I've discover my light box is too bright for the SBIG ST8300M, to avoid the shutter impacting the exposure the recommended duration is 4 seconds however the image even at 0.1 seconds is over-exposured.
The light source is a 69cm strip of these LEDs
http://www.ebay.co.uk/itm/12v-LED-STRIP-LIGHT-3528-SMD-300-LEDS-5M-UK-/320681128958?pt=UK_Home_Garden_Night_Lights_Fairy_Lights&var=&hash=item4aaa15b3fe (http://www.ebay.co.uk/itm/12v-LED-STRIP-LIGHT-3528-SMD-300-LEDS-5M-UK-/320681128958?pt=UK_Home_Garden_Night_Lights_Fairy_Lights&var=&hash=item4aaa15b3fe)
Inside one of these http://www.tesco.com/groceries/Product/Details/?id=277902897 (http://www.tesco.com/groceries/Product/Details/?id=277902897) (chocolates removed)
I tried covered intermittent LEDs to reduce the brightness however I only start seeing an impact on the image when there are too few LEDs leading to an uneven illumination on the diffuser.
I'm now going to try a different approach and add a potentiometer however I think my maths might be wrong in calculating the size of resistor.
The LED strip power is consuming 4.8W per metre so my strip should be using 3.3W
It has a 12V power supply so Power=IV so the current should be 3.3/12 = 0.276 A
Therefore based on V=IR the current resistance of the circuit is 12/0.276 = 42.5 Ohms.
As I'm trying to reduce the Power based on P=I2R, I need to reduce the resistance to reduce the power so I need a potentiometer 0-50 Ohms, is this right?
Yup should work if you wire them up in series, so that the potentiometer takes half the voltage at full resistance, you also need to
consider that when the pot is at 43 ohms it will be disapating .8 watt, so you might need to get at least a 1W pot.
The problem you will run in to is that as you increase the pot so the voltage to the leds will dim, to a point where they will
probably not turn on at all, this might not allow them to be as dim as needed,
The other way i would do it is to build a variable mark space ratio timer using a 555 chip,
design it so its running at 1000hz,
very simple, and then use this to power the strip, that way you have an adjustable dimmer for it from 0 to 100%,
and as all you are doing is turning the leds on and off fast, its fully controlable,
The leds will always light as they are getting the full supple voltage.
Mac
Thanks Mac. The smallest pot Maplins have is 1K but it is only .4W, so I'm not that will work then based on your calc
Quotewhen the pot is at 43 ohms it will be disapating .8 watt, so you might need to get at least a 1W pot.
BTW how did you calculate this?
Not sure where else to buy this locally.
Quote from: Mac on Oct 01, 2013, 14:47:32
The other way i would do it is to build a variable mark space ratio timer using a 555 chip,
design it so its running at 1000hz,
Otherwise known as Pulse Width Modulation (PWM). Like Mac said, easily done with a timer chip very cheaply too. Read up on 555 and PWM.
Ivor do you own a tablet computer? I have found a completely white screen and minimum brightness on my iPad makes a perfect flat panel.
Calculations.
You are correct in that your strip is 43.6 ohms.
If you get a 50 ohm pot and run in in series, then when the pot is at 43.6 ohms the voltage drop is exactly half the supply
the other half being over the 43.6 ohm strip.
You can the calculate the power as W = V2/R
which is 6*6 / 43.6 = 0.82 W
As the pot resistance is reduced so the voltage across it is also reduced.
So when its 10 ohms, the voltage will be 2.2V
The power will be 2.2*2.2 /10 0.48W
Mac.
Or you can buy some ND sheet from fleabay.
http://www.ebay.co.uk/itm/Neutral-Density-Filter-Sheet-299-1-2ND-/110601846109?pt=UK_Sound_Vision_Other&hash=item19c0623d5d
I have some of this for my flat panel which was to bright.
