should I use a barlow to magnify small objects for astrophotography?

[gerardsheldon]

I have a 130 Skywatcher pds f/5 and use a modified DSLR 1000D.  Lots of astronomical objects, like the ring nebula and many galaxies, are really small  in the sky.  when I take pictures of them, they occupy a very small area in the photograph. Magnifying them with a barlow (I have 2x televlue one) would also make them fainter and my tracking would have to be good. 

Does anyone have any advice about whether I should use a barlow or not?

[Pompey Monkey]

If it doesn't cost you anything/much to try - why not have a go?

I think your limitation might be with your mount - what one is it?

Also, I think that @Stub Mandrel has probably done what you are suggesting - and he will know everywhere it could go wrong!

[don4l]

30 minutes ago, gerardsheldon said:

I have a 130 Skywatcher pds f/5 and use a modified DSLR 1000D.  Lots of astronomical objects, like the ring nebula and many galaxies, are really small  in the sky.  when I take pictures of them, they occupy a very small area in the photograph. Magnifying them with a barlow (I have 2x televlue one) would also make them fainter and my tracking would have to be good. 

Does anyone have any advice about whether I should use a barlow or not?

Try it.

I use a "focal extender" ( a barlow) and it gives good results.

 

[gerardsheldon]

Thanks.

My mount is a HEQ5, so it does a good job when autoguiding.  Using a barlow would mean the camera would be further away from the telescope which could cause balancing issues and shake possibly?

don41 has got good results from using a focal extender - are there any issues I need to be aware of?

 

[vlaiv]

In principle you can use barlow to extend focal length of your scope and capture "enlarged" target, but that really depends on ability of your setup to resolve enlarged target.

If your setup (and sky conditions) don't allow for that additional detail - you will essentially end up with the same image as you would get if you just simply shot without barlow and enlarged result by factor of x2 (resampled image to enlarge target).

Type of processing is also going to play a part here. In your particular case, I would say that with 130PDS mounted on HEQ5, if your guiding is good - meaning around 0.7" or less and your skies are decent - like 1.5" FWHM seeing, you should be able to sample at 1.5"/px.

1000D has 5.7um pixel size, which means that without barlow you would be sampling at 1.8"/px if you had mono sensor, but since you have color sensor, you are sampling at twice that - 3.6"/px.

You can comfortably use x2 barlow / telecentric lens.

You don't need to use coma corrector in this case (you will probably need to crop outer edges a bit since there might still be some coma in corners) and it is better not to use one - you will get better stars without one if you use barlow. Also - you will need to image for longer than usual because of reduced sampling rate.

[DKNicholson]

Using a 2" Antares x1.6 Barlow into a 1.25" filterwheel then camera, curiously led to noticeable vignetting where I had hoped it would help avoid it. So whether the vignetting was a result of that particular Barlow or just an inevitable result of using a Barlow, I don't know, but it is perhaps something to watch out for.

[vlaiv]

3 minutes ago, DKNicholson said:

Using a 2" Antares x1.6 Barlow into a 1.25" filterwheel then camera, curiously led to noticeable vignetting where I had hoped it would help avoid it. So whether the vignetting was a result of that particular Barlow or just an inevitable result of using a Barlow, I don't know, but it is perhaps something to watch out for.

Barlows can vignette depending on their design and size of sensor.

In order to avoid vignetting it's best to use barlow that is meant for photographic applications - like Baader VIP barlow. It is designed to illuminate full frame sensor according to specs.

Of course, depending on how much vignetting there is one can either crop or use flats to correct small amount of vignetting.

[don4l]

When I first tried the extender I was horrified by the vignetting.  However, when I examined the background ADUs around the frame I realised that a lot of the problem was just the way that CCDStack displayed the data.  Flat fields fixed the display issue, and I didn't find that exposure times were unduly long.

 

I'll stick by my "try it" advice, because the results will speak far louder than any predictions.  If it doesn't cost anything, other than 30 minutes,  then it is worth a go.