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GEM28 Guiding, is this acceptable?

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#1 joeytroy

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Posted 09 August 2021 - 09:56 AM

So I have just got my GEM28 and it seems to be worse on the guiding than my EQM-35 Pro?!?!? I am not sure what is going on but here is the PHD2 Log from my first and second run with the ASIAir Pro. This is also with my RedCat51 that was recently repaired in Taiwan.

 

https://www.dropbox....V5_H8KsMoa?dl=0

 

image.png

image.png

 

 

I have since updated the firmware to V210105 on the GEM28 and also bypassed the USB cable to the hand controller and using the Wi-Fi to connect to the ASIAir Pro. While the clouds were kicking my butt last night I was still unhappy even at 1m39s of guiding when I was able to get a clear patch of sky

 

image.png

image.png

 

I have it pretty darn balanced with DEC but but maybe not? Here is a quick video on my balance. When pointing to north with the clutch unlocked it pulls to the right.

 

https://youtu.be/KoJ57BaJhps

 

I have also been leaving the legs closed so I am not extending it at all. Here is an image of the current setup. Like always any help is appreciated, I have a ticket open with iOptron and OPT but still no responses undecided.gif

 

mount.png


Edited by joeytroy, 09 August 2021 - 10:20 AM.


#2 OldManSky

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Posted 09 August 2021 - 10:07 AM

It depends on your image scale, but in general -- no, that's not very good.  The mount is certainly capable of better.

 

The video above shows DEC balance, not RA balance.  Balancing is tough, but it will certainly help performance.  Work on the DEC balance so it doesn't spin right out like it does in the video, then work on RA with the DEC axis in various positions (like it will be when you're shooting).

 

I'd also check cable dragging or grabbing or something similar -- one of your log summaries shows 38.52" of RA drift per minute...that ain't right.  Something's grabbing or sticking or sliding. :)


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#3 randcpoll

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Posted 09 August 2021 - 10:28 AM

I also have a GEM28 that I bought this year and am not satisfied with the guiding. When it is good it is about equivalent to an AVX I had, but it is inconsistent and occasionally gets pretty big spikes in RA. I also have a ticket in with iOptron but no response yet. undecided.gif   



#4 Tapio

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Posted 09 August 2021 - 10:30 AM

In the log there's Focal length = 190 mm.

And it looks like you are using OAG.

I believe Redcat 51 has 250 mm focal length.


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#5 Tom M

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Posted 09 August 2021 - 10:33 AM

Ways to fix the Z axis balance include rotating the entire assy in the rings or adding an offset counterweight. I chose the latter and added some large washers in a 3D printed shell for aesthetics - comes to roughly 200g. ADM can provide the vixen adapter and also modify if for the side mounted threaded rod. You just need to order the basic parts and note on the order to do the modification for the side mounted rod.

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  • VPA-EAF-1.jpg

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#6 blazek

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Posted 09 August 2021 - 10:35 AM

No, that is not balanced. Good balance will be when you have both RA and DEC unlocked and the scope stays and points wherever you move it . Yo will have to add some additional CWs  (like i.e. magnets on dovetail and on RA counterweight, ) 

You need one more dimension to balance each axis , as there is no chance you'll balance this only by moving either scope or counterweight forward/backward. That being said, perfect balancing is of course not the only contributor to the guiding error. 

There is something else adding to total RMS in your case.  Good PA and balance are base requirements ....


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#7 joeytroy

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Posted 09 August 2021 - 10:38 AM

It depends on your image scale, but in general -- no, that's not very good.  The mount is certainly capable of better.

 

The video above shows DEC balance, not RA balance.  Balancing is tough, but it will certainly help performance.  Work on the DEC balance so it doesn't spin right out like it does in the video, then work on RA with the DEC axis in various positions (like it will be when you're shooting).