Last night I placed some white card over the diffuser which made a different but the image duration was still only 0.75s which is likely to lead to the shutter impacting the shot. In addition this approach only gave a time range of 0.75 - 0.78s to play with so I need to look at either adding the pot or the PWM to give sufficient control over the brightness.
Trying to find a pot which is 100ohms and 1W rating is quite hard, Farnell have some but it will be £10 including delivery.
Can anyone think of a local store that might sell components other than Maplin?
My wife has an iPad and it is just big enough to fit over the FLT110, however I found that the touch screen appeared to respond to the telescope touching it, so I need to investigate some more. I don't really do Apple products so I found it quite strange being in an alien world of IT when I normally familiar with most things.
The only consolation is the poor flat I did last night highlighted that the problem I'm seeing in the light subs is caused by something else. Two problems great!!
Ivor, put a sheet of paper in front of the iPad if that is happening.
You can definitely build a pwm switch for less then £10.
Here's a quick circuit straight off of google.
(http://www.armory.com/~rstevew/Public/LM555/555pwm.gif)
You would need to probably have the PWM controll a darlington pair for the control of the LED strip.
(http://3.bp.blogspot.com/_j96DXtidZWo/TMaF98ccBpI/AAAAAAAAAaE/Tpu507viHAg/s320/darling.gif)
Instead of feeding an output, you would just have your strip instead of the resistor.
Mac.
I've used these PWM controllers from Maplin for controlling dew heater bands and also for controlling Peltiers. Easily mounted in a plastic "project box".
http://www.maplin.co.uk/panel-mounted-speed-regulator-module-30310
Switches up to 3 amps continuous
Hi,
You do know you can cut some of the LED off the chain if needed?
Also I'm not sure all LED like to be dimmed either, but that will be easy to test, as for the 555 idea, you could end up with inconsistent flats if the flicker is not consistent between flats.
I'm having a negative day...
The LEDs I'm using can't be dimmed so does this mean the pot and PWM solutions are ruled out?
I know I could cut some of the LEDs out, however as the current strip has the LEDs in parallel and the strip goes around the inner circumference. to get around this I covered 1 LED every 3 to ensure even illumination.
This solution produced a lovely flat for the canon http://forum.orpington-astronomy.org.uk/index.php?topic=8941.0 (http://forum.orpington-astronomy.org.uk/index.php?topic=8941.0) it's just the ST8300 is too sensitive.
I will play with the ipad solution some more tomorrow and I will see if a 100ohm resistor added to the circuit adjusts the brightness before investigating the pot or PWM.
I think I has most of the bits for the PWM in the garage from my failed attempt to built a dew heater last year so it would be pretty cheap if it was viable.
Why can't they be dimmed?
if you use the pwm you are not dimming them, you are just turning them on and off fast using a variable mark space ratio.
the wave form output looks something like this (per cycle).
on for 10%
__
|
|
|____________
on for 50%
_______
|
|
|________
on for 90%
____________
|
|
|__
so at 10% they are only illuminated at the full voltage for 10% of the time, therefore their brightnes will only be 10%
at 50% they are only illuminated at the full voltage for 50% of the time, therefore their brightnes will only be 50%
you get the picture.
Mac.
QuoteWhy can't they be dimmed?
It an assumption I'd admit, I'm basing this on a LED needing a minimum voltage to fire I'm not sure what the voltage range is for this type of LED, maybe I'm being too pessimistic.
Why not just get a pack of resistors from Maplin and experiment
Or
Chris's ND film
Quote from: Ivor on Oct 03, 2013, 08:40:39
QuoteWhy can't they be dimmed?
It an assumption I'd admit, I'm basing this on a LED needing a minimum voltage to fire I'm not sure what the voltage range is for this type of LED, maybe I'm being too pessimistic.
I've yet to come across an LED that can't be dimmed. If they couldn't LED TV's wouldn't be much use as pixel brightness is essential for it to work.
As Mac says, PWM is just turning them on and off for a duty cycle anyway. It is just so fast it deceives your eye (and if fast enough, your camera too) into thinking it's not so bright.
