Barlow lens

[brrttpaul]

Quick question guys, I didnt realise that I could use a barlow lens with my scope/ camera. I was thinking of trying the dumbell nebula at some point (maybe tonight). What sort of setting should I be looking at with a barlow lens?  amount of subs, length etc. Im not 100% I will use the barlow lens but its a nice option to have. My scope is a skywatcher 150, camera Nikon D7100

[Tommohawk]

The idea of using barlows for DSOs seems to generate mixed views.

My view is that using a 2x barlow will obviously give an image thats twice as big (4x the area) But because the light is spread over 4x the sensor area you will need 4x the exposure duration to get an image of the same brightness (per unit area.)

This isnt such a problem if you're using a CCD, and/or if you have a bright target, and/or if you're guiding. But if you're not guided, I think you will struggle with subs of more than 30 seconds, depending on how good your HEQ5 is, and how well you're polar aligned.

Short answer, assuming no guiding, try unbarlowed ISO 1600 30 seconds. I'd be interested to see your results, and especially your experience with a barlow if you decide to try that.

Ahhhh - just saw your pinwheel post with exposure 3 mins - so I'm thinking you are guided. I used ISO 800, 5 minutes (unbarlowed) which seemed to work OK. That was with 1000mm scope though so maybe 4 mins would do with the SW 150mm.

PS I'm sure you know this but Dumbbell is probably easier in July.

[brrttpaul]

Hi Tom thx for the reply, no i,m not guided just messed about with it, the main reason I was asking besides being bigger was the T ring is a [removed word] to get off afterwards whereas on the Barlow lens its a lot easier (dont know if anyone has experienced this). I am going to do one sub at 4 mins long using the Barlow just to see, if it turns out bad then I can quickly change over and still have time 

[Tommohawk]

You got your Pinwheel with 3 minute exposures unguided?? Amazing - I never got close to that. Your HEQ5 is definitely a keeper... and your polar alignment must be spot on too!

Good luck - post your results. Will be interested to see, and especially the difference between barlowed and not. Do a few subs just to make sure its not flukey.

Flats, darks and bias a good idea too if you havent done before.

[brrttpaul]

more luck than judgement Tom lol, a nice big cloud has just come over just as i was lining up polaris

[RikM]

Tom's correct on the exposure. You will need sub 4x as long to reach the same brightness with the Barlow in place and your polar alignment and tracking will need to be twice as accurate compared to without the Barlow.

Do give it a try though. You have nothing to loose except a bit of time.

[brrttpaul]

Stuck it till 1 but the clouds havnt budged