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Switching OTAs

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#1 takiwa

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Posted 29 May 2020 - 11:58 AM

Hey yall, I'm Jeremy from Texas, and this is my first post here.

 

My wife recently spent a bunch of reward points from her work and got a Meade StarNavigator 114 NG telescope https://www.meade.co...r-ng-114mm.html

 

My question is this...I would like to swap the OTA out with a bigger one. I'm looking at a Celestron 6" Advanced VX OTA https://telescopes.n...e-assembly.html.

 

Problem is I can't find any information on the weight limits of the Meade mount, or the motor assembly. The Celestron OTA weighs 10lbs. The weight of the Meade OTA isn't listed, only that the entire scope assembled is 14.72lbs. I have tried to call Meade but they have shut down their CS due to Covid.

 

Will this Celestron OTA work on the Meade mount? Or will it be too heavy? Will the Meade GoTo motor track correctly without lag with this extra weight?

 

Thanks in advance.



#2 Don W

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Posted 29 May 2020 - 01:43 PM

It should work. But bear in mind that this mount has a pretty wobbly tripod.



#3 takiwa

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Posted 29 May 2020 - 02:28 PM

I was planning on beefing up the legs a bit, not a fan of aluminum legs. I don't care about weight as much as I do stability. But thank you, I will get the OTA :)



#4 EFT

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Posted 04 June 2020 - 03:24 PM

I'm late to this, but that scope is very unlikely to fit on that mount.  It is too long and too wide.


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#5 takiwa

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Posted 10 June 2020 - 05:50 AM

I'm more concerned with the width than the length. The rings can be shifted back to allow it to traverse upward, although I'm not sure what stresses this will put on the motor drive (which was my initial question). But the width is something I can't fix. Granted, a 6" tube is only 3/4" wider all the way around than a 4.5" tube, but I'm sure I will lose some aiming because of this. Still, I am hoping for the best. The Meade hasn't come in yet, so for now I have the 6" OTA on my original Celestron German EQ mount, with only a 7lb counterweight. It's heavy, but usable. I've already got some real nice views of Jupiter, Saturn, Mars and the Moon.



#6 macdonjh

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Posted 10 June 2020 - 08:43 AM

takiwa,

 

Welcome to Cloudy Nights.

 

EFT knows mounts...

 

As you've already wondered, one of the risks of mounting a 6" Newtonian on your Meade mount is stress on the mount's motors.  If you are able to balance a 6" scope properly on the mount, that won't be a problem.  However, as EFT says, you may find the balance point of that scope on your mount may result in the scope not being able to point all over the sky.  

 

The other road block I think you may run into is actually mounting a C6N to the Meade mount.  I looked at the photos in the link you provided.  It looks like your scope is attached "directly" to your mount, there doesn't appear to be a dove tail bar.  The C6N you are considering has a dove tail and needs a Vixen-style saddle on the mount.



#7 EFT

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Posted 10 June 2020 - 02:16 PM

After doing a little more research, I will probably have to correct myself here, but I am having to make some assumptions regarding the scope sizes and weights (e.g., the Meade OTA is similar in size and weight to one they sell without a mount, the older Celestron OTA is similar to the current OTA of that size and configuration neither of which may be true).  The differences between the OTA's are hard to nail down but I dug around some and using what I assume is a similar Meade OTA, I found that the Meade OTA weight is probably about 11.35 pounds while the Celestron 6" is only 10 pounds so that is actually good.  Meade SCT's are always heavier than Celestron's but it looks like the same may hold true for the Newtonians as well.  Since that Celestron tube is at least 1.5" greater in diameter, that moves the center of gravity out from the altitude axis by 0.75", but the weight difference may make up for that.  The lengths of the scopes are probably within about 1/2" of each other so that is OK.  While I didn't find any good pictures of it, it does appear that the Meade uses a V-series saddle which would then fit the Celestron OTA dovetail as well.

 

One thing to be careful of though is significantly shifting the location of the tube rings on the tube in order to clear the base.  The more out of balance from front to back the scope is, the more stress it will place on the altitude axis drive system and the less stable it will become.  These mounts are not particularly robust in that regard and balance in the altitude axis is generally critical to performance.

