One for any electronics experts

[robin_astro]

I can see this wavelength in the daylight sky with my spectrographs.  How long exposure does a daylight sky "T shirt" flat need at this wavelength?  (Unless it is ridiculously high and as  long as you can keep stray light out, exposure time does not matter for flats as long as you expose long enough and average enough exposures to beat the noise down (and correct with suitable bias and darks of course which you should do any way)

Robin

[Demonperformer]

11 minutes ago, robin_astro said:

How long exposure does a daylight sky "T shirt" flat need at this wavelength?

No idea. But as I will be imaging at night it is not really a solution as I have to set up/take down every session.

[robin_astro]

 Do you tear down completely every night or leave the OTA, camera and filter wheel connected? If the latter a flat should last a fair while 

Robin

[Demonperformer]

3 hours ago, Stub Mandrel said:

Even if it emits light evenly in all directions, a short distance will give you very uneven illumination. At 60 degrees off axis the illumination at a plane ahead of the source will only get 75% of the illumination of on-axis. The LED strip suggestion is a way to get more even results.

Diffraction isn't an issue as they aren't point sources, the LED chips are significantly bigger than the light wavelength.

If you get clear perspex and  gently attack it with fine wet and dry paper you shoudl be able to make a suitability fine diffuser.

 

My preference would be for multiple light sources at least as long as it is wide, lined with slightly uneven aluminium foil. That will reflect the off-target light randomly and make a much more even field.

I uses this arrangement with four UV tubes  in a box for my PCB/etch resist exposing setup, with a very thin sheet (2mm) of glass (not frosted) and it works very well.

 

Thanks, Neil. This is really helpful.

Looks like a 150mm strip (9 LEDs) cut into three strips is the way to go (vast expense - £3.88). Add a red LED outside to show when it is on. I like the idea of lining the inside of the tube to produce random reflections to produce a more even field.

Will have to work out the power connection & any resistors when I see how the strips are powered. Also need to consider if they should go in series or parallel. Thinking one piece of perspex with the LEDs underneath, single hole for power to come through, everything else on top with another piece of perspex over the top to hold everything in place? Wonder if cutting circles from a suitably sized tupperware box might be the cheapest way of getting the plastic disks?

Thanks.

[Demonperformer]

2 minutes ago, robin_astro said:

Do you tear down completely every night

Yes.

[alan4908]

3 hours ago, Stub Mandrel said:

At 60s you are probably going to want to use dark flats.

It really depends on your camera and the temperature.

For instance with my camera (SX Trius 814) which has a low thermal noise + I capture all my flats at -20C, you have to be careful not to increase the noise by subtracting darks. In my case for flats, the signal is very high, so I choose not to subtract the dark but rather I subtract the bias.  However, for my light frames which can go up to 1800s, the signal to noise can be quite low, so I do use dark frame subtraction generated from 1800s Dark Masters. To reduce noise,  I use adaptive subtraction where the where dark scaling factor is determined by overall noise minimization rather than time.

Alan

[Stub Mandrel]

Here you go, it's crude but effective:

 

[Demonperformer]

The postage prices are ridiculous.

So far I have doubled up the uv led order in order to reduce the postage cost to less than 50% of the total cost and the perspex discs include a postage cost that is 70% of the total cost. Now looking for a red through hole led and (if I buy 5 instead of the 1 I need) the postage cost is 65% of that order ... mind you even that is better than one company that would sell me one led of 35p as long as I paid £12 for it to be fedexed to me (cheapest delivery option they offer).

How I miss Maplins.

[John78]

12 minutes ago, Demonperformer said:

The postage prices are ridiculous.

So far I have doubled up the uv led order in order to reduce the postage cost to less than 50% of the total cost and the perspex discs include a postage cost that is 70% of the total cost. Now looking for a red through hole led and (if I buy 5 instead of the 1 I need) the postage cost is 65% of that order ... mind you even that is better than one company that would sell me one led of 35p as long as I paid £12 for it to be fedexed to me (cheapest delivery option they offer).

How I miss Maplins.

RS is free delivery on everything, can get them for like 12p each if you buy 5.

[Stub Mandrel]

26 minutes ago, Demonperformer said:

The postage prices are ridiculous.

So far I have doubled up the uv led order in order to reduce the postage cost to less than 50% of the total cost and the perspex discs include a postage cost that is 70% of the total cost. Now looking for a red through hole led and (if I buy 5 instead of the 1 I need) the postage cost is 65% of that order ... mind you even that is better than one company that would sell me one led of 35p as long as I paid £12 for it to be fedexed to me (cheapest delivery option they offer).

How I miss Maplins.

Bear in mind that for a big company they probably lose money on any transaction totalling less than £10.

[Demonperformer]

29 minutes ago, John78 said:

RS is free delivery on everything, can get them for like 12p each if you buy 5.

Thanks for this - tracked that down now. A pity RS don't offer the 'sort' option of total cost rather than each.

[John78]

1 hour ago, Demonperformer said:

Thanks for this - tracked that down now. A pity RS don't offer the 'sort' option of total cost rather than each.