LED dimmers
This company could help, you can also find them on ebay
http://www.myled.com/led-accessories.html?cat=236 (http://www.myled.com/led-accessories.html?cat=236)
That's a lot cheaper than Maplin!
Pretty much anywhere is cheaper than Maplin.
I had a 100K pot at home and tried this and it appears to work in principle but because it was too big it didn't give me the sensitivity I needed so I've got a 1K to try out instead.
This process has been postponed as the flats I produced highlighted the bigger problem was with the ST8300. My subs have been coming out like this for a while and I could work out why.
(http://farm9.staticflickr.com/8208/8159164954_f397383bbc.jpg)
Well to keep a long story short it appears the ST8300 can suffer from condensation inside the CCD chamber, there is a desiccant pack which is inside the camera casing which should last about a year (which explains why the problem started to appear last April) which you have to drying out in the oven @ 170 for 4 hours! Also SBIG will send you a heater strip for free which can be soldered into the camera.
So I now have to cook a roast dinner for the family so I can get the camera working again and I'm waiting for SBIG to send the strip through.
Once the desiccant pack is reset I'm going to open up the camera and see if I can clean the glass screen in front the of the CCD.
I'm hoping to get the strip this month as I was planning on going to the Nov uDSC, but I'll have to wait and see.
Always make sure you store your camera somewhere at ambient temperature. Don't store it in the warm indoors. Keep it in a shed or garage.
When cooling the camera make sure you do a stepped cool down process, don't just go straight to the set point temperature. E.g. let it cool to -10, wait 5 mins, then down to -20, wait 5 mins, then down to -25, etc. If your camera has a warm up feature and the software will allow you to use it then always warm up the camera prior to turning it off. All the above will minimise the chances of it reappearing. Rapid cooling from a warm camera and rapid heating up again afterwards are a sure fire way of getting condensation inside no matter how well it is sealed. Keeping the camera indoors and then taking it out into the cold and cooling it to well below zero rapidly would be main culprit of this. Same as bringing a cool camera into a warm house afterwards. Bad idea.
Quote from: Ivor
you have to drying out in the oven @ 170 for 4 hours!
On that note, knowing that gas produces lots of water as it burns, I've often wondered how much higher the humidity is in a gas oven (compared with electric) and does it matter when drying out dessicants.
For desiccant bags I always find the best place is to leave it in an airing cupboard for a few days. No moisture in there.
Alas I don't have an airing cupboard, but I do have an electric oven. Mark's comment does make me think doing this with the roast won't be a good idea as it will add moisture to the environment.
I put my dessicant on the radiator to dry out. Also keep my camera in a plastic box with extra bags of dessicant.
Regarding the light box, can you not put pieces of paper in front of the light source to dim it?
Carole
Yes I can Carole, however it needs further experiementation with the smaller pot to decide whether it would be better to place the paper in front of behind the diffuser or even at a distance from the diffuser. I also want to experiment with some sticky back plastic to aid easy removal of dirt from the paper.
It's still in a prototype phase but I can post up some pictures if you are interested. I'm planning to do a tutorial for this once I've ironned out the issues. The current design should be able work with any scope up to 25cm, bigger scopes could be accommodated with a large selection box :).
Look here
http://forum.orpington-astronomy.org.uk/index.php?topic=9376.0 (http://forum.orpington-astronomy.org.uk/index.php?topic=9376.0)
cant you just buy a flat panel & put sheets of paper under it
QuoteLook here
http://forum.orpington-astronomy.org.uk/index.php?topic=9376.0
Looks interesting i guess it depends on the size of the resistor inside on whether it is viable. I'll be interested in how others get on with them as it is certainly a cheap altnerative to a dewheater.
Quotecant you just buy a flat panel & put sheets of paper under it
A flat panel costs at least £60, my solution cost less than a tennor, I got to be manly with a soldering iron and eat a tub celebrations. When you look at the choices there's only one worth picking :D