Starlight OAG & OAG questions

[Astroscot2]

Hello all.

I know a few of you fellow imagers are using an OAG in your imaging train and as an imager who has always used a guidescope for guiding im finding myself scratching my head trying to guide with an OAG on my newtonian.

I recently purchased the SX OAG ( the newer version without the guide turret flexure issues ) My imaging train is as follows ASA 0.73x corrector - OAG - SX filter Wheel - and H694 camera.

Last night I spent th best part of 3 hours focusing and trying to guide with the loadstar and Astroart - the guiding was terrible and I suspect im not using the OAG correctly.

Normally I can get my head around mechanical or software issues and resolve them but on this occasion Ive gone a bit thick and need some guidance.

The OAG did not come with any instructions on its use what-so-ever, I think SX beleive if your buying one then you should know how to use it ;-)

Issue 1 . Guidestars, I see plenty of them, however defocused they are all little rectangles in focus they are not tight stars, the tighter the focus they spread out into a line of pixels. Now is this due to the position of the pick off prisim or could it be an issue caused by the ASA corrector? The spacing is fine from corrector to main camera chip 62.5mm . I positioned the pick off prism so that it clears the 1.25 inch filter, should it be deaper into the light cone for better guidestar shape?

Issue 2. guide camera possition, I spent some time thinking about the best position for the OAG and the orientation of the loadstar. In the end I rotated the imaging train so that the loadstar pointed back towards the main mirror - simillar to the position it would be on a guidescope, I also rotated the scope tube so that the loadstar ship could be orientated to the Dec and RA axis. This semed to work but the guiding was woefull.

Ideally i want to persevere with this OAG as I beleive I have some Diff flex issues when using the guidescope.

Would be interested in hearing your thoughts and any images of your OAG in situ on your Newt.

I think in going wrong somewhere ?

Many thanks

Mark

[pete_l]

Issue 1 . Guidestars, I see plenty of them, however defocused they are all little rectangles in focus they are not tight stars, the tighter the focus they spread out into a line of pixels. Now is this due to the position of the pick off prisim or could it be an issue caused by the ASA corrector? The spacing is fine from corrector to main camera chip 62.5mm . I positioned the pick off prism so that it clears the 1.25 inch filter, should it be deaper into the light cone for better guidestar shape?

That sounds like a great systems you've got there (envy!).

I don't have the ASA corrector, but its documentation says the corrected view is only 25mm in radius. Presumably, the pick-off mirror has to be positioned within that circle.

I hope you get it working, as I might be tempted to follow in your footsteps.

[Astroscot2]

Thanks Pete, looks like I need to seek the answers elsewhere ;-(

[steppenwolf]

I use the SX OAG but wasn't aware of an upgraded version that resolved the turret flexure issue - I'd be interested to know what SX have done to correct this although my own solution of a grub screw bearing on the front of the turret with a thin metal foil shim at the rear of the turret seems to work pretty well.

Guide star shapes will not be good using an OAG for two reasons. Firth the prism adds its own artefacts and secondly by its very nature, it is showing stars on the perimeter of the telescope's light cone which is where you'd expect the worst shape stars to be found within the system. My guide star shapes are quite oval but I still achieve good guiding because the guide software still the correct star centroid.

Issue 1 . Guidestars, I see plenty of them, however defocused they are all little rectangles in focus they are not tight stars, the tighter the focus they spread out into a line of pixels. Now is this due to the position of the pick off prisim or could it be an issue caused by the ASA corrector? The spacing is fine from corrector to main camera chip 62.5mm . I positioned the pick off prism so that it clears the 1.25 inch filter, should it be deaper into the light cone for better guidestar shape?

For best guide star shapes the inner edge of the prism should be just outside the light cone falling on the imaging camera's sensor but be careful of diffraction artefacts if you get too close.

Issue 2. guide camera possition, I spent some time thinking about the best position for the OAG and the orientation of the loadstar. In the end I rotated the imaging train so that the loadstar pointed back towards the main mirror - simillar to the position it would be on a guidescope, I also rotated the scope tube so that the loadstar ship could be orientated to the Dec and RA axis. This semed to work but the guiding was woefull.

The orientation with regard to the telescope should not be an issue but it does make sense if you have a rectangular imaging sensor for the pick-off prism to align with the long edge of the sensor. For my own satisfaction, I arrange for the guide camera to share this orientation to make best use of the prism which is also rectangular. Your guiding software will allow for any orientation of the guide camera when you run the calibration routine.

[ollypenrice]

Like Steve I needed to sort flexure with the earlier SX offering.

Just a thought, since you are using AA5 to guide; are you using bin2 or are you in the bizarre default binning mode they call Hires fast? (By which they mean High Res fast. I was wonderng who hires what from whom at first!)

This bins 2X2 on one axis and is unbinned on the other. The idea is to have full unbinned resolution on the RA axis. If you wanted to use this system (which is an idea too far in my view) you'd want to orientate the unbinned axis along RA. Personally I just choose 2.2. This might account for your elongated stars...

Olly

PS, While AA5 guiding involves a promiscuous orgy of open windows and an arcane sequence for opening them all, it has one supremely redeeming feature. The statistics window gives you the usual linear guide trace for both axes but it also has the concentric bullseye display which records the position of every single guide hit, so you can see after 6 hours if you have had a bad hit or not.

