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Celestron Cosmos 90GT upgrade (nexstar)

accessories beginner Celestron mount refractor tripod astrophotography equipment
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#1 Vickmann

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Posted 18 February 2019 - 11:17 PM

Hi guys, this is my first post, im glad to be here smile.gif

 

On christmas I bought a Celestron Cosmos 90GT in offer for only $200 on amazon, I saw this telescope years ago for more than $600, and I read that now it had a mayor update and is compatible with the new wifi app Sky Portal (before it was compatible with an app that disappeared), this was the first telescope with wifi capabilities on the low/middle range. Is almost the same as the new astro fi telescopes, and use the same nexstar drivers, in fact... is a GT (maybe SLT) nextar version with wifi capabilities.

 

Ok, I read so much about the weakly mount tripod, but nevertheless I taked the risk because the offer, but its true, its very cheapily and plastic, maybe I will be able upgrade with this cheap mounts from explore scientific with a screwdriver and faith:

For only $79 https://explorescien...fl-twinanot1-00

 

I see that Cosmos 90GT dont have so much payload, some pages talk about 5 pound and other talk about 8 pounds (like astro fi), so... this payload it is not very suitable for astrophotograpy, but I want to try this, I bought a Canon 450D for about 100 bucks in ebay, after I will buy a orion off axis guider and a cheap but good guider cam (1.5 pounds maximum), I think this lightweight combination leave me some pounds free, my goal is to not exceed 3 pound, so I will have 2 or more free for accuracy, so... my question is:

Is there a way to maximize the payload?, maybe with a better motor?, is this a better motor?: https://www.celestro...otor-assy-azmra Maybe the payload calculation is about tripod, and well... we know that the cosmos-astro fi tripod is very weakly, and maybe I will increase the payload with a new tripod

 

I live in third world so I had to take advantage of this bargain (it was inevitable), yes I live in a socialist paradise wich much taxes, dictators and weapon restrictions, so... please dont ask me to change my new beloved telescope, only just try to adapt to my drama wink.gif

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#2 Michael_Swanson

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Posted 18 February 2019 - 11:56 PM

Hi,

 

You are correct that the Cosmos GT is essentially the same as the Astro Fi.  Here is a brief description of the Astro Fi from my book, The NexStar User's Guide II:

     The Astro Fi series is an introductory-level model similar in physical design to the
SLT. The tripod is constructed primarily of aluminum and is not quite as stable as
the steel-legged model provided with the SLT; be sure all the bolts are tight to
improve stability somewhat.
     The mount features a standard CG-5/Vixen dovetail that allows the use of a wide
variety of small optical tubes; the weight capacity is about 8 pounds/3.6 kg. The
supplied optical tubes come with a red-dot unity finder on a quick-release bracket.
     Power is provided by 8 AA cell batteries in the provided power pack or by another
12 V DC external power. The provided power pack must be placed on the tripod tray
or hung from an opportune location on the mount or tripod. The best solution is to
utilize hook and loop (Velcro) straps to affix it to the fork arm (Fig. 3.9).
     The unique feature of the Astro Fi series is its built-in WiFi to make use of the
Celestron SkyPortal app on an Apple or Android tablet or smartphone. The
SkyPortal app takes the place of the physical NexStar computerized hand control.

In fact, the Astro Fi, unlike all other models discussed here, does not include a hand
control. It is, however, compatible with the standard NexStar hand control as well
as the StarSense AutoAlign hand control and camera. That said, one who purchases
the Astro Fi can bypass the cost of the hand control by making use of an existing
tablet or smartphone. A word of caution: before buying an Astro Fi, be sure to test
the SkyPortal app (available for free in Google Play and the Apple App Store) on
your smartphone or tablet to ensure it is up to the task of running this large app.
     The Astro Fi series includes inexpensive Kellner eyepieces, and the refractor
models come with a low-quality mirror diagonal. The optical tubes in the Astro Fi
series can provide much better views with improved eyepieces and diagonal.

 

 

The only significant differences are that it only has a single AUX port and it has an older WiFi module that has lower power and may experience drop-outs (disconnects) from your smartphone/tablet.  The first Evolution mounts had that same WiFi module but the Evolution suffers more from it as the module in the base and obstructed more by metal.  In the Cosmos GT it is on the side of the fork arm and suffers less obstruction.

 

Regarding autoguiding - this is only possible on NexStar/Cosmos mounts if they are mounted on a wedge and polar aligned.  There are two notable options for the Cosmos mounts.  Mederos CNC makes a wedge for these mounts (http://www.ebay.com/...eros_cnc/m.html). Additionally, it is possible to mount these scopes on the NexStar 4SE/5SE tilt-head tripod.  Note that the 4SE/5SE tripod has no provisions for fine adjustments so polar alignment is very difficult.

 

THAT SAID - the gears in the GT/SLT/Cosmos mounts are low quality with a LOT of backlash and do not lend themselves to accurate autoguiding so personally, I wouldn't spend much money on that.  Instead, spend time perfecting the art of capturing 15-25 second exposures with your camera and then stacking them to increase detail and reduce noise.

