Modded camera and sub times - Filter or lower ISO?

[swag72]

Last night I decided to try to sort out the focal tubes issues I seem to be having, that is giving me elongated stars on the edges of the image. As per suggestions, I took a sub, looked at it and examined the corners, adjusted the screws and took another pic. I did use CCD Inspector as well, but we really didn't know what we were looking for, so I did it more by eye instead.

Anyway, I digress!! The subs for checking the above were nice and short. So once I was happy, I decided to push through a couple of 5 min subs looking at M51 just to double check my happiness! Well, I really can't totally comfirm this as I was really surprised to find my 5 min subs at 800 ISO totally over exposed.

I am often seeing modded camera pics taken at 300sec or more, so are the good folks of modded camera land always using filters? I had just about resigned myself that I didn't need my CLS filter and that I could manage 300 sec quite happily without it. It looks like that is not the case afterall. I guess that I could just lower the ISO right down - Would that be a good idea?

I hope that I have sorted out my elongated stars woe and I just have to get my head around the difference between a modded and unmoddes camera now.

Thanks for reading this rather rambling post and I hope that there are a few out there that can help me. Cheers as always.

[Dave Moulton]

Sara,

This sounds like local Light Pollution ispart of the problem. I've imaged with a modded DSLR from home which suffers from average LP. My limit before the subs were washed out due to LP were in the order of 2.5minutes. After adding my 2" CLS filter I could image at ISO 800 for a maximum of 10 minutes.

I would start by putting your CLS filter on and try imaging at 5 mins at ISO 800 or 10 mins at ISO 400.You should see a vast improvement. Take some dark frames and stack them with the lights, Deep Sky Stacker is free and works well. Don't expect to see too much in a single sub as on their own they all look a bit washed out.

Dave Moulton

[AndyUK]

Hi Swag - I have a modded 40D which I also generally use at ISO 800 but as I suffer from bad light pollution I do always keep the CLS filter in the light train...

Sorry if this is a silly question, but where was the moon? I know it's only about 40% at the moment, but I was amazed how much effect it has - On a "no moon" night (using ISO400) I managed to take subs of 600s, but on another night with a full moon, I was lucky to get 180s and even then the histogram was WAY to the right.

Although not a direct comparison (different subject and different ISO), hopefully these 2 straight unprocessed subs will show the difference? (Oh - The top frame is 180s @ ISO800 with full moon and the lower frame is 600s @ ISO400 no moon)

[swag72]

Thanks for that guys - I really was hoping to try to avoid the clip in filter, unless you have a solution for the following.

Issue 1) - I decide what I want to image, use goto, then take a 30 sec sub to ascertain framing etc and keep moving the mount from there untill I get framing that I want. With the CLS filter, this will be nigh on impossible, due to the extended subs time for me to be able to see the subject. As it is now, I can often get a star as a reference point to what I am looking at and get it all fairly central. What a pain if I have to do all of that, take the camera off the back of the scope, add the filter, put it back on, hope that the focus hasn't changed since before the filter was put in ................ And so it goes on!! How do you frame with a clip filter in place?

Regarding the moon - There wasn't one, which was nice!!!

[Psychobilly]

if you wallop the ISO to max you should be able to strt capturinng a hint of the target in 30s subs... just remebr to drop it again before shooting the nights data ...

I also frame using star patterns... plan you session before you go out and either take a printout or a sketch of the brighter stars...

I usually go out witha shot list of RA and dec cords of the center of the FOV I want and "shock horror" camera rotation for some target ... although most of the time the camera is either North Up or rotated 90 degrees....

In the obs the scope is just controlled from the handbox and a pc doing the guiding... this stopped me flitting from target to target..

All this is from memory though as it was 4 months since the obs scopes last saw starlight

Billy....

[AndyUK]

Hi Sara - I follow pretty much Pete's process...

After getting the scope polar aligned and then 3 star aligned, I slew to a few objects around the area where I want to image (using PAE) and once it seems fairly accurate, I then move to a nearby bright star, attach the DSLR (inc CLS clip filter), focus it (using a bahtinov mask - wonderful invention !) and lock the focuser.

I then slew back to the target and set the ISO to 1600 and take a 60s single shot which I compare to a Stellarium view. I then "fiddle" with the camera rotation and framing, taking the ocassional shot to check... and when happy, set the ISO back to 800 (or 400) and then take a "long" frame to check the histogram's within bounds... and when it is, set the timer and take as many as I can before a) the clouds move in or one or both of the lenses dew up or c) the battery drains.

Having said all that, I've only taken 4 guided images to date, so I may be doing it totally wrong myself . APT might be worth a look if you have a laptop available - It has nifty "liveview stacking" and also a framing mode... I haven't tried it myself yet (still trying to get to grips with other "issues" ), but it's something I'd certainly like to try out...

[swag72]

Just another question than Andy - In my 1 page Aide Memoir for setting up etc, I always focus at the very start after balancing the scope. I use the Bahtinov mask for focusing, it's a real breeze. With your clip filter in, do you find stars bright enough through it to be able to focus with Live view?

[AndyUK]

Hi Sara - Yep... No problem... Admittedly, I do pick a "bright" star though... usually one that I know (which sadly isn't that many ).

With ISO set to 1600, first I slew to it, and then centre the star roughly using live view and then use the x5 zoom and centre again... and then x10 zoom. I then focus to get the star as small as poss and then pop the mask on for the final tweak... Then slew to the target and take the first framing shot...

(and then take another but this time with the mask removed )