DSI II and 'Green' Stars

[Interested]

Thought I would try guiding for the 1st time Friday evening - that was a success with M51 staying 'framed' for 3 hours.

However, using the DSI II wasn't so successful!

I have used the CCD previously for short exposures (15s max) and noted a few 'green' stars in the image - this was with the CCD readout temp at 8 Deg C.

Friday night the CCD readout temp was 17.5 Deg C and the image was very grainy and with plenty of green stars. There was also a glow in the top left hand corner of the image.

I am assuming the problem is noise, due to the CCD running 'hot', and hence the reason people buy 'cooled CCD cameras?

I know the scope is ok as there is no false colour when I set up the camera in daylight and, if it was the scope optics I would expect false colour to be consistent.

M 31 seemed like bright ball of light in the imager - I assume that, as I increase exposure time, I need to decrease gain/saturation accordingly?

Wondering if I should now look at buying a QHY8L or similar and relegate the DSI to planetery imaging!

Any advice on the above would be greatly appreciated

[Merlin66]

A series of darks (same length exposure as the ones on the stars but with the telescope covered) should remove most of these artifacts.

[Si W]

Did you take any dark frames ?, it's a must with uncooled cameras like the DSI, also to get rid of any noise you will need to stack at least 10 subs to get rid of the noise.

[Interested]

Yes, I took darks at the same exposure and used the 'dark substitution' option in the software. I ended up with some 'all white' images and some 'all black' images.

Am I right that increasing exposure time means decreasing gain? (seems to make sense to me....... ).

[Si W]

Every time you increase the exposure time you need to take Dark frames for the exposure time and that temperature, i am assumming you have do that. decreasing the gain is also dependant on how faint the object your imaging it's finding the right balance, then fine tune it with the historgram.

[Interested]

Yes,

I did that. As a default, if you are taking exposures of 15 secs or under using the DSI and take darks first it takes a set from 1s-15s. I tried a 4 min exposure and took darks at 4 min just before taking images. I think the 'white' images where because I had the gain set to high and the 'dark' images because I set the gain to low.

I binned the images I took on Fri evening as there was not a lot that could be done with them - they were aweful ! The 'grainniness' was so bad (like white noise on a TV set) that M51 was barely visible!

I think what might help me a lot is answers to the couple of sentences that I have put question marks against in my original text.

[Si W]

A problem i have found and this was the other night, because of the lack of dark due to this time of the year, i did see at lot of noise on my images, and that was in Mono mode, as i have found this time of the year (except between midnight and 1 am), if i use colour it picks up alot of blue light that is scattered in the atmosphere. Have you tried using just Mono, you will still see noise though, but you can tweek that out. I'd give it till the end of July beginning of August and give it another go.

[Interested]

What you are saying about 'blue' light makes a lot of sense Si. I noticed blue light in the images as well, especially from 2am onwards. Coupled with the fact I live in a fairly light polluted doesn't help matters.

The 'grainy' image I was getting was also evident in 'live view' (in fact, what I saw in live view was what I got in my image). I also noticed that when taking the darks there was an awful lot of coloured pixels on the screen (far more than usual). I thought this may be down to the CCD not liking the temp it was operating at (+17.7C).

I think the way to go will be a cooled CCD and only using at my girlfriend's place (a very dark sky environment), and, as you say, wait until the darker nights start rolling in.

[Si W]

Your certainly right about getting a cooled CCD, i have the ATIK16ic (great little camera), the chip is smaller than the DSI 2, but noise reduction is very low in comparison, and the plus is it's mono, so more sensitive than the Colour. I'm currently going though a revamp of my kit (but thats another story), so soon the DSI is be made redudant to a guide camera and the ATIK will be the main one, and using PHD helps. What i would say about light polution is get a filter if you dont already have one, i live 4 miles from Manchester city centre so it's a must.

[Interested]

I was thinking about getting the Opticstar DS-615C XL or the QHY8L (both use the same chip).

I use the QHY5 + PHD for guiding. The DSI will probably be used for planetary once I have upgraded.

I have a UHC filter and the DSI has an IR filter fitted but they seem to make little difference!

Anyway, thanks for all your input Si. Confirms I am not the only one getting 'blue' images at the moment. Seems to me the DSI only seems to cope with dark skies!