First light with G2-8300 and ext FW - disaster!

[DougAtDallington]

Since re-starting my astrophotography hobby last year I've been using a mod'd Canon 450D and a Celextron Nightscape one-shot colour camera with reasonable results.

But I wanted to get better details in images of nebulae and galaxies so I got myself a proper Moravian Instruments G2-8300 CCD camera with external filter wheel.

A couple of nights ago I hooked up my new camera and tried some LRGB shots to test it out. But I must be doing something terribly wrong because they call had, to varying degrees, a pattern of halos centered on a bright star in the field (see attached). The darks, flats and bias subs are free of the halos, and the red subs show it only faintly.

Can anyone tell me what I'm doing wrong? Could it be condensation on the filters?

Thanks in anticipation.

Doug.

[ChrisLX200]

Did you switch the CCD chamber window heater on?  I'm assuming the camera has one (my Moravian G4-16000 does) but the setting for enabling it is a bit obscure. It can be found in the ASCOM driver properties sheet, and the on the SIPPS camera tab. I failed to do this and found a had a big foggy donut in the middle!.

ChrisH

[Herra Kuulapaa]

I remember seeing similar some time ago. That time it was reflections.

Here's one case I found from web:

http://www.progressiveastroimaging.com/viewtopic.php?f=60&t=8338&sid=9d74c0d9e42a44f95b96089b7bb4ecd2

[ulrichbenzing]

My guess (!) would be moisture producing the primary halo and reflections causing the other halos... i had similar halos due to dew on my coma corrector

[DougAtDallington]

Thanks everyone for your ideas.

After checking the link from Herra I think I've nailed it.

The guy at Progressive Astro Imaging Group noted that his image had been '...stretched to ridiculous levels to show the effect...'. The handbook for the capture software (SIPS) I was using with my G208300 camera says that the software automatically stretches the images taken for focussing (which was the mode I was using), so I guess it stretched the images enough to make these reflections visible.

The PAIG guys feel the reflections happen between the CCD chip surface and the back face of the filter, differing wavelengths of light being reflected at differing angles, hence the changes in spacing of the reflections. The images I posted were one form each of the L-R-G-B-Ha filters and so this seems to confirm the hypothesis.

Anyway, I'm now taking my camera off the scope when not in use and keeping it in a sealed plastic bag with a desiccant pouch - just to be sure its not a moisture issue.

Ah well, just need to sort-out my PA problems now - and get some clear skies!

Thanks again for your thoughts.

[DougAtDallington]

And there's more...!

On 365Astronomy's 365Telecopes blog there is an entry on Newton's rings caused by reflections in the otpical train,

http://www.365telescopes.com/how-to-deal-with-newtons-rings-in-cmos-imaging/

and containing a link to David Cortner's page

http://www.davidcortner.com/slowblog/20130513s.php

This explains very well what is probably happening in my 'scope. Problem is, how do I arrange a 'tilt' of my filters with respect to the camera? David Cortner has added soem adjustment bolts, but that seems a little beyond my engineering abilities.

Thinking head on....

[ChrisLX200]

And there's more...!

On 365Astronomy's 365Telecopes blog there is an entry on Newton's rings caused by reflections in the otpical train,

http://www.365telescopes.com/how-to-deal-with-newtons-rings-in-cmos-imaging/

and containing a link to David Cortner's page

http://www.davidcortner.com/slowblog/20130513s.php

This explains very well what is probably happening in my 'scope. Problem is, how do I arrange a 'tilt' of my filters with respect to the camera? David Cortner has added soem adjustment bolts, but that seems a little beyond my engineering abilities.

Thinking head on....

I don't think these are related to the phenomenon you observed, and you definitely do not want to be purposefully tilting your CCD.

ChrisH

[DougAtDallington]

Hi ChrisH.

No, you're probably right. I haven't got the skills to do it anyway.

Re: CCD chamber window heater; I don't think my G208300 has that facility - at least, there's nothing in the camera handbook about it.

When I've next got it on the 'scope I'll see if there's anything in the SIPS software (Camera tab wasn't it?). Although wouldn't this heating be fighting with the chip cooling?

Regards.

[ChrisLX200]

Hi ChrisH.

No, you're probably right. I haven't got the skills to do it anyway.

Re: CCD chamber window heater; I don't think my G208300 has that facility - at least, there's nothing in the camera handbook about it.

When I've next got it on the 'scope I'll see if there's anything in the SIPS software (Camera tab wasn't it?). Although wouldn't this heating be fighting with the chip cooling?

Regards.

To a small extent it does, but the idea of the window heater is to counteract effects of the cold sensor just mm away from the chamber window. I looked on Moravian's website and they show a graphic of SIPPS working with your camera, and the window heater tick-box appears to be enabled:

ChrisH

[DougAtDallington]

Aha! Thanks ChrisH - found it in the manual too.

I'll try it our the next time the sky is clear enough for some imaging.

[DanL]

Just out of interest, would you have to use SIPPS to enable the window heating?So would need to have that program running in the background alongside any other sequencing app you might use? (Eg SGP)

[swag72]

I have suffered from the big ring reflections that you posted on only one occasion and that was with Alnatik. I would say that once you get imaging for real, there's every chance that you'll be no where near such a bright star and so you won't get any reflection issues to speak of. 

Just be aware that big bright stars can do this when you are looking at framing your targets.

[ChrisLX200]

Just out of interest, would you have to use SIPPS to enable the window heating?So would need to have that program running in the background alongside any other sequencing app you might use? (Eg SGP)

No you don't (with my Moravian camera anyway), if you open the ASCOM driver properties sheet there should be a tick box available, mine has two power settings.

It is sort of either On or Off - no graduated power settings other than my 2-stage heating. I've left mine on by default now, it does reduce the amount of sensor cooling available slightly but not hugely, I can still use -20degC with just a few more % power required to get down there.

ChrisH

[davew]

Hi Doug,

As a Moravian G2-8300 user of over four years, I have to say I've never had condensation problems with my camera. As far as I'm aware, the series of camera I use doesn't have a chamber heater. All I need to do is stick the silica gel in the oven every now and again as per the instructions.

It's an internal filter wheel type and you have an external. That may be the difference and I notice you asked about the filters and not the camera on your original post. It could be that there's moisture on them. As I set up each time I use the camera I also ensure I don't expose the filters to direct damp air by connecting outside. I always connect to a scope or lens inside the house.

I've never had reflections from filters or the chamber cover glass and would not recommend you try and tilt anything.

As a last line of defence I'd contact Pavel Cagas at Moravian as he's responsive to inquiries. 

Dave.

[DougAtDallington]

Thanks all for your comments - very useful.

Dave (and any other G2-8300 owners); I'm starting a new thread about adding/changing filters to an ext G2-8300 Filter Wheel, I'd be grateful if you could offer any gems of wisdom.

Regards,