 

Regarding stability, the fork arm and motors are quite stable up to that 8 pound limit.  It is the tripod that shakes.  Be sure to tighten all the bolts and only extend the legs are far as necessary to maximize stability.  I did replace the tripod on my NexStar 102 SLT with a NexStar 4SE/5SE tripod as shown in this image.  I don't actually use the tilt-head for EQ alignment, but the tripod itself is much more stable.  Unfortunately, the tripod isn't sold as a separate item so about the only way to get one is on the used market.  Note that the original NexStar 4GT and Tasco StarGuide 4 both used this same tripod so your search could include those models as well.

 

The motor you linked to on Celestron's website is what is already in the mount.  Again, the motors are sturdy, but the gearing is a bit rough.  The gears will last for many years but they don't provide ultra smooth movement as you would want for longer exposures.  There are no after market upgrades to change this.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com

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#3 Vickmann

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Posted 19 February 2019 - 12:21 AM

Instead, spend time perfecting the art of capturing 15-25 second exposures with your camera and then stacking them to increase detail and reduce noise.

 

Wonderful advice Mike, I read your page a lot to make a good beginner (third world problems) choise, I will try to find a NexStar 4SE/5SE tilt-head tripod (maybe you can help me with that), to ​​grow a bit more in my user experience and when im ready... I will move to a better mount and telescope, a lot of thanks.



#4 Michael_Swanson

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Posted 19 February 2019 - 12:58 AM

Here's one for a reasonable price:

https://www.cloudyni...-price-reduced/

 

I'm afraid shipping costs might be your downfall.

 

If you get one, let me know and I'll explain how I mounted my SLT on that tripod.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com


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#5 Vickmann

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Posted 19 February 2019 - 01:06 AM

I'm afraid shipping costs might be your downfall.

I have contacts that are smugglers, lol ... "Science justifies the means".

On 20th of this month I will do that buy, thanks a lot again ;)



#6 Vickmann

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Posted 27 February 2019 - 07:29 AM

If you get one, let me know and I'll explain how I mounted my SLT on that tripod.

I already have it, please explain me smile.gif

 

Celestron NexStar 4/5 SE mount

 


Edited by Vickmann, 27 February 2019 - 07:29 AM.


#7 Michael_Swanson

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Posted 27 February 2019 - 08:25 PM

 

I already have it, please explain me smile.gif

 

I won't be able to provide measurements; my NexStar SLT with this modification now lives on another continent with my son.

 

First, you will find you can remove the center pin on the tilt plate on the new tripod - use "Vice-Grips" or good pliers and it unscrews.  You will need a hand drill and a set of drill bits capable of drilling through that tilt plate.

 

If you look at the bottom of the COSMOS mount (remove it from the tripod and also remove the optical tube), you will see there is a small metal bar which the mounting bolt on the original tripod screws into.  By "COSMOS mount", I mean the fork and base.  Hold the mount centered against the top of the tilt plate in the position shown in my picture above.  Insert a small object up through the center hole in the bottom of the tilt plate until it reaches that threaded bar so you can get a measurement on the size of bolt you will need to secure the mount to the tilt plate.

 

You will notice as you do this that there are 3 small protrusions on the bottom of the mount which prevent the mount from rotating when it is on the tripod.  You will need to mark 3 corresponding locations on the tilt plate and drill holes the size of the base of those protrusions, maybe even a little smaller.  I would start with a 2mm smaller and increase from there.  When I drilled these holes, I created a paper template to mark the locations.

 

IMPORTANT - follow the age-old advice of "measure twice, cut (drill) once".  You can't put metal back after you remove it :-)

 

Next you will want to go to a hardware store and find a nice stainless steel bolt (common name is "machine screw") of the correct threading and length for holding the mount on the tilt plate.  I originally thought I would get a machine screw with a countersink head but it turned out a round-top screw head was best.  At issue is that the tilt plate must return to a flat position and so you need to make room for the bolt head to protrude down into the tripod head.  I found I didn't need to increase the size of the hole in the center of the tripod head as much when I used a round-top machine screw.

 

Once you have the machine screw at home, the next task is to drill out the center hole of the tilt plate so the machine screw slides through.  Then you just need to open up the hole in the middle of the tripod head to allow the tilt plate to sit flat.  Attach the mount, tighten the machine screw and you are all set.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com


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#8 Geo.

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Posted 02 March 2019 - 03:29 PM

IMPORTANT - follow the age-old advice of "measure twice, cut (drill) once".  You can't put metal back after you remove it :-)

 

Best regards,

Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com

Oh, you can if you have a TIG welder or are very skilled with gas. It's just that aluminum, unlike other metals, likes to go from a solid to a liquid without any warning=8)



#9 Michael_Swanson

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Posted 02 March 2019 - 08:19 PM

Oh, you can if you have a TIG welder or are very skilled with gas. It's just that aluminum, unlike other metals, likes to go from a solid to a liquid without any warning=8)


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