Off Axis guiding.

[alan potts]

I tried my OAG last night after about a year of being in the box, I also bought the focuser just in front of Christmas from FLO. Now the 290MM mini (2.9um) was working on a SW ED50 guidescope and with the new multi star selection was getting very nice results.

I did however wish to try my 183mc (2.4um) on my 805mm APO and though an off axis guider would be required for this, I realise the match between the two is not good but wanted to try it anyway. Now I got the stars to focus and select in PHD-2 and made calibration without issue, though they did not appear as sharp as I normally get. The guide though was very up and down, I assume because the longer F/L of the scope, the SW ED 50 is only 242mm, so this is over 3 times longer.

Simple question is this to be expected, i,e, the guide appearing what would be about 4 times worse than normal. On the first test of the new PHD-2 it was a straight line much of the time, using the OAG it was anything but.

Results of 4 minute subs were not great but on a few stacked images the result looked reasonable from the point of view of using the 183MC with this scope, going to try the SW ED50 tonight to see what happens with that.

Alan

[Hallingskies]

The first time I used an OAG, I was also disappointed with the result.  However, I quickly traced it down to the fact that I hadn’t tightened down the height adjuster on my Atik OAG.  Results were a considerable improvement over a guidescope after that.  You’ve probably checked it all over three times already, but you need to make sure that everything in the imaging train really is rock solid.

[alan potts]

38 minutes ago, Hallingskies said:

The first time I used an OAG, I was also disappointed with the result.  However, I quickly traced it down to the fact that I hadn’t tightened down the height adjuster on my Atik OAG.  Results were a considerable improvement over a guidescope after that.  You’ve probably checked it all over three times already, but you need to make sure that everything in the imaging train really is rock solid.

Fairly sure I covered everything and I did tighten up the alen screws, however as I took things apart I thought there may have been some movement in the many rotating parts of the 3.7 inch focuser, I will continue to play with it over the coming months. I feel my biggest problem is I am always chopping and changing scopes and focal reducers, too much gear not enough clear sky.

Alan

[ollypenrice]

Probably nothing to worry about, Alan. Our OAG guide graph at 2.4 metres FL looked Himalayan when it was presenting the trace in pixels. In fact it was running at about 0.3 arcsecs RMS which is brilliant. If you plug the guiding focal length (805mm) and the guide cam pixel size into PHD it will give you the guide value in arcseconds and you will probably be greatly reassured by this!

It won't do any harm to use an OAG. It's probably a good idea, but I guide my TEC140 (1015mm/0.9"PP) with a guidescope.

Olly

[alan potts]

2 hours ago, ollypenrice said:

Probably nothing to worry about, Alan. Our OAG guide graph at 2.4 metres FL looked Himalayan when it was presenting the trace in pixels. In fact it was running at about 0.3 arcsecs RMS which is brilliant. If you plug the guiding focal length (805mm) and the guide cam pixel size into PHD it will give you the guide value in arcseconds and you will probably be greatly reassured by this!

It won't do any harm to use an OAG. It's probably a good idea, but I guide my TEC140 (1015mm/0.9"PP) with a guidescope.

Olly

Yes of course I didn't change the guide scope focal length in PHD, so therefore I imagine the calibration would be wrong as well, so much to remember with this dark art, plus the grey matter is not what it once was. I was considering having a play with my 180mm Mak, even though it's F15 and I don't have a reducer I thought it may be good on globulars, well the brighter ones.

Alan

[ollypenrice]

14 minutes ago, alan potts said:

Yes of course I didn't change the guide scope focal length in PHD, so therefore I imagine the calibration would be wrong as well, so much to remember with this dark art, plus the grey matter is not what it once was. I was considering having a play with my 180mm Mak, even though it's F15 and I don't have a reducer I thought it may be good on globulars, well the brighter ones.

Alan

I don't think PHD uses the FL information you give it for guiding. I can't be sure, but being a lazy so-and-so I never bother telling it! It calibrates perfectly happily though.

Olly

[vlaiv]

21 minutes ago, alan potts said:

Yes of course I didn't change the guide scope focal length in PHD, so therefore I imagine the calibration would be wrong as well, so much to remember with this dark art, plus the grey matter is not what it once was. I was considering having a play with my 180mm Mak, even though it's F15 and I don't have a reducer I thought it may be good on globulars, well the brighter ones.

Alan

This is the reason for larger errors in the graph.

PHD2 does not use guide scope focal length for guiding, but it does use it to report error in guiding. Error was reported to be x4 as bad as it really is because you usually use 242mm of focal length and now you used 805mm - more than x4 longer focal length and for same pixel size - more than x4 resolution in arc seconds per pixel.

Error is reported in arc seconds (on the graph and in the stats, although you can choose which one you prefer - in pixels or in arc seconds) as that is comparable between different setups (not all have the same pixel size and focal length).

[newbie alert]

Did you re run the guiding assistant Alan? You would need to adjust the focal length and pixel size of the guide cam within the settings