Guide Corrections for RA and DEC Delayed

[KJRitch]

I use a C8 SCT. Recently, I purchased a Hyperstar V4, which changes the C8 to F/1.9 with a Focal Length of 390mm. I also purchased a ZWO 30mm F4 guide scope that has a focal length of 120mm. The main camera is a ASI071MC Pro and my guide camera is a ASI174mm mini. I've only had two sessions with my Hyperstar but I'm finding the guiding is off compared to when I use my OAG in F/7.1 or F/10 mode. My mount is an iOptron HAE43-EC strain wave.

I created a new PHD2 profile and set the focal length to 120mm. RA is Hyteresis and DEC Resit Switch. I just started with the defaults: RA Aggression 70% Min/Mo 0.17, Max RA Duration 2500ms. DEC Aggression 100%, Min/Mo 0.17, Max DEC Duration 2500ms. Guide exposure is 1 second.

Calibrations are completing without alerts or warnings.

What I'm seeing is when either the RA or DEC moves away from the zero line on the guide graph it takes a long time for PHD2 to generate a correction. This occurs in RA and DEC. The axes may move 2 arc-secs from the zero line before a correction is made. When I look at a PHD log from an OAG session I see lots of corrections for both axis. While I can typically maintain a sub 1 arc-sec RMS, 0.5 to 0.7 for my OAG sessions, I can't on my Hyperstar sessions, so far the best I've gotten is 1.14 arc-sec RMS

If a solution can be found for why guide corrections are delayed that should improve the guiding.

I've never used a guide scope before. When I started this hobby in 2023 I jumped straight into OAG guiding. I kinda of expected guide scope guiding to be easy but I under estimated again.

I've attached a PHD2 log from Aug 31. It includes a Guiding Assistant log.

Thank you for your advice.

[Bruce Waddington]

What you're seeing is a consequence of the guiding arrangement you're using.  It has a guider image scale of over 10 arc-sec/pixel.  So even with a low MinMove value of 0.1 px, PHD2 won't issue a guide correction until the star is displaced by more than 1 arc-sec.  So you can't expect that your overall guiding rms will be less than that and it will probably be somewhat higher - like the 1.1 - 1.2 arc-sec results you saw. Some people use this kind of guiding arrangement, but they generally also have a very coarse image scale on the main camera.  You should probably take a close look at your final images to see if they have acceptable quality in terms of star sizes and elongation.  If they don't, you will need to substantially reduce your guider image by increasing the guide scope focal length or by moving to a guide camera with smaller pixels.

Good luck,
Bruce

[Brian Valente]

Just to illustrate Bruce's feedback, because your guidescope image scale is so coarse, your min moves are very high. Dec min move is 1.75" so you are going to end up with that kind of RMS. 

It may be possible to reduce your min moves but keep in mind about 0.10 pixel is starting to get into the minimum for PHD. To get Dec to 0.5" min mov, you would need .05 pixel at your current image scale. 

--
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 visit https://groups.google.com/d/msgid/open-phd-guiding/be646271-b514-443c-b7b2-3d4c6a06669en%40googlegroups.com.

--

Brian 

Brian Valente

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

[KJRitch]

OK. That makes sense. I see in the guide log the guide scale is 10.07 arc-sec/pixel. When I look at a log from an OAG session that value is 0.84 arc-sec/pixel.

I did some research and found the formula for calculating the pixel scale, (sensor size * 206.265)/Focal length. For my ASI174mm mini and 30mm guide scope its (5.86 * 206.265)/120 =10.07 arc-sec/pixel.

So what pixel scale would you recommend for an SCT at F/1.9 390mm? Do I aim for the lowest pixel scale. I notice is some instances owners of SCTs but some large guide scopes (80mm) but its because they are imaging at higher focal lengths and not using an OAG.

I have an ASI585MC planetary camera with a sensor size of 2.9 microns. Using the equation above that would give me a 4.98 arc-sec/pixel. What should I expect from that pixel scale? Will I get corrections occurring at 0.5 arc-sec deviations?

If I were to purchase a 50mm guide scope with a FL of 190mm the pixel scale with my ASI174mm mini is 6.36, with a ASI220mm mini (4 microns)  = 4.34 and ASI120mm mini (3.75microns) 4.07. I would have to purchase one of this last two cameras. If I use my ASI568MC at 2.9 the pixel scale is 3.14.