It also shows patterns very coherently. Eg two separate clusters of hits, looking the like the Double Cluster, instantly says 'Backlash' with the hits lying on the different sides of mesh. It's a great bit of graphics. Brilliantly informative.

[Astroscot2]

Steve, Olly thank you for your replies.

The SX OAG I received had the cormers of the turret and turret hole chamfered which prevents the rocking I experienced on the first unit I recvd - a simple solution that works really well.

Petes initial reply got me thinking on the position of the pick off prisim and the quality of the guidestars Im seeing, I think I need to go a little deaper into the light cone and hopefully I should see an improvement ?

Steve, yes I am aligning the loadstar long side of the chip with the longside of the prisim, however you mention the calibration orouting should sort out any orintation of the guidecamera and here I have a confession, Ive used AA3 for about 6 years add ive never hit the calibration button, I suspect now that im using a longer FL instrument and OAG that this is something I need to learn on how to use.

Olly, im guiding using 2x2 bin adn 2 second exposures, watching the guidestar prior to hitting the guide button it sits pretty steady, Ive always been able to play around withthe X&Y axis options and aggresive options and get good smooth guiding. Olly, can you give me a rundown on your calibration routine ? nothing in the AA literature.

Thanks

Mark

[robbieince]

I use the sx oag and find it really nice if you dial it in correctly. Focus setup is always an issue but once set is fine. my scope is f3.8 so is very subject to collimation errors too, this also affects star shapes if not right.

The farther out from the centre you place the prism, the more you get non ideal star issues, but these are not a problem if the shape is consistent as the software (I use PhD) calculates the centroid and bases moves on that. Don't forget there is an oft repeated tip that out of focus stars give better guiding - its the same principle.

I orient mine so that the axes match ra to LR, and on my Newtonian, with the rig as near the axis of rotation as possible to minimise the need for balance weights.

best regards, Rob

[Astroscot2]

Many thanks for the reply Rob, I would be interested in seeing a photo of your set up with the Camera train in situ?

Do you have an image you can share?

Thanks

Mark

[ollypenrice]

Steve, Olly thank you for your replies.

The SX OAG I received had the cormers of the turret and turret hole chamfered which prevents the rocking I experienced on the first unit I recvd - a simple solution that works really well.

Petes initial reply got me thinking on the position of the pick off prisim and the quality of the guidestars Im seeing, I think I need to go a little deaper into the light cone and hopefully I should see an improvement ?

Steve, yes I am aligning the loadstar long side of the chip with the longside of the prisim, however you mention the calibration orouting should sort out any orintation of the guidecamera and here I have a confession, Ive used AA3 for about 6 years add ive never hit the calibration button, I suspect now that im using a longer FL instrument and OAG that this is something I need to learn on how to use.

Olly, im guiding using 2x2 bin adn 2 second exposures, watching the guidestar prior to hitting the guide button it sits pretty steady, Ive always been able to play around withthe X&Y axis options and aggresive options and get good smooth guiding. Olly, can you give me a rundown on your calibration routine ? nothing in the AA literature.

Thanks

Mark

Sure. I just use the AA auto calibration. (You have the guide star visible in its window but don't click to mark it.) In the guide window you just click calibrate and wait. You've probably done this.

Next I look at the bullseye target in statistics, and at the graph, to see what's happening. That 'double cluster' pattern usually means backlash so I load the east side a bit heavier. Over correcting means too much movement from a guide signal so you tell the software that the mount moves faster than it really does. ie you increase the values in Telescope Speed (pixels per second.) A higher number means less movement per correction and vice versa.) I often find that auto calibrate sets the number far, far too low so it over corrects. The length of guide sub that gives the best result does vary. In bad seeing go for longer subs to avoid chasing it. I usually set the rest period between guides to zero and the 'Don't guide when error is less than...' to the minimum of O.25. Sometimes I may raise it. Here's my setup screenshot in action. It isn't always like that!!!

Olly

Olly

[BlueAstra]

I don't know if your corrector is similar to my Wynne corrector, but if it is, the out of focus image is quite astigmatic. So instead of a focussed spot just growing symmetically either side of focus you get an oval or line focus either side. So if your OAG is slightly out of focus you may see ovals or lines rather than circular spots. I also line up the prism to one side of the CCD.

[Astroscot2]

Thats a nice guide graph Olly. Do you calibrate using the same guidespeed and is there any option for adjusting the length of time AA makes the moves for the calibration?

Thanks for the image blueastra, difficult to tell the orintation of the loadstar in the image due to the cable strain protector.

Mark

[BlueAstra]

I think the plugs are parallel to the camera axis, so parallel to the plane of the photo, so to speak.

[robbieince]

Many thanks for the reply Rob, I would be interested in seeing a photo of your set up with the Camera train in situ?

Do you have an image you can share?

Thanks

Mark

Here's a piccy of my setup taken at Kelling this year. The guide camera is an Atik 16ic, the image camera is an Artemis 4000 (Atik 4021) and the scope is my Orion Optics AG8 on my EQ6 Pro

[Ibbo!]

I use a TS 9mm OAG and have successfully guided when the guide star looks something akin to a seagull with both the FR in the 1800mm f/l RC and the fs102 and the SX H9

I'm using maxim and a 716 to guide with don't know if maxim guides in a significantly different method