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A 24-incher - and be quick! Cobbling up an f3.4 in the depths of the shed

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#176 tommm

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Posted 03 September 2020 - 12:12 PM

Thanks JT! I will note that down so I can make some informed requests down at the shop. Any ideas on a rechargeable battery for this?

I use a DeWalt 5Ah 20 Volt nominal tool battery for my Goto system and to drive an 8" Noctua fan through a DC/DC converter to drop it down to 12V.  Both draw very little current. After 4 or 5 viewing sessions I've typically used less than 2Ah.  Note that power is the product of current and voltage so the current out of the battery to the fan running at 12V is a factor 12/20 of the current through the fan.  You can find different Ah sizes with a charger on Ebay for a good price.  Iirc I paid $60 for my 5Ah battery and a charger. 

 

They last a long time, and no worries about sulphate buildup killing them when the sit unused for a long time as in lead acid batteries.  I've been running the lithium batteries in the car I converted to electric for almost 11 years now, and that is a much more demanding application.  You can get DC/DC converters in various form factors from Digikey or Mouser for cheap.  I paid a bit more for one mounted with terminal screws ($20?) to make hooking it up easy.



#177 jtsenghas

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Posted 03 September 2020 - 01:41 PM

I use a DeWalt 5Ah 20 Volt nominal tool battery for my Goto system and to drive an 8" Noctua fan through a DC/DC converter to drop it down to 12V. 

That's a GREAT IDEA!  I recently added a few more Porter Cable cordless tools to my stable, including a couple of 5Ah batteries and could do the same.  Any of us who have cordless tools in the 18-20 Volt range could do that for little more than  the price of the converter. 

 

Did you simply modify a charger for the electrical contacts  and make a 20V plug for your DC/DC converter for hooking it up to your scope? 


Edited by jtsenghas, 03 September 2020 - 01:42 PM.


#178 jtsenghas

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Posted 03 September 2020 - 01:56 PM

Yeah! That's the ticket.  If we buy one of those chargers that has the connector separated from the AC/DC transformer like this.....

 

Screenshot_20200903-144829_Amazon Shopping.jpg

 

... then we could cut the cord and install simple two pole mating connectors between them. 

 

If a matching connector to the one on the transformer side is wired to a 20Vdc/12Vdc converter it's done!

 

The batteries could be hooked up to the original transformer for charging, even for the original tools, and then hooked up to the dc/dc convertor for observing!

 

Edit- Auto parts stores sell a variety of connectors for wiring repairs that are designed to attach only one way and not permit accidental shorting of the hot terminal on either connector. 


Edited by jtsenghas, 03 September 2020 - 02:01 PM.


#179 tommm

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Posted 03 September 2020 - 10:53 PM

That's a GREAT IDEA!  I recently added a few more Porter Cable cordless tools to my stable, including a couple of 5Ah batteries and could do the same.  Any of us who have cordless tools in the 18-20 Volt range could do that for little more than  the price of the converter. 

 

Did you simply modify a charger for the electrical contacts  and make a 20V plug for your DC/DC converter for hooking it up to your scope? 

I bought the charger with the battery on EBay - brand new for less than the cost of the battery alone at Home Depot.  The DC/DC is in front of the yellow battery in the photo below. There are thousands of different types and form factors of them available at Digikey, Mouser and others.  I got that one because it was easy to mount and connect to. The actual device is small, they are usually mounted on PC boards. The battery slips into the charger upside down. Even after several nights of observing it typically takes less than 45 minutes to recharge.  The 200mm Noctua fan pulls < 80mA, and OnStep is set up for maximum motor driver currents of 300mA in "hold", 400mA during tracking, and 500mA in Goto slews at 3.5 deg/sec, so current draw is small. The motors are NEMA 17.

 

Alt and Az drive components.JPG



#180 Aperturefever

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Posted 04 September 2020 - 04:48 AM

Thanks so much guys for the great ideas. I will see what I can scout out over the weekend but now I have some idea of where I am headed. The learning curve continues ...

#181 CrazyPanda

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Posted 04 September 2020 - 12:21 PM

My suggestion is that you investigate pulse width modulation (pwm) DC speed control using simple electronics designed for LED lights and the like.  Some are sold as "LED dimmer switches".  I've used such with direct wired and wireless controllers to very good effect. 

 

These flicker on off at high frequency for smooth speed control and good torque on fan motors.  When you adjust the speed control up the fraction of each cycle that the voltage is on increases.