 

I'd also check cable dragging or grabbing or something similar -- one of your log summaries shows 38.52" of RA drift per minute...that ain't right.  Something's grabbing or sticking or sliding. smile.gif

I updated my post also going to look at some balance video's

 

I also have a GEM28 that I bought this year and am not satisfied with the guiding. When it is good it is about equivalent to an AVX I had, but it is inconsistent and occasionally gets pretty big spikes in RA. I also have a ticket in with iOptron but no response yet. undecided.gif   

Suck...that is not reassuring

 

In the log there's Focal length = 190 mm.

And it looks like you are using OAG.

I believe Redcat 51 has 250 mm focal length.

Good catch, I will fix that right now! Oddly enough ASIAir Pro see's the focal length like 247 after using the EAF based on what it's seeing, even if I change it to 250 for the main scope it still sets it down to 247 so I will adjust that for the camera, that maybe the main issue, that and proper balance.



#8 joeytroy

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Posted 09 August 2021 - 10:45 AM

Ok with both clutches unlocked I am able to get perfect balance now, and I just updated the focal length for the ASI290mm. I will post back after the smoke clears in our area maybe a day our two...

 

East

east.png

 

West

west.png


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#9 bobzeq25

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Posted 09 August 2021 - 10:51 AM

People get bent out of shape by PhD2 numbers for no good reason.

 

Do you like your stars?  Then, you're done.

 

The only use for PhD2 numbers is diagnosing issues with your stars.  They are not goals in themselves.


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#10 scadvice

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Posted 09 August 2021 - 10:52 AM

Wrong F/L contributes to large RMS totals  RA seems to be your main problem. Level mount as well as possible and work on perfecting polar alignment to make sure they are not adding to the error.. The cable bundle you have may be contributing to the larger error. Standing over or moving around the mount while imaging on some ground and surfaces can flex quit a bit and usually shows up as elongated stars in single frames and would coincide with a mount correction on the graph. You might try out the new PHD2 multi star routine.


Edited by scadvice, 09 August 2021 - 10:58 AM.

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#11 joeytroy

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Posted 09 August 2021 - 10:55 AM



People get bent out of shape by PhD2 numbers for no good reason.

 

Do you like your stars?  Then, you're done.

 

The only use for PhD2 numbers is diagnosing issues with your stars.  They are not goals in themselves.

Bob,

 

It's funny you mention that. It's true I do get bent out of shape with numbers lol.gif however even with those numbers from night one here is my 57 minutes of exposure on HOO and I will say I am pretty happy with the stars at 180s exposures. However I did have a lot of bad frames in the set 19 good, 12 bad.

 

M27-HOO.jpg



#12 Tom M

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Posted 09 August 2021 - 10:56 AM

Ok with both clutches unlocked I am able to get perfect balance now, and I just updated the focal length for the ASI290mm. I will post back after the smoke clears in our area maybe a day our two...

 

With the RA axis horizontal, point the scope straight up and see if it stays there or in any other position other than horizontal and the DEC axis unlocked.



#13 joeytroy

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Posted 09 August 2021 - 11:17 AM

With the RA axis horizontal, point the scope straight up and see if it stays there or in any other position other than horizontal and the DEC axis unlocked.

Negatron scope keeps moving in DEC when the RA is horizontal. Which adapter from ADM did you get?



#14 Tom M

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Posted 09 August 2021 - 11:30 AM

I ordered the following.

 

TR- Stainless Steel Threaded Rod. 3” or 5” Long - 3inch, size: 3inch
VPA- V Series Dovetail Adapter.

 

You do not need one of the counterweights as they'll be way too heavy. I used 4 of some 3/4" flat washers I picked up and designed a printed shell for them that threads onto the rod so it's easy to adjust.

 

The other benefit is that the VPA can be adjusted front/back to fine tune the balance as well w/o having to slide the entire tube assy.


