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iOptron GEM28 Advice

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#1 Ntro

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Posted 24 June 2022 - 01:21 AM

Hello everyone,

 

This is my first post. Nice to meet you all.

 

I consider myself a newbie. I was into astronomy years ago, visual only. But a few months ago I decided to take a serious dive into AP. So, I bought the GEM28, and I've been imaging almost every clear night since.

 

My setup:

GEM28 Mount

C6 with a F6.3 reducer and a ZWO ASI533MC Pro camera

Guide Scope is 240mm F4 w/ the ASI290MM mini

Using PHD2 and NINA software

I live in a Bortle 7

 

My question is about the guiding. I'm trying to figure out if the performance is above, below or average for this mount class. It seems to be very inconsistent. I get guiding anywhere from 0.5 to well over 1 arc secs. It will guide great for few minutes (0.5-.0.7), then shoot up well over 1 for no apparent reason. I find myself throwing out about 25% of my images because of guiding errors. Now I know there are a lot of factors which will influence guiding. So is this typical for this mount class? Is it all I can expect, or is there something I can do to improve the consistency? Tweak PHD2 settings, tune the mount, travel to a dark site to test, or buy another mount?

 

Any advice would be greatly appreciated.

 

Clear Skies!

 



#2 Tapio

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Posted 24 June 2022 - 04:20 AM

There can be many things causing this - mount backlash, wind, clouds, traffic vibrations etc.

Some are easier to fix.

Like guiding settings for example.

Have you run guiding assistant (for several worm periods in order to measure backlash), typically over 10 minutes ? And then followed suggestions.

Are you using multi star guiding ?

Are you using PPEC RA guiding setting in PHD ?

 

Backlash can be tweaked and there are several videos how to do that.

Some GEM28 expert might tell which is good or give hints here so I won't go there.

 

Also replacing guide scope for OAG can make a difference with SC scopes.



#3 GaryShaw

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Posted 24 June 2022 - 07:24 AM

All good questions from Tapio above. I’d just add that the best place to get help is the PHD2 forum. You should post there and fully describe the steps you’ve taken to establish good PHD2 settings for your gear and include a recent guiding log showing the range of issues that concern you. You’ll get valuable help if your post includes all the info ( and logs) needed for the experts to diagnose your situation. 
good luck!

Gary



#4 Ntro

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Posted 24 June 2022 - 01:14 PM

Thanks for the replies.

 

So here is what I've tried.

 

Ran GA, many times (up to 15 minutes), and tested the new settings, plus adjusted manually. Slight improvements, but still inconsistent. There is quite a bit of backlash in the DEC (1800), and I have that turned on.

I am using multiple star guiding, I believe it's set by default.

I tried recording PPEC in the mount itself and turning that on, but that made things much worse. Not sure what's going on there.

PA is ~1-2 arcmin)

I considered wind could be the issue, but I recently bought a tent (Explore Scientific), and it did seem to help on breezy nights. But even on a perfectly still night, under the tent I'm getting inconsistencies.

I have watched some videos about mesh adjustment etc., but haven't tried anything. I'm reluctant to mess with anything until I run out of other options.

Also, my current target is M101 (I have about 20 hours), which is up high earlier in the night, and drops later. Guiding does get a lot worse later in the night as I would expect.

I calibrate on a star near the CE (Dec 0). But guiding is much worse there, not even doable.

 

That's all I can think of right now.

 

Thanks again

 

Clear Skies!



#5 Tapio

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Posted 24 June 2022 - 03:19 PM

Couple of thoughts/questions more based on you reply.

Are you using ASCOM/pulse guiding or On-camera?

If former then you need to calibrate once near equator, if latter then in/near target.

 

Try PHD2 PPEC setting and let it run couple of worm gears.



#6 Ntro

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Posted 24 June 2022 - 04:13 PM

Where would I find out if it's ASCOM/pulse guiding or On-camera? Just FYI, I have the ASCOM drivers installed on the computer. I have a USB hub connected to the computer. Then I have the Main Camera and Mount Controller connected to the  hub via USB. The guide camera is connected directly to the Main Camera via USB. In the Connect Equipment dialog in PHD2, the Camera is set to ZWO ASI Camera (i.e. ASI290MM mini), and the Mount is set to iOptron GEM28 Mount (ASCOM)

 

Regarding PHD2 PPEC setting, where is that in PHD2? I can't seem to find it.

 

Thanks



#7 Ntro

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Posted Today, 02:14 AM

Hello again,

 

I think I figured out my problem, incase anyone else is experiencing something similar. I believe it was a transmission issue, because the guiding was stalling ever so slightly, and throwing it off. I was using a passive USB hub, which I believe was the issue. I switched to an active hub, and the guiding has greatly improved. There are still some glitches every now and then, but it's down around 0.5-0.6 RMS on average tonight. I still need to do more testing on a target closer to the meridian though.

 

Thanks again, and clear skies!


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