Using RGB filters with a DSLR?

[blackparticle]

Cheers Jamie.. I didn't really want to get into a windows vs osx discussion anyway.

Back to the filters.

I managed a few shots of Andromeda just as it was getting light with the filters and got a reasonable amount of data back from each one although certainly not as much as compared to an unfiltered sub.

It was a good test for me as I now realise that by adding a filter you also have to re-focus the scope slightly as well. By the looks of it, each filter changes the focus differently compared to another one. It's a small variance between RGB but it's noticeable.

That knowledge alone has probably just saved me hundreds of pounds in the long run because if I have to manually re-focus between filters, there is obviously no requirement for an automated filter wheel and a manual one will happily suffice.

I've also come to the conclusion after trying a series of long (albeit smeared) exposures on M27 that the lack of colour depth in the red range is actually exposure dependent. Working with 30-40 second subs is nowhere near enough time for the camera to properly register a decent amount of red data to get it above the noise levels compared to the green and blue parts of the spectrum.

So really, my problem isn't about filters, I think it's the fact that I'm not guiding my scope and able to get decent (5 mins+) exposure lengths.

Thanks to everyone for the help anyway. I know now what to do to improve the colour in my images. I think guiding will also increase the lifespan of the DSLR no end.. It appears I've already racked up over 12000 subs since last month.. At that rate the Pentax will probably suffer shutter failure before Christmas.

Now to research the merits of a Synguider vs Imaging Source cam + PHD.

Alan.

[RobH]

Hello Alan.

Some filters are designed to be parfocal, meaning that refocus isn't supposed to be required between filters. Astronomik and Astrodon are 2 makes that are like this.

However, if you use a refractor, and have anything less than a top end apo, you'll probably have to refocus for blue, and checking focus regularly in the early part of the session is needed anyway due to changes while the ambient temperature drops.

Cheers

Rob

[Psychobilly]

Synguider...Only really comes to "life" on a fast guide scope the ST80 is ideal 400mm f5... it wont work on a finder guider... I was an early adopter with the Synguider and if truth be told I dont think I would buy it knowign what I know now.. I wanted it as a computerless guiding solution for an EQ3-Pro ideally on a finder guider... but the Syngiuider is useless with a finder and the EQ3-Pro has "ST-4 guideport" issues

IS cam + PHD works fine plus you have a decent lunar and planetary cam as well...

Which IS cam do you have or thinking of buying?

With regards to Red response even with a modified Canon I try and get 5-10 min subs at f4.8 and for Ha I need 15-30 min subs and in both cases lots of them...

Peter...

[blackparticle]

Rob -

The Orion filters I have came as part of an astronomy 'job-lot' and I presume they are down the budget end of things. I don't mind re-focussing to be honest and will check the camera every 20 mins or so anyway to make sure it's not slipped out. I've found a bargain Atik manual wheel with 5 slots and t-threaded both ends so have gone for that as it will come in useful down the line.

Peter -

I've read your synguider report and your EQ3 woes. I've got a HEQ-5 and have noticed a few others using that combination (with the occasional niggle). For guiding I was thinking of using the 130P as the guide-scope for the APO66 or the APO66 for the 130P / MAK150 depending on what I want to image. In combination with a x2 or x3 tele-convertor for the camera rather than a barlow, as these appear to give me better results, that should give me imaging focal lengths from 400mm all the way up to 5400mm.

I've looked at the DMK 21/31/41 firewire options and believe they work very well with PHD if I get a GC-USB-ST4 box which has native OSX support with PHD. As you say, this would also give me a better cam for high-magnification lunar/planetary rather than use the SPC900.

I did manage to capture a hint of red with the DSLR in M27 when I stacked the better of the shorter subs so I do think there is a lot of mileage left with using the DSLR for primary imaging.

This is 21 x 27 seconds @ ISO 1600

Alan.

[Psychobilly]

You want a reasonably short FL for the guider if you go too long you can end up chasing seeing... longer guiding exposures of a couple of seconds help...

In the old days of manual guiding you would want a longer FL guidescope with modern autoguiding solutions the star centroid is calculated to sub-pixel levels so a short FL wide field guidescope is a realistic possibility...

Imagign at 5400mm is going to be "challenging" .. I have tried using the CPC800 at f30 ... soemtime I guide on planets when imaging them at long FL... keeps them in the FOV nicely...

Good luck with your plans

Peter...

[blackparticle]

heh.. I did manage to get some shots of the moon with a x2 + x3 tele + x4 imagemate on the Mak150.. Focal length = 43,200mm. Not very good images mind for a single frame shot but possibly stackable.. Something to try when not standing out in the middle of a field full of thistles I think.