2X Telextender - effect on speed?

[rocketandroll]

Hi all

Ok, so i got myself one of the Meade 2X telextenders a while back to try and do some planetary imaging with my 127mm Apo...

Now I know this pushes the effective focal ratio up to f14.5 or so from the native f7.5... and that's a bad thing for imaging DSO's... but I did wonder whether there is any chance of still using it on perhaps globulars or something which are fairly bright?

I did some shots of M3 the other week and was getting reasonable detail with 10 sec exposures at ISO 800....

if I go up to 60 sec exposures, should I get any joy?

I am afraid the math isn't my strong point so I'm struggling to work out just how much this will affect the required length of exposure to get the same effective brightness in the image?

Oh, and I have no guiding at the mo so, at this sort of magnification even 60 seconds is going to be REALLY pushing my luck :-)

Thoughts?

Ben

[riklaunim]

I used 5 min exposures at f/10 some time ago with C8 and DSI III Pro, but that was guided. Soon I'm planning to try C11 at f/10 on a globular cluster which may be quite fun :]

[jgs001]

Ben, I've used a 2x Teleconverter on Globular clusters with my 80ED (f/15) and it worked ok, but I was using 5 minute exposures with ISO800 to do so.

The problem is, when you double the focal ratio, you actually need 4x the exposure duration to get the equivalent exposure. Max out the ISO (it'll be noisy, but it'll work), that'll help even things out a bit... and I guess should a shedload of lights... no more than that...

[rocketandroll]

Ben, I've used a 2x Teleconverter on Globular clusters with my 80ED (f/15) and it worked ok, but I was using 5 minute exposures with ISO800 to do so.

The problem is, when you double the focal ratio, you actually need 4x the exposure duration to get the equivalent exposure. Max out the ISO (it'll be noisy, but it'll work), that'll help even things out a bit... and I guess should a shedload of lights... no more than that...

Yeah, I was thinking this may well be something I need to consider doing when I have the guiding set up later in the year.

Hmmm.... well, I may have a little stab at it tonight if I get a spare few minutes... if I take one 50 - 60 sec sub at ISO1600 and cant see much in it, I know it's probably not worth bothering :-)

Trying to decide what to shoot tonight if it's clear as I have a chance to go out to our astro club's dark site.... may just stick with my original plan and shoot M81/82 with the scope as-is. Should be nicely framed in the 127. Just waiting for a 0.8X focal reducer to get galaxies a bit quicker... but that hasn't turned up yet.

Ah well, might have a play around, see what's looking good :-)

Ben

[Catanonia]

2x barlows = squared = 4 times the exposure time needed.

[rocketandroll]

2x barlows = squared = 4 times the exposure time needed.

Thanks, thought it was something like that :-)

Hmmm, well, if I was getting ok data out of 10 - 20 sec exposures... it might be viable at 50-60 secs with the 2X?

Will have a stab tonight I think... moon may be too bright for getting galaxies.

Ben

[jgs001]

Ben, I'm not certain on that... but give it a try... push the camera settings too... and capture many many frames... (I picked up the bubble nebula with an unmodded camera with 40 second exposures.. actually I should add... it was 168 exposures at ISO1600... and the bubble nebula is an infrared target... I didn't even know I'd got it till I stacked the images)...

[Catanonia]

Thanks, thought it was something like that :-)

Hmmm, well, if I was getting ok data out of 10 - 20 sec exposures... it might be viable at 50-60 secs with the 2X?

Will have a stab tonight I think... moon may be too bright for getting galaxies.

Ben

Off course it means you need much better guiding too for long subs at longer range.