Very faint and blurry stars using OAG and PHD2

[John Natale]

Hello,

I have a C8 Edge telescope, and recently began using a Celestron OAG and ZWO asi 174mm guide camera. I am usually getting only a couple very faint and blurry guide stars with the OAG.  No matter how hard I try, it is impossible to get them into sharp focus by either moving the guide camera or by using the OAG's focuser.  I am sure the OAG's prism is set correctly to the top of the main camera frame. 

Why do I only get a couple stars to guide on, and why can't I focus them? And why does PHD only manage to find one of them usually when I select "auto select star?" Often times when PHD does find a star even that one is lost.  This is far different from the many clear and bright stars I get when guiding with my ED50 guide scope and this same camera. I think my PHD2 settings are probably wrong for the OAG configuration.

I attached screen shots of my advanced settings tabs. In my attachment, there is one screen shot on each page for each of the tabs. I also included a link below to the guide and debug files from my attempt at imaging M101 last night. For some additional information, there was a very bright full moon and I was using 1-sec exposures in PHD.  (Note - the log and debug files show two set-ups, but the one I am interested in correcting is the one showing the asi 174mm and OAG).

Between the log and debug files and these screen shots perhaps someone can recommend different settings, or otherwise indicate what is wrong here. I appreciate your help!

https://openphdguiding.org/logs/dl/PHD2_logs_ZXL7.zip

[Brian Valente]

Hi John

>>>Why do I only get a couple stars to guide on, and why can't I focus them?

The short version is nothing is 'wrong', this is about what to expect from your equipment. 

SCTs are challenging in many ways, oag guiding is one of them. The C8 is a fairly slow telescope (f/10) so it's already not letting a lot of light through. It also has a lot of light and sharpness falloff towards the edges where the OAG is located. Distorted stars can be a reality for setups like yours. Generally it should not impact guiding (PHD is pretty good for calculating centroids)

All is not lost. There are some things you can try:

- increase your guide camera exposure time

- increase your guide camera gain

- try to position your oag pickoff mirror as close as possible to the center without causing obstruction

- consider a reducer that will reduce the f-stop and increase image brightness

- a full moon isn't going to help things. You might also try testing as far away from the moon as possible, or during a session where the moon isn't yet up

Brian

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/35b32357-e1a8-49b6-ad52-001de7a5607dn%40googlegroups.com.

--

Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

nature portfolio brianvalentephotography.com 

other astronomy https://www.brianvalentephotography.com/astrophotography/ 

[Bobby]

 Brian's tips are good.  A couple of other thoughts:

• Try to set up both your scope and OAG in your planetarium software's "telescope" setup.  Stellarium has perhaps the most simple way to do it.  You may find that if you rotate both your camera/oag a certain amount of degrees, you can get a star in the OAG frame.  Don't forget, calibration will need to be redone every time you settle on a new angle.

• Check your scope's alignment.  I'm not familiar with the Edge, but my OAG image improved significantly after aligning my Vixen VISAC. 

Keep at it and remember that even thought there are "billions upon billions of stars", none of them are in the right place.  (Sorry, Dr. Sagan.)

Bobby

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i5NFOWBbi8hFbafC%2BOYgTHDM-DPX6FYLcn%2BAXr7%2BsXcvg%40mail.gmail.com.

--

Bobby Sapovits
www.equusbicycle.com

[Brian Valente]

Yeah good tips Bobby

Also ensuring the SCT is well collimated can help improve things as well.

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/6f89f315-825c-459f-8a86-b604748fc323%40equusbicycle.com.

[mj.w...@gmail.com]

Hi John

Guide stars are invariably distorted on OAGs, that's due to being at the edge of the image circle, not because they are out of focus.

1. The FL in the OAG part of the GuideLog was 1488mm, which I take to mean you are using a 0.63 Reducer ?

Do you have the Backfocus of the FR at 105mm, to get the best star shape at the edges of the image circle, where the prism is ? 

FRs reduce the size of the image circle, are you sure the prism really is at the edge of the usable part of the image ?

As I believe the Celestron OAG has limited prism insertion depth.

2. Your last OAG Cal was on the 14th March - are you sure the OAG setup hasn't rotated since then ?

 I don't see an OAG Cal in the Log.

3. Polar Alignment errors were large on all sessions, over 40arcmins.

How are you Polar Aligning ?

4. Try disabling "Star Mass Detection" and rely instead on the Star HFD setting.

And OAG Pixel scale was only 0.81 arc-sec/px,.

So you could safely Bin the guidecam, which would increase the guidecam sensitivity.

5. As Bobby said, use a planetarium to aid rotating the OAG setup to find best guide stars:

Michael

Wiltshire UK

[John Natale]

Thanks everyone for your suggestions, I appreciate it.

Michael- here are the answers to your questions:

1. I have a C8 Edge telescope and use a 0.7 reducer. Since it is an "Edge" scope, my understanding is that the stars should not be distorted anywhere in the field of view, but maybe the reducer plays a role in star distortion? I have the camera and guide camera's sensors both set at 105mm behind the reducer (or very close to that). I looked at the OAG prism and it looks to be positioned at the top of the main camera's frame, but I'll try lowering it a bit and see if it interferes with the frame.

2. I am sure the OAG has been rotated several times since I imaged on March 14th. I'll be sure to calibrate again when I set up next time.

3. I polar aligned with sharpcap, and got a result of "excellent," but maybe something caused it to be off after polar alignment. 

4. You said to rely on the star's HFD setting. How should I do that? I see there is a HFD minimum and maximum setting in the advanced settings guiding tab. They are set to 1.5 minimum and 10 maximum. Should I change these? I read somewhere that this has to do with hot pixel detection. Someone said to set the max lower than 10 (I think they said to set it to 2 or 3). Do you recommend that?  Also I changed the binning to 2 - thanks for recommending that.

Also- Brian, you recommended that I increase the guide camera's gain and exposure. What should I increase them to? Should I try 2 secs for exposure and maybe 90% gain? I was previously using 1 sec and 60%. 

Thanks again!

[Brian Valente]

>>>Also- Brian, you recommended that I increase the guide camera's gain and exposure. What should I increase them to? Should I try 2 secs for exposure and maybe 90% gain? I was previously using 1 sec and 60%. 

you'll have to experiment here. you can try boosting it 2x and see how many stars PHD can find. Your suggestion sound reasonable

Also FYI binning a cmos camera is not going to improve the sensitivity, so I would not bother there

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/f2be3fe2-8ea7-4ebe-a74f-6e3198bfbec0n%40googlegroups.com.

[John Natale]

Thanks.

What should the HFD minimum and maximum settings be set to? Right now they are at 1.5 minimum and 10 maximum.

[mj.w...@gmail.com]

Hi John

1.  "stars should not be distorted anywhere in the field of view,"

Perhaps that is true of Edge images with the imaging camera.

But the prism is outside the image circle.

2.  " I looked at the OAG prism and it looks to be positioned at the top of the main camera's frame, "

Not sure what you mean by "looks to be".

Take Flats, lowering the prism until you start to see a shadow from the prism.

Back off until the shadow is just gone.

3.  " What should the HFD minimum and maximum settings be set to?"

Like many PHD2 settings, it should work on those defaults.

4.  "I polar aligned with sharpcap, and got a result of "excellent,""

I'm not an Expert, perhaps that PA error was a result of rotating since the last Cal ?

Michael

Wiltshire UK