For some things there is a MOQ filter, but it doesn't seem to be there for the LEDs - so you just need to order by price and scan down the MOQ column find the cheapest realistic option - bit frustrating in this day and age.

[Demonperformer]

All right, I'm guessing there must be a simple solution to this and it is just my inexperience that is responsible for my complete failure to work it out.

The UV LED strip has arrived. It consists of a 30cm strip, 6x5cm sections. At one end is a power supply connector. The rule is that you can cut the strip at any of the 5cm intersections. BUT, the entire thing is enclosed in a thick plastic casing with (as far as I can see) no way of powering any of the bits that you cut off. If it wasn't for the casing then one would be able (with care) to solder wires to go from one strip to the next, but this casing seems to make that impossible. Below are some pics that hopefully make the setup clear.

Now, my aim was to use three of these strips (each of which is 50mm x 10mm) in a 50mm square with a 10mm gap between each pair of strips. Surely this cannot be designed in such a way that it can only be used as one long strip - if that was the intention, why would they say that you can cut it at the designated points? That would surely make no sense (except possibly to the retailer, telling them they can only sell 'quanta' of 5cm).

With the power supply already connected to the strip I am also going to have to come up with a way of adding the red 'indicator' led (if I still go down that route).

I suppose one positive is that they appear to run on a 12v system and so I won't have to worry about additional resistors to get the power right. There are already a load of resistors on each strip.

I did warn at the beginning of this thread that this whole area was unnatural territory for me. So, any ideas?

Thanks.

[Stub Mandrel]

You solder a 12V supply to one pair of two gold plated pads on each section, or you can supply one section and 'daisy chain' the other sections together, always + to + and - to -.

[Demonperformer]

But I can't see a way of getting at the terminals with the plastic casing in the way. Unless I'm meant to try to slice it away without cutting into the strip? (I could see that ending badly!)

[Gina]

I guess it depends how well your fingers work.  I've done it without too much difficulty.  Forget what I used now but maybe small nail scissors or craft knife.  If you want a really tricky job, try soldering a tiny SMD chip with 50 or even 25 mil leg spacing onto a carrier to use with 0.1" stripboard!!!

Oh - if you don't know what I mean my "mil" - it's one thousandth of an inch.  50 mil = 0.05" etc.

[Dr_Ju_ju]

Buy a scalpel & 10A blades, will make light work of the plastic sheathing over the terminals...

[Demonperformer]

Thanks for the suggestions. I guess it's a good thing I got a few more sections than I need so I have a few spare to mess up on.

[Stub Mandrel]

Get a decent small soldering iron tip. Or get a bit of thick copper wire (like in fixed earth wire) coil it around the soldering iron bit and use the tip of it as a mini-iron.

[pete_l]

3 hours ago, Demonperformer said:

But I can't see a way of getting at the terminals with the plastic casing in the way. Unless I'm meant to try to slice it away without cutting into the strip? (I could see that ending badly!)

Yes, that would seem to be precisely what you are expected to do.
If you don't want to cut away the plastic, just melt it off with your soldering iron.

[Gina]

But stay away from the fumes!

[Demonperformer]

Well, found a local hobby shop that sells 10a scalpels so will have a trip out this morning.

8 hours ago, Stub Mandrel said:

Get a decent small soldering iron tip.

Just had a look at my soldering iron - it has been so long since I used it the tip has actually gone rusty! So I guess homebase or b&q will be on my shopping trip as well. Not actually going to tackle this until the perspex discs arrive, but want to get everything ready.

I've also been giving some more thought to my red indicator led. I'm guessing that I would need to do something at the end of my "daisy chain" to connect the + & - together to complete the circuit anyway, so should I just be able to connect it there to complete the circuit? This is the one I am planning on buying.

I'm also starting to think about a "tube" to hold it all together. I was thinking about a postal tube, but none of them are quite the right size. So I'm now thinking the simplest solution is to design something that is exactly what I want (to hold the disks and to sit firmly over the end of the scope) and get it 3D printed.

Thanks.

[NEM]

I don't know the specifics of this strip, but I doubt you need to connect + and - together at the end of the strip.  I suspect that would damage your power supply or burn out the tracks on the strip.  I'd try it without connecting them first, that shouldn't harm anything.

You could add your red LED there, but remember you'll need a current limiting resistor in series with it.

Noel

[Gina]

No, you don't want to connect the + and - together at the end of the strip.  As for the red LED, just connect it to the 12v supply with a 1K resistor in series taking note of the polarity (shorter lead is the cathode, -ve).  If that's too bright use a 2.2 K resistor or even higher.

[Stub Mandrel]

3 hours ago, Demonperformer said:

I've also been giving some more thought to my red indicator led. I'm guessing that I would need to do something at the end of my "daisy chain" to connect the + & - together to complete the circuit anyway, so should I just be able to connect it there to complete the circuit? This is the one I am planning on buying.

No, don't connect them, they are connected together via the LEDS, much sorrow will follow if you short them together.

You can, however use them  (or any othrr point along the chain to 'feed' your red LED. I'd bump the resistor up more than 2K2 with modern LEDs as they are so efficient.