WalterT If you are interested in doing something like this let me know and I'll provide some more details.
Looks interesting, I'd like to know the details .
Steve
OAG field of view too narrow for guide stars.
glancey3
Hi,
Guiding really needs some technique, I have a few suggestions you may want to try;
- Check the mount and telescope balance.
- And point the telescope to the mid sky, the star moves faster there.
- Adjust the exposure time to get a brighter star.
- Also you can adjust the calibration steps/Max DEC Duration/Max RA Duration in the guiding setting.
You can also refer to the following posts and articles.
https://openphdguiding.org/Analyzing_PHD2_Guide_Logs.pdf
https://openphdguiding.org/man-dev/Trouble_shooting.htm
https://www.cloudynights.com/articles/cat/column/fishing-for-photons/what-to-do-when-phd-guiding-isnt-push-here-dummy-r2677
https://bbs.astronomy-imaging-camera.com/d/11318-guiding-asistant
I made some adjustments as suggested and PHD2 was able to auto-select a guide star, but it kept stopping after a minute or 2 of star movement. I let PHD2 run for over an hour as it repeatedly selected a star and stopping. Guiding never started and what's more, it failed to calibrate. There was no calibration data.
- Edited
@tech@zwo#47645 Guiding really needs some technique
You don't seem to understand.
It is not a case of knowing how to guide. The ZWO OAG is simply problematical when used with a larger sensor like the IMX174, or even an IMX290 sized sensor. The aperture is simply too small for the sensor, and that leads to aperture vignetting, and causes the shape of stars to be even more distorted than the usual optical aberrations at the edge of the OTA's image circle.
Even for smaller sensors, as Walter has shown, the aperture of the prism stem already needs to be enlarged to avoid guiding problems.
The mirror size alone (at 8mm x 8mm) will pass no more than 70% of the light to the IMX174 sensor, not to mention additional aperture vignetting. Aperture vignetting is a Fourier domain problem; a non symmetrical vignetting problem will cause irregular diffraction pattern and spikes, etc -- it is not simply the case of the reduction of the illumination of the image circle, but also additional distortion of the star shapes.
Check out this thread on what a simple bilateral aperture vignetting (symmetrical on only one axis) does. The kind of vignetting that the original poster saw is completely unsymmetrical and will cause much worse problems (as Walter mentions, effects such banana star shapes):
https://stargazerslounge.com/topic/275921-star-shape-artifacts-black-wedges/
Notice that distorted stars will spread the star's flux over a large area. That alone will reduce the signal to noise ratio of a star, something that is crucial in finding centroids of stars for multi-star guiding, assuming that the centroid finding algorithm can even find a stable centroid of a star that is banana shaped and stretched over 20 pixels.
When a star is not radially symmetric, moving it by sub pixels will cause the true centroid to move irregularly when the guide scale is not over sampled (and if you over sample, you also loose signal to noise ratio -- so there is no win there).
@glancy3's latest seems to point in the direction of PHD2 (which is a reliable gold standard) of just such a problem. He obviously could not calibrate because PHD2 could not get a reliable fix on the centroid.
@glancy3: you might be able to reduce the problem by trying to get the image circle to at least fall into the center of your guide plate instead of that oval in the corner. It might involve some filing to manipulate that stem around. Also, try to insert the prism stem closer to the optical axis of your OTA; that will do two things, it gets you more illumination, and it reduces the effect of aperture vignetting. But there is a fine line there: if you stick the prism in too far, you will be creating a shadow in your main OTA (removable by flats, not of the time). Try to stick the prism in as far as possible, but no farther (some quote from the Da Vinci Code :-).
A flattener might help with the illumination, but will not help with the vignetting caused by the OAG's structure. Unfortunately, a reducer will usually make matters worse at the edges of the illumination (unless it is designed as a reducer/flattener).
The Celestron OAG that Walter mentioned has a 12mm x 12mm prism, and is large enough for telescopes with large f-numbers.
@tech@zwo: do the modification that Walter suggested (or 3D print a stem with an enlarged aperture), give it to your algorithms person to test, and you will find that guiding will suddenly improve by leaps and bounds, even with a small ASI290MM sensor.
Chen
StevenEvan Hey Steve, to be clear I used (and still use on my small refractor not my SCT) the moded oag with my 290mm, the hole would still be too small for the 174mm sensor. Also, great care should be taken to not scratch or crack the prism.
What I did was remove the stem-prism from the OAG and removed the tiny recessed hex screw that is on the side providing additional friction to hold the prism. The prism is not glued or bolted in place it's held in place by friction. So I got a nice bowl of hot water and submerged the stem/prism in the water for about 5 minutes. Then with dishwashing gloves on I took it out and by only small finger pressure popped the prism out, no tools or prying objects. The stem is made of a soft metal, not aluminum, probably brass so it was very easy to file. I already had on hand a set of small files, which can be purchased from a hardware or auto parts store for $5-$10. I slowly enlarged the hole into an oval shape using a round file until it was the size of the gap of the "ears" that hold the prism on the other side. Then I "squared" the hole using a triangle file. I took care to not file all the way to the edge on the flat side of the stem as the wall is very thin and it would have ruined it. Also, the hole should not be made wider or longer than the distance between the "ears" on the other side that hold the prism down. Otherwise you run the risk of not having a little "ledge" to hold the prism level or having gaps that will let stray light in. Then I painted the newly exposed shiny metal from the new hole with flat black primer and then with flat black acrylic paint. Once dry I submerged the stem into a bowl of hot water again for five minutes (not the prism, keep that one at room temperature). After the five minutes I took out the stem and wiggled the prism into place and let everything cool, then replaced the recessed hex crew on the prism, put the stem back on the OAG, and tested it on the sky the next time I had a chance.
thanks for the info.
