the centre of the OAG to the main imager is the same distance needed from the center (maybe bottom) of prism to the guide cam. Then the fine adjustment by hand or even better if you had like the Baader 1.25" / T2 EP holder with Helical focuser built in would make things easier.

The other bit of advice is to make sure the prism is not in front of the main imager chip. I usually made sure it sat on top of the chip on the side where the chip is longest, that is usually where you have more room for things like a prism and means you can get the prism into the image fov better where star shapes are better.

[kens]

I've been through this recently. In my case I have a reducer rather than a flatteneer but they both increase the difficulty of getting the spacing right. You'll need to make some calculations and be prepared to get spacers and shims to get everything right. 

You'll need to know the optical distance from the prism to the two camera connections, camera back focus for both cameras  and the spacing for the FF/FR in addition to the OAG optical length. Start with the optical path from the FF/FR to the imaging cam to get that right. The use the optical distance of the prism to guide cam and guide cam back-focus to work out where you need to "tap into" that path.  To maintain the FF/FR spacing, any spacer you add to the front of the OAG needs to be removed from the back of the OAG for BOTH cameras.

Ideally you want the guide cam to be as close as possible to the prism . Given you don't have a filter wheel this should not be an issue. (A filter wheel increases the optical path to the imaging cam and requires the same increase to the guide cam)

You may need shims of various sizes to get the camera orientations to your satisfaction. Im a bit OCD and like to have the cameras aligned to RA/Dec. The T threads are 0.75mm pitch  so shims of 42mm ID between 0.1mm and 0.5mm give good fine tuning.

[Aidan]

Thanks, some great pointers there,   Regarding focus, I’d read that some found putting a T2-1.25 helical focuser on the OAG helped to get fine focus.  

I’ve got two rigs, but only the one guide cam.  So, I’ll have to swap the cam each time I change rigs.

 

[Allinthehead]

I just started guiding recently using a zwo oag too. One piece of advice that helped was to try to get everything focused during the day on a far off object. 

These videos helped with my settings to start with. 

 

 

[Aidan]

Im quite happy with PHD2, i think once it’s all focused as you recommended, the guiding part will hopefully be quite straight forward....Famous last words!

Still waiting for the OAG to arrive.  There seems to be a trend these days with sites showing items “In Stock”  but when you call to find out why it’s not arrived the excuses become quite bad.

[ollypenrice]

You can set the prism depth (on the long side of the chip as Ian said) by taking flats in the daytime. They don't even need to be decent flats, the idea being to get the prism as far in as possible without its creating a shadow.

Olly

[alacant]

On 4/12/2017 at 21:00, ianaiken said:

Focus and prism position. That's it to get these things to work.

+1. Just did it. Ignore the horror stories and just do it! HTH.

 

On 4/12/2017 at 18:44, Aidan said:

What are they like to get in focus for use in PHD2? 

Easy. I used Mirfak; loadsa stars around it.

1. Focus the main camera.

2. Set PHD2 to loop say, 0.2s

3. Push in the guide camera slowly until you see stars.

4. Tighten the thumbscrews.

That's it. HTH.

[Aidan]

The OAG arrived today along with the Lodestart x2 camera.

Just got everything setup and getting the focus sorted on both the main cam and guide cam.  So far its not been too bad.   One thing i have noticed is the image displayed in PHD2 from the guide cam.  Not sure if this is trait of OAG or if i am doing something wrong here.

The two images are as follows.  The perfectly square one is the Lodestar in my 60mm guidescope..  The oval one is the Lodestar in the OAG.   Is this a typical view when using an OAG?

A bit out of foucs, but its the view which i am concerned about at the moment

 

[alacant]

1 hour ago, Aidan said:

The oval one is the Lodestar

Hi. With my zwo120 via the OAG on a 150mm f8, PHD2 displays edge to edge. What are the binning settings? HTH.

[Aidan]

I’d have to check, but I think I’m at 1x1

[Aidan]

All sorted :-)   Was  case of moving the OAG as close to the Flattener as possible.   Orignally i had it setup =  

Camera OAG - Spacers

now

Camera Spacers OAG.   

 

Full FoV in PHD2.  Both the imaging CCD and Lodestar nicely in focus.  Other good thing i have found.. Regrding lining up the prism with long edge of the CCD chip on the camera.   I did that originally and got it working, then tried rotating the camera..  No matter where the camera is rotated, i get a clear shot from the imaging camera.. Result!

[Jammy]

I've got a QHY5l-ii mono guide cam and the ZWO OAG for my SCT and dslr.

As I can see in the above thread, you've managed to set up ok.  I found it easiest to get both cameras in focus in daytime.

One thing to note, I found that there was a little bit of flex on the stalk that the guide cam attaches to.  I solved this by removing the stalk and putting it back in with a tiny shim of tin foil between the stalk and its housing.  It was a really small gap, but enough to give movement that you don't want.  Maybe it was just the machining tolerance on the one that I bought, but best to check.

So far its been easy finding guide stars because of the slightly larger prism.  Its far better than the old style Celestron OAG.

I've seen forum threads where people say they're awkward to use, but I quite like using mine.

Best of luck!!

[Aidan]

I know exactly what you mean about the small movement with the cam holder.  I fond this and used a smell bit of insulation tape to get rid of it.  I just need to find a permenant solution - Foil sounds just the job.

i got my setup outside on Friday and did the first few practice and setup runs.

i would agree testing in daylight to learn how to set it up is well spent time.  However, I encountered (but didn’t know at the time) using a pinhole on the main optics caused me lists of problems - the cam image in phd2 was terrible.  As soon as I tested when it was dark, all the problems went away... and that was just by testing on the mount in my garage at a distant object.

Overall the first night was ok.  A bit of fine tuning.. A non rotating helical focuser would certainly help to get the guidecam in focus!

I too read all those threads.  If you understand the basics and with a bit of patience, it’s relatively easy to get working.