Question about CCDs

[MishMich]

I'm not sure how to phrase this question best. Imaging with an unmodded DSLR I will have to take longer exposures than a modded DSLR with some/many objects? With a dedicated CCD, is there a further advantage over a modded DSLR in terms of lengths of exposure?

Not that I am thinking of getting one, just wondering. Only I look at some of the exposure times, and it seems like I am having to expose longer than I see for others on some objects, and running into noise problems I suspect are beyond the sensor itself. I do use an LPR filter, although TBH, I find that tends to require even longer exposures, and I am wondering if that lends itself to an increase in noise that entails a loss of any benefit from using the filter.

M.

[MishMich]

I suppose what I am wondering is whether it is worth sending the camera away for modification (and have it unavailable for a while), or hang on to it as is and hope that one day I will be able to find the money for a CCD camera. I did buy it cheap with the intention of having it modified. I have seen the site that walks you through the mod. Has anybody here done it themselves? How was if for you? I am not a particularly technical person - do you need to be pretty skilfull to do this successfully? I think I'd want to go down the route that replaces the IR-cut with a clear filter, and get the CLS clip, so I can still use it with lenses if I want to.

M.

[Euan]

The two main advantages in a dedicated CCD are in sensitivity and noise. The sensitivity is from the QE (Quantum Efficiency) of the chip itself, and the noise reduction is mainly due to the fact they have active cooling of the CCD.

Lower noise + higher sensitivity = higher signal to noise ratio, meaning more usable data and shorter exposures

I'm no expert so no doubt some more knowledgeable people will either confirm or deny this....

[SteveP]

Hi MM

I'm not aware that DSLR's are intrinsically any less sensitive, thus requiring higher exposure times, than CCD's. But of course what you can do with a DSLR which you cannot do with a CCD is change the 'ISO'. So what ISO would you typically use? Increasing the ISO will reduce exposure times but the trade-off for that is more noise

As a benchmark, there's a very good image on here of M42 taken with a Canon 350. I'm sorry but I can't recall who produced it but from memory it was a very good image and used something like 2 hours of subs. If you've taken any shots of M42, were your exposures significantly more than that?

Steve

[Euan]

I suppose what I am wondering is whether it is worth sending the camera away for modification (and have it unavailable for a while), or hang on to it as is and hope that one day I will be able to find the money for a CCD camera. I did buy it cheap with the intention of having it modified. I have seen the site that walks you through the mod. Has anybody here done it themselves? How was if for you? I am not a particularly technical person - do you need to be pretty skilfull to do this successfully? I think I'd want to go down the route that replaces the IR-cut with a clear filter, and get the CLS clip, so I can still use it with lenses if I want to.

M.

If assuming you have the 1000D from your sig, the mod is relatively straightforward. Although I say that based on someone having a basic technical knowledge

I'd say if you can strip down a PC confidently, then this should be doable if you watch what you are doing

[SteveP]

MM

Apologies, I gave you some duff information earlier. There is a significant difference in the sensitivity (as measured by QE) between DSLRs and CCDs

Have a look here

EOS40D / EOS50D comparison

The article is in French but the graphs are universal. It shows how much additional QE is created by the removal of the IR cut filter - not much in red & blue wavelengths but significant in red wavelengths

Steve

[adamsp123]

I suppose what I am wondering is whether it is worth sending the camera away for modification (and have it unavailable for a while), or hang on to it as is and hope that one day I will be able to find the money for a CCD camera. I did buy it cheap with the intention of having it modified. I have seen the site that walks you through the mod. Has anybody here done it themselves? How was if for you? I am not a particularly technical person - do you need to be pretty skilfull to do this successfully? I think I'd want to go down the route that replaces the IR-cut with a clear filter, and get the CLS clip, so I can still use it with lenses if I want to.

M.

Mishmich, the procedure for modifying it is actually quite straight forward, here is the method

http://ghonis2.ho8.com/rebelmod450d1.html

I printed it off and it took me less than a grand prix to do the job, it looks very difficult but if you follow the method systematically it is not difficult.

[MishMich]

Thanks,

I use ISO 800, but am trying to get this down to 400. I have taken over an hour's worth of M42, and captured quite a bit, but sky noise starts to encroach. Ditto with the Flame (& Horsehead). With an hour of 2 minute frames I only just get a hint of the Horsehead, and a much fainter Flame than I am trying for. However, at that kind of length, before the moon is over the rooftops, I get an awful lot of noise, and gradients I cannot eliminate.

Maybe I should move on to galaxies & clusters for a while? M31 is not really available to me at the moment. What other galaxies would be worth trying to get some practice with?

Thanks,

M.

[jgs001]

What about M101. I tried an image with ISO200 the other week. Now bearing in mind I do not have a modded camera, and was using 5 minute subs, I picked up a whole heap load of red Ha neb in and around Orion, the processing was silly easy (it took under 10 minutes), and what noise there was, was really easily dealt with, I'm sold on the lower ISO settings. My next time out, I'm going to do M101 at ISO200 with the C80ED, probably using 10 minute subs, to offset the change in focal ratio.

[jnp]

Yes, M101 is good and M51 is also a very nice. Saw them for first time last week.

I have similar poor results with Horse head (also M1). I think the mod is needed to get the contrast. I am therefore also contemplating the mod to my Canon 1000d (well that is after 'error 99' gets repaired =No camera at mo ). The main objective of mod is increased sensitity to Ha, if anything this means more signal less noise?

Actually I have noticed that I get more colour (especially red) saving to JPEG rather than RAW format. Is that normal?

cheers

John

[jgs001]

John, what happens is, when you save to jpg, the camera applies a batch of settings, which increase, contrast, sharpness, saturation etc. At the same time as downsizing to 8bit and compressing the data. a RAW file is capturing what the sensor picks up. There is (in theory) no processing in camera carried out. This means, that regardless of what you are shooting, some processing will be required for raw files. It's far better to shoot in raw than jpg.

[SteveP]

MM

You have some good suggestions for galaxies. An alternative might be to try some clusters, preferably globular but also some open. They're a bit brighter than either nebulae or galaxies and will therefore take less exposure time. Alternatively, they might be a way to try out a lower ISO setting

Steve

[MishMich]

Thanks, M51 or M101 look good, although I suspect I'll need to try between 2 a.m. & 5 a.m., because the direction they lie in is for me the most light polluted, and they are highest between those times (anything East-North-West is problematic for me).

This has given me something to think about, and John's comment about his ISO 200 has reminded me of his Orion & Barnard's loop image. One thing I did want to try was using the Canon with the 50mm lens and capture constellations. So, I could start off with Orion, using my Onyx to guide, and later in the evening the Plough. That would be a good way to get the guiding sorted out. At the moment, the combinations I can manage on my setup (Astrodevelopments M3 main and support bars) are DSLR + 5", DSLR + ED80, 5" + DSLR. I'd need to change the scope rings on the SPX to use that with the ED80. I found the HEQ5 was struggling a bit with the SPX, although I was up to 4 this morning stripping it down, cleaning the gears & bearings, regreasing and reassembling. It sounds much sweeter now, but not tried loading it up yet.

M.