I notice the sensor sizes of the ASI120 and ASI220 are much smaller than the ASI174mm and the ASI585MC. What is your opinion of sensor size or is it just the focal length of the guide scope and the sensor pixels size that is most important. I guess with a larger sensor PHD2 will see more guide stars, which will help when using the multiple star guiding feature?

I utilized the Astronomy.tools Guide Scope suitability tool and I figured the ratio of 1:1.38 was good enough. I see the guide scale of 10.07 arc-sec/pixel. 

When researching what guide scope to use with my Hyperstar, I only remember reading user comments online about guide scope choice for their  main scope FL and a ratio number of 1:5 as a maximum. No one was talking about the guide pixel scale for PHD2 performance. When I went to visit the Starizona shop to purchase the Hyperstar they recommended the ZWO 30mm. I don't remember if I mentioned my ASI174mm mini would be used but I bet I did.

The two sessions I've tried with the Hyperstar so far the stacked images don't look too bad for star shapes even in the corners. Running BlurX correction only feature cleans up any bad stars. 

 I'll give the ASI585MC with my 30mm guide scope tonight. I finally have some clear skies. 

I'll report back.

Thank you.

[Bryan]

This has come up before.  

Read Bruce's comments and rule of thumb at https://groups.google.com/g/open-phd-guiding/c/xE5SDi5S_1I

Bryan

[KJRitch]

Thank you. I did try to search but didn't find this type of information. It would be good if this  information was in the PHD2 manual. 

I did the calculation on my set up from last night using my ZWO 30mm and my 585MC as the guide camera and it appears even with 2.9micron pixel size and 120mm focal length I'm still below the Rule of Thumb focal length calculation of 153mm. So a 50mm scope at 190mm or 200mm should be what I need with my 585MC or if I decided to get a ASI120mm or ASI220mm in the future.

Even if I use my 533 camera with Hyperstar a 50mm guide scope should be adequate as the Rule of Thumb focal length calculation for a 533 main camera and 585 guide camera is 117mm FL.

When I plugged in the ASI174mm (5.86 microns) the Rule of Thumb focal length calculation was 310mm FL

I think I'll just get a 50mm guide scope at 200mm FL and continue using my ASI585 I can use my ASI071MC Pro or Player One Ares-C (533).

Thank you

[KJRitch]

I think in my last post I did the math wrong. My GP and IP values were wrong. Instead of the pixel sizes I used the resolutions for the Guide and Image camera/telescpe combinations.

If the Rule of Thumb for guide scope focal length selection from the link provided above and is correct
Guider focal length > 0.2*(GP/IP)*IF

with GP = the guide camera pixel size and IP = the image camera pixel size and IF is image telescope focal length 

ASI174mm = 5.86microns, ASI071MC Pro = 4.78 microns

C8 Hyperstar V4 FL = 390mm

0.2*(5.86/4.79)*390

0.2*(1.23)*390

0.25*390

95

Therefore my ZWO 30mm guides scope with 120mm FL is greater than 95

But when I calculated the guide scope resolution of 5.86*206.265/120 =10.0726, this value is too high for guiding.

I'm still confused how you choose a good performing guide scope/guide camera combo.

Thanks

[Brian Valente]

Kenn

you are better off calculating the image scale for each and then comparing, aiming for about 3:1 image to guider

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/36221fb9-efd0-41c4-9240-2ef64c8a0e5dn%40googlegroups.com.

[KJRitch]

According to Astronomy.tools Guidescope Suitability  calculator when I chose my C8 with Hyperstar and my ASI071MC I get a resolution of 2.53 arc-sec/pixel.The ZW0 30mm guide scope paired with the ASI174mm I get a resolution of 10.07 arc-sec/pixel and a ratio of 1.398. If I substitute the ASI585 I get a guide resolution of 4.98 arc-sec/pixel and a ratio of 1:1.98.

I decided to purchase a 50mm guide scope with a 200mm focal length. Using the ASI585MC with 200mm FL guide scope and with the ASI585MC the guide resolution is 2.99 arc-sec/pixel and the ratio is 1:1.18 

Just for comparison if I use my ASI174mm with the 200mm guide scope the guide resolution is 6.04 arc-pixel and the ratio is 1:2.39.

Will I notice any difference between 1:1.18 and 1:2.39? I guess the better question is will PHD2 perform better at 1:1.18 vs 1:2.39.

I never expected guiding with Hyperstar to be such a rabbit hole.

Thanks