CCD (QHY8L) First Experiences, Set Up and Review

[JSeaman]

 

Below is a summary of my experience with a QHY8L over the past few days. This is my first CCD and is a definite improvement over a DSLR but it is not without its challenges! This isn’t meant to be an expert opinion, just hoping to save some time for others. Please feel free to add feedback and comments below.

 

CCDs (Mono vs OSC and the QHY8L)

CCDs, are they all they are cracked up to be? That was the question I kept asking myself before making the decision to switch from an SLR (Canon 1000D for a couple of years then a 70D for a couple of years and more recently a 5DMk3) to a QHY8L. You’ve fundamentally got two choices of CCD camera which is mono or colour. With a mono camera you need to use filters to capture luminance, red, green and blue whereas on a ’one shot colour’ (OSC) camera you take a picture like you would with your SLR. So why would anyone want a mono camera if they are black and white, more expensive and require lots of photos of the same object with different filters? Low noise and high sensitivity is why, the images are fantastic but you have to invest more time and money in these.

 

For me, a OSC was the next logical step from my SLR so I opted for a QHY8L, these are an OSC with low noise and a sensible price tag (~£1k new or £500 second hand). They have had a few issues in the past but the 8L and Pro are generally well liked if looked after properly. So what changes when moving from the SLR to the CCD?

 

• The QHY8L is quite a large device, it fits straight into a 2 inch scope and is about 6 inches long. This is a factor if it hangs out the back of your scope and points up at a steep angle as it can start to clash with things so be aware of its size!
• You will need to refocus considerably; on my ED80 I couldn’t wind the focus back anywhere near far enough to get my CCD in focus. You can either use extension tubes (~50mm) or a diagonal to address this. This will effectively bring the focal point much further forward and allow you to achieve focus
• Cooling, this is where the QHY and other CCDs come into their own. If you take a short exposure at an ambient temperature of 20 degrees there will be lots of white dots which is ‘noise’, when you cool it down (-25 degrees) these tend to disappear. Cooling is great. There is a Peltier cooler integrated which is capable of cooling to <30 degrees below ambient. Best results are around -20 and below. If you are going to use the camera all year round then it is worth using a set point you can achieve in both summer and winter e.g. about -15 to -25 degrees in the UK.
• There are various software options available to you: APT supports CCDs, is free and has good plans (multiple exposure durations, vertical plans etc). EZCap ships with the QHY products and seems fairly robust, if a little crude and poorly documented. Nebulosity has a good reputation and performed the core functions very well (cooling and taking a series of pictures).

 

Connecting things up

With the QHY8L, it is quite fussy about how it gets plugged in. There are very clear steps in the manual which say you should …

i.                     Connect the USB to the camera

ii.                   Connect the USB to the PC

iii.                 Connect the 9 pin to the camera

iv.                 Connect the 9 pin to the DC201 (ferrite end here)

v.                   Connect 12V to the DC201

 

In reality, a piece of wire that isn’t connected to anything isn’t going to do an awful lot so my connection process is:

Leave the USB cable connected to the camera along with the 9 pin cable which connected to the DC201 at all times (with the ferrite bead, the bulge in the cable, closest to the DC201 end not the camera)

Connect the driven end of the USB to the PC when I’m ready to image then connect the 12V to DC201 to switch on the fan

 

Disconnection is just a case of switching off the 12V to the DC201 and removing USB cable. This works well if, like me, you’re using a timer to control power to your mount.

For the first power up, with the QHY CCD drivers installed (and EZCap and the ASCOM driver), connect the USB cable to the PC. Connect the 12V DC supply to DC201 before  you try and talk to the camera. At this point you can use EZCap to connect, select ‘Camera’ from the menu and you should see QHY8L enabled. Click on this and then check the camera settings to set the temperature to where you want it to be.

 

Cooling Down

Once you have the camera physically connected, use your software to connect to the device. In EZCap, go to the ‘Camera’ menu and you should see the QHY8L enabled, click on this. For nebulosity, select the QHY8L from the CCD drop down list. Now set the temperature to your chosen value e.g. -20. The TEC light should be illuminated on the DC201 supply to the camera when cooling is on. It takes about 10 minutes to stabilise at temperature and it is normal that it overshoots then undershoots. For example, you might set a target of -20 and it will bounce to -23 then up to -17 and so on until it settles at -20 +/- about .2 of a degree.

 If you find cooling isn’t working and the TEC light isn’t coming on, check the 9 pin cable from the power supply (DC201) on the QHY8L as it is notorious for coming out. Cable ties are your friend!

 

Gain and Offset

Gain

Gain is used to set the low end of the camera’s sensitivity, making sure black is black. To set up your gain you should cover the camera (like when you take darks). The exposure time can be set to 0 and a picture should be captured. You are interested in the noise associated with the image which is available through the “Noise Analyze” menu in EZCap, we are looking to get something in the region of 500-1000 when taking a 0 length exposure.

 If you find a number much higher than 1,000 you will need to reduce the gain. QHY8Ls are often running at a gain of 0 or 1 to get to this sort of region.