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#15 limeyx

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Posted 09 August 2021 - 12:59 PM

Ways to fix the Z axis balance include rotating the entire assy in the rings or adding an offset counterweight. I chose the latter and added some large washers in a 3D printed shell for aesthetics - comes to roughly 200g. ADM can provide the vixen adapter and also modify if for the side mounted threaded rod. You just need to order the basic parts and note on the order to do the modification for the side mounted rod.

Curious where you got the black weight from and if they come in M8/1.25 thread

 

I added an M8 bolt and washers to my CEM70 and it's barely enough with two bolts taped together and I wanted something a bit heavier if possible



#16 Tom M

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Posted 09 August 2021 - 01:05 PM

Curious where you got the black weight from and if they come in M8/1.25 thread

 

I added an M8 bolt and washers to my CEM70 and it's barely enough with two bolts taped together and I wanted something a bit heavier if possible

I made it myself as most of the available weights are way too heavy or too expensive. It's a 3D printed shell with some 3/4" flat washers inside and comes to about 200g when assembled. It's threaded for the rod (ADM uses 1/2x13) so is also easy to adjust. Since the shell is 3D printed it would be relatively easy to adjust to any number of washers or change the internal thread size.


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#17 joeytroy

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Posted 18 August 2021 - 10:26 AM

So just a small update, I was able to use the mount again over the weekend and after the balance, and fixing the focal it was better. After going over the settings and looking at the logs looks like I interposed the focal length to 274 and not 247 lol.gif. It was a long night as I spent about 1:30 messing the the OAG outside since this was the first time I did a run after getting my RedCat back from warranty work in Taiwan and I took apart everything thing to clean. Had to come back inside and readjust everything and was able to get it to work so I am sure in my "trying to push through" I messed up the focal length again. It's now set right but like most astro problems it's user caused lol.gif Anyways I was happy with the out come as all night long I was well under 2" RMS errors and as low as 0.77". Still some work to do like Z-Axis balance but it seems a lot better now. Thanks everyone for the input!

 

32m54s

RA: 0.70 (0.32 px) Peak: -3.15" (-1.77 px)

DEC: 1.06" (0.49px) Peak: 4.59" (2.11 px)

Total: 1.27" (0.58 px)

 

RA Drift
+3.92"/min, +1.80 px/min
Dec Drift
-0.08"/min, -0.03 px/min
Polar Alignment Error
0.3'


Edited by joeytroy, 18 August 2021 - 10:28 AM.

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#18 rgsalinger

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Posted 18 August 2021 - 10:50 AM

When guiding in RA you are accelerating and decelerating the mount. Even though the mount is balanced, there's still a moment arm involved and the motors may simply not be able to respond well enough to the guiding commands to get better numbers. That system looks great but piling things above the saddle is asking a lot from this mount, IMHO.

 

If this were my system I'd be looking at ways to reduce the height of the system. Get rid of that red box and the risers and figure out how to configure the system differently. I think that I'd also get rid of the Vixen dovetail and get a Losmandy dovetail (if the GEM28 saddle will take one). 

 

I'm sure that 3d balancing might also buy you something but my money is on getting the center of mass better distributed as the better alternative.  Remember what Bob said earlier. Once you get the RMS guiding well under the image scale you should be getting round stars and never looking at the guide graph. Having something good to read when imaging is one of the best things you can buy for your system. 

 

Rgrds-Ross


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#19 joeytroy

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Posted 18 August 2021 - 12:26 PM

Ross,

 

I do have Losmandy dovetail coming in with a Stellarvue SVX80T mounted on it waytogo.gif At that point the RedCat may go on the chopping block to help fund a possible jump to the ASI2600MM. for now I will move the AAP from the top and get rid of the little saddle bar to help bring down the height. If I happen to keep the RedCat I will also look at getting the black cat mount which moves the EAF to the right side of the mount with the AAP so I can remove the Deep Sky Dad connector which will bring down the height another inch or so.

 

https://agenaastro.c...-telescope.html

 

Thanks for the info, time to get some more books to read...




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