 

You simply need a controller that can handle the current of all the fans wired in parallel, using 12 volts for that model. Four of those would draw only half an amp at full speed. 

One thing to be aware of is some fans are 4 pin that are meant to work with computer-controled PWM from a motherboard. I've found that these will not always work with normal PWM controllers.

 

However, 2 and 3 pin fans are fine (the third pin is just for RPM information to be sent back to the computer, and can be ignored).

 

Else, Noctua sells a 4 pin PWM controller https://www.newegg.c.../1W8-001X-00014 that would presumably work with all 4 pin fans.

 

This is the fan I bought

 

https://www.amazon.c.../dp/B00KFCQY94/

 

There are 4 pin versions of this so you have to be careful to make sure you're getting the 3 pin version if you want to use your own PWM controller.

 

That fan is a beast and despite my attempts to isolate it from the structure, it still vibrates the view like crazy even at modest observing powers. But I don't run it when observing, I just run it to initially cool down the scope - something it excels at. Maybe when I get it wired to a PWM controller I can keep it on since it won't be at full speed.


Edited by CrazyPanda, 04 September 2020 - 12:29 PM.

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#182 Aperturefever

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Posted 16 September 2020 - 10:15 PM

I have been busy wheeling the Hulk in and out of the garage over the past few nights for some shake and bake testing and come up with the following points moving forward.

1. Motions are nice, and with the finders attached the scope is perfectly balanced. So that's a win.

2. Pole lengths are right, but there may be minute slippage of poles on the bottom clamps over the course of a few nights. I might employ aluminium pole collars to keep things in position.

3. Collimation holds true, but collimation needs to be absolutely spot-on.

4. Taking it out of the garage into the early evening air and giving it just a couple of hours to adjust is a true torture test of the primary.

5. I need my thermal management system finished and running ... like now. That, and star test at about midnight.

6. Star tests on a big rig like this need to be averaged out over many nights.

7. As expected star tests show overcorrection, which I would expect to ease off as the night progresses.

8. No 'stig that I can discern.

9. Omega Centauri is a big ball of resolved stars, even with an uncoated primary.

10. Jupiter shows a bunch of banded detail around thermals. A splash of coating, some plugged in fans and it should look amazing.


Edited by Aperturefever, 16 September 2020 - 10:17 PM.


#183 maroubra_boy

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Posted 16 September 2020 - 11:22 PM

Hello mate! laugh.gif

 

Is your garage airconditioned?  If the Hulk is kept cool from the start it will have a big impact on its thermal properties when you want to use it.  It doesn't mean you need to keep the aircon on all the time either.  If you don't use Hulk when the Moon is up, you may like to turn the aircon off.

 

If you do employ fans, be careful how you use them.  Fans can cause thermal problems too with glass.  Have a fan blow directly on to glass, and you will induce strain into it because of the uneven distribution of heat in the glass, and this will screw up its fine figure.

 

Alex.


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#184 Aperturefever

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Posted 17 September 2020 - 01:04 AM

Alex, imagine seeing you here grin.gif .

 

Mate, much care has gone into the fan design, but hey - if it's no good I can send this monstrosity back to the maker and ask for my money back lol.gif .

 

Some form of air conditioning within a storage cabinet or some such has crossed my mind. The thermal gradient isn't too bad over the winter months but I really notice it getting a little steep heading into spring. I vaguely recall a thread about someone doing something with an air conditioner and a big dob. It's certainly an idea, and you could pick up a small portable unit second hand relatively cheaply I reckon. The trick would be not over-chilling it. But at least you could keep your observing beers cool!



#185 tommm

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Posted 17 September 2020 - 11:03 AM

...The trick would be not over-chilling it. But at least you could keep your observing beers cool!

I wonder how many peoples' judgement of "The best view of X I've ever seen!" has been influenced by that?
 



#186 Aperturefever

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Posted 18 September 2020 - 06:27 AM

Well Tom you know beer goggles are the poor man's night vision! 😄
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#187 jtsenghas

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Posted 18 September 2020 - 07:46 AM

Well Tom you know beer goggles are the poor man's night vision!

Averted imagination enhancement? 


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#188 Stardust Dave

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Posted 18 September 2020 - 06:11 PM

"If you do employ fans, be careful how you use them.  Fans can cause thermal problems too with glass.  Have a fan blow directly on to glass, and you will induce strain into it because of the uneven distribution of heat in the glass, and this will screw up its fine figure."

 

Can you be a little more specific on fan use. I may be reading that wrong.

 

Is this advise specific to a large / thin mirror?  




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