 Offset

The offset is a bit different, start with a value of 130 and open the lens cap with the camera facing a uniform light source (just like when taking flats). You will need to increase the exposure time until you saturate the image and then check you are getting a value of around 60,000. Essentially you don’t want to reach 65,535 which is the maximum value the sensor can record, because it is ‘clipping’ (losing) data. If you are clipping at 65k, turn the offset down.

A good starting point on a QHY8L is a gain of zero and an offset of 120-130. I ended up with GAIN=0, OFFSET=111.

 

Once you have set the offset, go back and check the gain until both figures are in spec.

 

Capturing Images

With a CCD camera you still use lights, darks, flats and bias files but you can re-use these a lot more when the temperature is controlled. After you first set up the CCD on the ‘scope and achieve focus,  you can take your bias, darks and flats which you can use for all images taken at the same temperature and orientation.

Bias

Your bias frames should be taken as a 0 length exposure, a quantity between 10 and 50 are typical. Check each bias frame with a stretch before committing them to a reference library, you may find your first image is poor with a wavy line at the top when using the QHY8L.

Dark

Your dark frames should be of the same length as the lights, I’d recommend you start with 5 or 10 minutes on a QHY8L. A lot of people comment on the fact that the QHY8L is very quiet when it comes to noise so you can actually skip this phase and get away with it, I found significant noise was addressed through darks when you start looking at 10+ minute exposures. For the sake of a few hours to build a dark library, I would definitely recommend it.

Flats

There are differing views on flats, some say flats should have an ADU of ~30% of the dynamic range (i.e. 20-30k), others look for a similar ADU as the lights (which may be achieved with 1-2 seconds of exposure) and others will aim for about 20k as an arbitrary value which is likely to be ~0.5 to ~3 seconds of exposure. Flats will remove the vignetting you get at the corners of your image and improve detail. Note if you find a reverse effect (centre is dark and corners are bright) then your flats are too light.

 

Capturing

There are a few software options available to you for capturing images, I have looked at 3:

1.       EZCap which is free and comes with the QHY8L. This is a great place to start when you first plug the camera in to get an image and have a play with the set point cooling. You can either manually configure this by changing the power output or (recommended) set a temperature and allow auto control. The ‘Planner’ feature to set up a sequence of exposures is poor compared to other options.

2.       APT is the tool I used for my DSLR imaging and found it very useful but, whilst it can control a CCD camera, it wasn’t quite as slick as some of the other methods. I found the cooling control was erratic and the stretching and viewing capabilities weren’t as good as …

3.       Nebulosity is a very simple tool which allows you to capture a series of images with a pause in between. This is more basic than APT’s profiles but adequate nonetheless. I do have one issue with Nebulosity, when you press ‘Abort’ mid-way through a series of captures, it will not recover and control temperature properly until the camera is power cycled.

 

Stacking in DSS and FITS

When stacking in DSS you need to ensure you can use kappa sigma clipping to remove hot pixels and also debayer it with an GBRG CCD selected. This converts the apparently black and white image you see through tools such as Nebulosity to colour.

The file format generated by the QHY is FITS which you will appreciate when stacking because it will process much more quickly than huge CR2 files. Unfortunately it isn’t as easy to view/manipulate the files so you will need to get a tool such as FITS liberator (which is free) to view the files.

 

Other Points

 

-          If you find any kind of condensation is appearing, use a desiccant (silica gel) tube to try it out. Leave the filter on the QHY8 at all times.

-          Cool the camera down in stages, the software tends to look after this for you but avoid ‘thermal shock’ where you make big temperature changes quickly

-          Heat the camera up in stages! You have to warm it steadily once it has been cooled for several hours taking your pictures

-          You may see the ‘amp glow’ phenomenon where a corner of your pictures is lit up due to the QHY circuitry being active during image capture. This is largely removed during calibration with bias, flats and darks.

 

Worth It?

 The ultimate question, is it worth investing £500+ in a new camera which you can’t use for ‘normal’ photography? Yes. If you love astrophotography this is a great middle ground between DSLR and even more expensive mono cameras. I would definitely recommend this as an interim step and expect to be happy with the QHY8L for a few more years at least before I think about a mono.

 

Don’t expect miracles. It is better than an SLR but it isn’t going to suddenly make all your images incredible, it’s an evolution that is all. You will find you see better colours, more definition in gas clouds and a cleaner image but the rest of the obstacles that come with astro imaging are still there. You have to achieve good focus, light pollution is still light pollution and several hours’ worth of pics are still required to get a nice image. That said, it’s a good step forward from an SLR and I’m only on my first ~20 hours of use so there may be more tips and tricks to find yet.

 

Edited February 15, 2018 by JSeaman
typo

[sloz1664]

Firstly, congratulations on obtaining your first CCD. Secondly, thank you for your in-depth findings of your QHY8L. Regarding gain & offset, I would set your gain to around 8 - 10 and your offset seems ok. Have a look at the attached gain & offset proceedure:-

gain and offset.pdf

Steve

 

[JSeaman]

I'll have a read of that but note with a gain of 1 my values went up to over 2k 

My first picture is below, lots of work to do yet!

 

[Fangio]

JSeaman, so grateful for your lucid advice on using an QHY8L which I've owned new since May 2018 and have failed to nail a single image.  I've been told that my failure is an all-too- common experience. With your excellent guidance am going to try yet again. Many thanks.