 

So I will change my initial recommendation to say that, based on some assumptions, you may be OK switching the OTA's.



#8 aalmanni

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Posted 10 June 2020 - 04:54 PM

Technically speaking it should work, but at some point you'll stress and maybe damage the motors at some point

 

Myself, one day, I did put a 120mm f/8.3 refractor (weight almost 10 pounds) https://www.telescop...cope/p/9867.uts

On Astromaster tripod https://www.celestro...romaster-tripod

 

It worked but the 120mm is almost non usable due to stability issues.

My advise, go for a larger mount, and you'll never regret it.



#9 macdonjh

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Posted 11 June 2020 - 10:07 AM

 

One thing to be careful of though is significantly shifting the location of the tube rings on the tube in order to clear the base.  

And because of the offset in the mount's arm, that might not be necessary.  Alt-az mounts are hard to use near zenith anyway, so it might not be that big a deal if the scope couldn't be pointed straight up because of interference with the base.



#10 takiwa

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Posted 13 June 2020 - 03:05 AM

20200613-025202.jpg

20200613-025222.jpg

 

Forgive the messy garage...but it's on there smile.gif I tried to get the OTA centered on the mounting arm as best as I could, and it seems to work pretty good. It slews ok, but after about 10 minutes the object is out of view through a 26mm eyepiece. I'm not sure if that has to do with the OTA weight, or the fact that I didn't get the A/C adapter and am running on 8 AA batteries atm, or if I just didn't have the scope aligned properly...maybe it's a combination of all 3. All I know is I just got off work, I'm dead beat, and the manual is way too much for me to process right now, lol. I'll take another look in the morning. 

Thank you for all your inputs into this, I will re-read everything tomorrow to make sure I didn't miss anything in anyone's comments that might help me.

EDIT* And yes, I am looking for ways to make the legs a bit more stable. It isn't as bad as I thought it would be, but they could be a lot more beefy. Maybe adding counterweights to the bottom? I don't know...again, way to tired to think atm
 


Edited by takiwa, 13 June 2020 - 03:09 AM.


#11 takiwa

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Posted 14 June 2020 - 12:58 AM

Update

 

So, it seems that the batteries are good for about 3 hours. I am assuming that this is because of the extra weight of the OTA (I didn't weigh either on a scale, but the 114mm Meade is a good deal lighter in my hands than the 152mm). It tracks decent with a 26mm eyepiece, but doesn't center the object. It's still in the FOV, but it's off, either to the left or the bottom of the field (never right or up). I did order an A/C adapter this morning, so I hope that a constant power source will help with this, but I may end up putting the 114 back on this mount and just getting another goto setup for the 152mm. 



#12 SimonMiller

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Posted 14 June 2020 - 06:53 AM

I think you have a few issues to contend with:

 

A Newtonian scope uses cradles to allow the tube to be balanced. You may get it balanced at a point where the base interferes with the Alt/Az Mount; Refractor and Newtonian OTAs don’t play well with this kind of mount;

 

To align the image so the focuser is conveniently placed requires tube rotation. You may find an equatorial mount more convenient especially if it is on a short tripod. With the original scope the height of the EP stays almost constant. With a Newtonian scope the height of the EP varies e.g. you might need a step ladder or box to stand on;

 

A Newtonian has a large side profile that puts high demands upon a mount e.g. wind buffeting.The capacity of the mount e.g. carrying capacity is not just about weight. Some mounts will carry an OTA weighing only a few kilos. Compare this to an equatorial mount, that may carry 15-20kg OTAs and also suitable counterweights and you see that the requirements are different;

 

If you called CS, they may not want to give a recommendation, but it might work. Or they may be suggesting combinations that may only just work or even not work e.g. just look good on paper or in their catalogue;

 

If you later want to try AP, the weight of the camera can be offset by adding weights at the primary mirror end. Some focuser tubes are too short to allow a camera to focus. You might overcome this with a Barlow. Results may require a more stable mount. So it just depends...

 

Simon




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