Question, is the prism held at the correct angle by a recess, or do I have to align it when it is replaced in the stem?
Steve.
Helpful post - agree the M68 OAG is not really suitable for the 290 or 174. I am thinking to finding a different OAG, does anyone have a suggestion?
xthestreams I eventually purchased the celestron off axis guider 93648. It think it is the best option taking into account price vs value. It is built very solid, it has two points for 360° rotation, and the prism and opening are huge. The draw back (at least in my case) is that it is very difficult to achieve 55mm back focus with it because it seems that all of the ZWO gear assumes an OAG will take 16.5mm but this one takes a minimum of 38mm with the shallow adapters that come with it. But there are plenty of threads on CL on how to make it work with different setups.
- Edited
StevenEvan The best way I can explain it is that it rests on the ledges of the stem to keep it level and is held on by the ears on each side to keep it in place.
Hi guys,
Thank you for all good ideas. Your suggestion have been conveyed to our R&D.
Got it! I found that the prism in the OAG actually pivots a few mm's on the stalk and just a slight adjustment fully opens the FOV up to a full circle, with plenty of stars for PHD2 to work on. The other key to focus is to adjust the height of the stalk in combination with the guide camera to synchronize it with the main imaging camera.
The 174 works great with the OAG with this adjustment, so I don't understand WalterT's concerns. In fact, I would argue that the circular shaft in the stalk is the correct shape and size for the camera, providing an FOV only slightly smaller than the imaging chip and is uniform no matter which direction the prism sits with respect to the imaging chip.
The pivotal aspect of the prism was discovered by accident when I was adjusting the stalk and inadvertently moved the prism. I at first thought it was odd that it wasn't fixed on the stalk and only moved it a bit as a last resort to see if that would open the FOV. It did. There is nothing in the OAG documentation on this but it seems to be key to getting guiding working.
Note to ZWO support. Document this feature.
glancey3 The 174 works great with the OAG with this adjustment
Walter is correct. Measure your prism and it probably is 8mm x 8mm. The diagonal of your IMX174 sensor is significantly larger. The stars closer to the edge of the frame will be dimmer, and also have irregular shape. With multi-star guiding, PHD2 might need to select stars out to the edge of the frame.
By the way, watch out for the moving prism, make sure it stays perfectly put so there is no differential flexure between the guide camera's axis and the main camera's optical axis.
Chen
Chen,
You bring up a good point and one I am concerned about, since there is no way to tighten the pivot points on the prism. So far it's been spot-on.
a month later
I just ordered the new Large OAG, excited!
9 months later
A few months back I was having guide problems with my 290 camera. Not know about the soda-straw hole in the OAG stem I bought the 174 camera thinking a larger sensor and binning was the answer. Results have been generally worse.
I came across this thread last week and measured the prism stem hole at 6mm. I was able to widen this out to a square about 6.3mm per side. I did not want to get too close to the edges as you described. 6.3mm does not sound like much, but going from a round hole to a slighter large square hole increases the area of the entrance by 40%.
I was able to test last night. What a difference! Stars actually appear round in the guide camera. Easy to focus.
However with breezy winds running 5 to 8 knots I was not optimistic about seeing an improvement in tracking. But it happened anyway! The rms dropped 0.09 arc seconds in each axis compared to my sessions on the calm wind nights of May 1st and 2nd.
I still need to fine-tune my aggressiveness settings. The mount is the Panther alt-az and there are constant corrections in both axis, it is not quite as simple as an equatorial procedure. But I am expecting even better results the next time with calm winds.
If you have the ZWO OAG, this modification is very much worth doing.
It looks like ZWO has made a modification to the OAG. Here's a picture of the prism port on the unit I got a couple of weeks ago from Agena.
-chuck-
Good to see they recognized the issue.
It would be even better if ZWO offered owners of the older units a chance to purchase the new stalk at a fair price.
- Edited
ckoos Here's a picture of the prism port on the unit I got a couple of weeks ago from Agena.
That one has been out for quite a while now (check the documentation). The opening is still small (5mm x 7.4mm rounded rectangle); I had mentioned it in post #3 of this thread:
https://bbs.astronomy-imaging-camera.com/d/14298-oag-strange-shadown/3
While it will work for the ASI290-class camera, it is too small for an ASI174 or ASI178; which are arguably better for guiding than the ASI290.
Chen
w7ay Eyeballing the radius and plugging your dimensions into MacDraft I come up with an area of 32.85 square millimeters. Not a big improvement over the 6 mm round aperture.
DIY efforts will give better results.