pd mount discussion

[shirva]

Cat among pigeons time again. ..lol..now we all know that alt az tracking mount is ok on the cctv video camera going along up to 512 sense up so the discussion would like to start is,, the pd at full integration is double the sammy spec so how good is an alt az mount for this at full intregation, so would an eq mount be required and if so would guiding then be required as well...Davy

[rowan46]

Cat among pigeons time again. ..lol..now we all know that alt az tracking mount is ok on the cctv video camera going along up to 512 .....

I didn't know that

[Carl Reade]

Hi Davy I think most say field rotation dosnt kick in until around 60 secs so should be fine. If your processing later I think DSS can deal with it anyway. I wouldnt even consider guiding unless I was going for the dreaded long exposure, 2 hours of data etc that the folks nearer the top of the forums talk about lol

Just my thoughts Carl

[shirva]

It was just me doing a bit of thinking...saw somewhere somebody says 600 frames for a decent stacked image on cctv cams,,,/so I wondered at full intregation on pd cam .....would it be 20 second exposure per frame x 600 frames ....or am I getting wrong end of stick, that's why I wondered if it would then bring in field rotation issue. ...apologize if I'm off my head...lol..Davy

[Carl Reade]

Could do with more input on this one from the alt az folks

[johnno]

Davy I normally do 1000 frames, which if you look at it isn't long at all. The way I look at it is if your grabber is doing 25 fps a second , 4 secs is 100 frames 40 sec's 1000 frames. that's the way I look at it.

what I think Intregation is every 20 seconds the camera stacks a frame on top of another , but i'm might be just shooting in the air here, the image can only improve so much . ie ... say you did 60 sec grab the camera might have reached its optimum picture quality so if you carried on to 120 secs the picture would be just the same as a 60 sec one.. that's the way I see it, but i'm most likely wrong.

if you watch them on NSN you'll see after say 20 sec's or so the picture does not improve unless they alter it within the software.. I'm likely talking thru my backside... best shut mi gob and let the experts explain it properly..

Rock mallin says he only use alt/ az mounts and theres nothing wrong with his broadcast,  that's why I got a alt /az for video, getting polar alignment isn't that critical as with dsrl imaging....its the tracking that has to be right or you will get streaky images everytime it intregates,,,right i'm shutting up now...

[johnno]

yep Carls spot with the rotation , DSS will sort that out, honestly i'm not going to say another word...

[Spacedout]

Hello Davy,

I have only ever used Alt Az and I agree with Carl and for 60 seconds is fine. 

[DoctorD]

Hi Guys

In-camera integration (sensup) is in effect long exposure so does not have the noise advantages of stacking lots of frames in DSS (in fact the noise usually is worse as we are running the camera a MAX gain). Stacking frames in DSS needs unique frames, stacking lots of the same frame has no effect (I extract every 512th frame from the video - or better still use Sharpcap to record only every 512th frame).

I've only ever uses AZ and have had no problems up to 10 seconds with the SDC 453 - google search shows up 20-30 seconds being OK, however this is usually in the context of higher resolution cameras such as DSLR's so may be we can get away with more for our PAL resolution video cameras. 

Field rotation is dependant upon where in the sky you are pointing.

Some interesting maths (if you find that kind of thing interesting!) here:

http://www.astronomyforum.net/astro-imaging-forum/148319-time-takes-field-rotation-affect-alt-az-photos.html

Sammys with SNNR enabled actually stack the frames (average old frame and new frame) weighting the new and old frame dependant upon the SNNR setting - higher settings keeps more of the old frame, effectively increasing the exposure time, reducing noise and putting higher demands on the mount.

Try your camera in daylight with no and full SNNR , nudging the mount slightly and see what happens.

Hope this helps.

Paul

[johnno]

I'm getting a bit confused with this, if we are doing live view it does not matter if we use a alt/ az mount because after each integration frame the image will be updated to the lastest frame, so if the max intergration 20 secs,a new frame will be added to the image and the oldest frame of all the intergration frames will be discarded so you will not see any star trails but you might see rotation of the image over time.

When I took this photo of the monitor, it must have been tracking it for about 4 or 5 minutes as I had to go back in and get the camera from upstairs and drag the other half out to have a look at it..

this makes me think that a alt az is ok for live view . after all these video camera's are mainly for live view , if I wanted better quality images I would use a dsrl and a eq mount.

We need to do more tests.

[Laurie61]

Hi Johnno, the camera does add frames together and then updates the image, so at a setting of 128x you have128 x 1/ 50th sec=2.5ish sec integrated exposure ( 1056 x = 20.6 sec) you then have to wait another 2.5 sec to get the next frame. If the mount is not tracking star trails will increase as integration goes up. If you have an EQ mount properly aligned and tracking this will obviously remove both trailing and rotation, an ALT/AZ mount only removes object movement around the pole and not field rotation. It is dependant on camera pix size but field rotation should not be noticed at less than 30 sec exposure and probably longer (1 min?). So for the PD with 20 sec Max integration a ALT/AZ mount would be fine.

Even if you wanted to do better images, with a DSLR, you could still use an ALT/ AZ as long as your exposure time was short enough (sub 1 min) to eliminate field rotation and then use software to de-rotate multiple images when stacking.   

[shirva]

Paul ..if we used every 512th frame and stacked say a hundred frames I take it would be like stacking single dslr subs and should improve image if subtracted darks ??? Just like I do with my dslr...Davy

[johnno]

Davy pm Karl from Bedfordshire, he uses a AZ EQ6  mount and see what mode he uses plus he has a mallincam with long exposures, he'll put us all right for the pro's and cons

[johnno]

Thanks Paul for that info, so really it doesn't matter which mode of mount you use for video as long as the tracking is good.

I've tried it without tracking and it was like looking at one of them old space invader games every time it integrated the frame

[Carl Reade]

Great discussion I think all tracking mounts are fine for video given our short exposures live view.

Processing the results for keeps I like to do as well which is when the frame rate/capture comes in.

Carl