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CEM70/Edge HD 9.25 Beginner Questions

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#1 The Cat

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Posted 28 November 2020 - 02:04 PM

So, this is a variation on some questions I posted elsewhere and before, but I thought I'd ask it here since I know that my new equipment is popular among planetary astrophotographers. I've had the iOptron CEM70 for a couple weeks now and I've finally begun using it with the Edge 9.25. But I'm having some issues, to the point that both pieces of equipment have been rendered useless.  So: help!!

 

1) Focusing issues with the Edge HD 9.25. I wasn't able to obtain sharp focus of any celestial objects. I went through about 10 different revolutions of the focuser, back and forth, to lubricate it but that made no difference. So, I tried my hand at collimation. Disaster. According to instructions, I blew the star out of focus and the disk at the center was too far to the left. I tried to use the method of placing my finger across the lens (without touching) to identify which screw had to be turned. I think think I tightened that screw way too much. And I loosened the other two screws. Now the star is way, way too dim! I slewed to the moon and, although I could make out the craters etc, the surface was way too dim. What could have happened? Is there any way for me to collimate in the day time without a ball bearing and without an artificial star?

 

2) Balance issues with the Edge HD 9.25. I do have a dew shield on the way. Without it, or some other form of frontal weight, the Celestron is just too back heavy to be balanced.  Is the dew shield enough to balance? Does anyone have experience with those extra weights that hang from the base of the telescope? 

 

3) Challenge with fastening the Edge HD 9.25 to the mount. In the dark, when I try to place the telescope bracket on the CEM mount swivel tray thing, it slides around a lot before coming to rest in the correct position.  Any tricks to that? 

 

4) iPolar. I thought I was doing a brilliant job of polar aligning the scope because I was able to get it level and then use ipolar to make subtle adjustments to the scope so that the red dot became green! But I think it's all a failure because ipolar wasn't calibrated to begin with. When calibrating, am I supposed to take two, covered photos, one with the OTA in the zero position and one at the 90 degree position? If so, can I do that inside during daylight? 

 

5) GoTo Issues. GoTo is way off, perhaps because of poor polar alignment? Perhaps because of the balance issues? Any thoughts on how to compensate. 

 

Thanks in advance for the answers to these many questions. I was becoming better at planetary photography using my beginner, 100 mm Orion refractor. I have a Stellarvue on its way to learn more about DSO astrophotography. But I had figured that a high magnification SCT would be a good scope for planetary. 

 

Anyway, thanks again!

 

 



#2 mborland

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Posted 28 November 2020 - 03:19 PM

 

2) Balance issues with the Edge HD 9.25. I do have a dew shield on the way. Without it, or some other form of frontal weight, the Celestron is just too back heavy to be balanced.  Is the dew shield enough to balance? Does anyone have experience with those extra weights that hang from the base of the telescope? 

 

4) iPolar. I thought I was doing a brilliant job of polar aligning the scope because I was able to get it level and then use ipolar to make subtle adjustments to the scope so that the red dot became green! But I think it's all a failure because ipolar wasn't calibrated to begin with. When calibrating, am I supposed to take two, covered photos, one with the OTA in the zero position and one at the 90 degree position? If so, can I do that inside during daylight? 

 

5) GoTo Issues. GoTo is way off, perhaps because of poor polar alignment? Perhaps because of the balance issues? Any thoughts on how to compensate. 

Balancing SCTs is a hassle because of the heavy mirror in the back. It gets worse when you put an imaging train on. My LX90 is pushed all the way forward in the saddle. I also have a Meade tube balance set to fine tune things, plus some DYI weights to balance left/right. If you google around, you should find some solutions for weight systems to attach to the front of the dovetail. Here's an example:

https://sctscopes.ne...ing_an_sct.html

 

The iPolar calibration needs to be performed when stars are visible. By analyzing how the stars move when the mount is rotated, the iPolar software can figure out the offset of the camera axis relative to the mount axis.

 

I had GoTo issues with my SCT as well (Meade LX90) on several mounts, even with good polar alignment. I think it is just poor placement of the dovetail mounting holes on the SCT tube, in my case. After you get polar alignment sorted out, try a three-star alignment and look at the cone error. If it is large, you probably have a mechanical straightness issue in the mounting of the tube. These days, I never do GoTo or alignment with the mount itself; plate solving (using NINA, APT, or your favorite software) is a much easier and more reliable way to go.

 

--Michael



#3 RedLionNJ

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Posted 28 November 2020 - 03:31 PM

If you suspect the collimation is considerably off, start with just looking in the visual back (no diagonal!) in daylight. Line things up so at least your eyeball is centered.

 

Then in the dark, pick a really bright target at first. Mars would be fine for this these days. Adjust until there's a rough collimation.

 

Then switch to a fainter, more pointlike object (any brightish star) and refine further.  Don't forget the target will appear to move as you tweak the collimation. It would also be helpful to have the mount tracking semi-reasonably before you start this phase. Of course, perhaps you have a chicken-and-egg situation with respect to collimation and alignment....  but it's the MOUNT you're aligning, not the OTA.

 

Sounds like maybe you took on a little too much at once. It may help to straighten out one aspect at a time.  Planetary season is just about over for the current apparitions of the bright targets anyway.  A great polar alignment will stand you in good stead for DSO work, too.




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