Barlow DSO madness.....?

[stan26]

OK, I may have completely lost the plot with this one but there is "some" method in the madness......

I'm planning on using, or at least trying to use a 2x barlow for DSO imaging........and on my EQ3-2

My Canon 200L lens, and ST102 are fine for most targets, but for the more smaller compact targets, i.e M51, M27 etc the ST102 just doesn't have the FL. I could go out and purchase a longer FL scope, but there are two major problems involved here, 1, I don't have the money & 2, the EQ3-2 will be pushed well beyond its practical weight limits.

The theory is:- If I can barlow my way to a more suitable FL, I can keep with the ST102 thus keeping weight and cost down. Not ideal but I think its doable, and with the guiding tuned to a T, and the addition of a longer FL but light weight guide scope (i.e 60mm F10 plastic tube jobby) I think it could work....possibly....maybe....or not..?

I appreciate that the exposure time will have to be upped to compensate for the f/ratio, and that this will be hugely demanding for the guiding on such a mediocre mount, but I love a challenge!

Has anyone tried this method before..?

My first problem to overcome is achieving focus. There is not enough inward travel to bring the 500D into focus with the 2x barlow. Can I get around this by extending the actual barlow body, i.e pushing the lens deeper into the scope away from the T adapter.....?

Stan

[ArmyAirForce]

This is M13, taken with my Nikon D50 on a SW200PDS. The original M13 image I took was quite small, so I tried adding a Jessops x2 converter directly onto the camera before the T ring and focusser adapter. Effectively it is a x2 Barlow I guess, though an accessory from the world of photography rather than astronomy. I can't remember how many subs this image was made from, but it wasn't many, and because I'm imaging un-guided, the exposure time was probably no more than 60 seconds, probably 30 seconds ( my maximum auto long exposure ).

The x2 does cut down the light to some degree, as will happen any time you start adding more glass into the light path, but I say give it a go!

[Uranium235]

No need to even attempt this because the results will be awful, even if you take into account the quadrupled exposure times. I've tried this before, and the results were tragic to say the least. Stick with prime focus and you cant go wrong

Even with ED barlows, the edge of field performance will be poor.

[stan26]

OK fair enough uranium, thanks for the honest reply, maybe I'll give it a miss then. I'm glad I'm not the only one to think of the idea though.....

Stan:)

[laser_jock99]

No need to even attempt this because the results will be awful, even if you take into account the quadrupled exposure times. I've tried this before, and the results were tragic to say the least. Stick with prime focus and you cant go wrong

Even with ED barlows, the edge of field performance will be poor.

I disagree, if you already have an ed bar low why not give it a try? Many people use barlows on bright dso,s to get the extra resolution. Far from doubling the exposure you might want to consider short subs since you are also magnifying your tracking error.

M13 Hercules stack of ten 30 second subs, through a 2x ED bar low

[RikM]

I had though of having a go with a Barlow in the chain for small planetary nebulae. Even with four times the exposure time, it is still only going to be 4 or 5 min subs instead of 30 - 60 sec normally since these things are so bright.

[stan26]

Hmm ok thanks guys.

Also, Could I not just up the ISO on the camera. My usual style when DSO imaging is 2min subs at ISO400-800 depending on target and if I'm using my LP filter or not. So if I stuck to the same sub length (if doable) and upped the ISO to something like 1600-3200, surely things wouldn't be far out..? not sure if signal to noise ratio is a major factor here.....?

Stan

[RikM]

Signal to noise ratio is generally better at higher ISO. Increasing the ISO doesn't alter how many photons your scope collects. A 2 min sub is a 2 min is a 2 min sub. What the barlow does is spread the photons out over a larger area on the chip. These days I think of the ISO setting as a kind of pre-stretch of the data.

People do image at f/10 but with 2min as a max sub length, I would have thought it would be okay for bright high contrast targets like globs and small bright planetaries, but not so good for faint targets like galaxies where you need loads of signal to be able to stretch them up above the sky background.

As a comparison, imagine the results you would get with 30sec subs and twice as much visible trailing. That is the sort of shot that should be achievable using the barlow.

[NickK]

Has anyone tried this method before..?

http://stargazerslounge.com/imaging-discussion/178518-televue-powermate-2x-4x-imaging-105mm-scope.html

I would say it can work however you'll be limited to brighter targets.

With the EQ6 I have to get the balance right and alignment spot on for 1340mm fl (2x with 670mm native fl). The back focus stays almost identical. Focusing is a PITA at this focal length - you'll need a motor focuser really otherwise it will take a while to keep waiting for the scope to stop wobbling. The scope was operating well past it's dawes limit (1.14 theoretical limit, operating at 0.82 arcsec/pixel!).

Yes there is some coma - look at the m82 below, that's through a very good APO with a field flattener built in and a TV 2x power mate, and you'll not some coma at the edges. Personally I don't see this as a problem. £250 vs .. well something like £2-6,000 depending on the quality of the scope!

This does for (attached - excuse the dropped OAG prism causing the vingetting!), but note that this was a 20m exposure so you'll have to weight the time it takes vs the larger scope. I've included a badly guided M42 trap I used to test it out before SGL7. Not bad as you can make out E and F

[stan26]

Thanks guys, By the sounds of it my best bet is probably to stick with the wider frame considering what mount im using etc.....

Cheers

Stan

[jgs001]

It can be done, if you can increase the tracking time considerably. I've used a 2x teleconverter (TC) on small targets and it works, but I had to use 10 minute exposures, instead of 5, and that needs a guided mount. a 2x barlow will give more than 2x magnification on an SLR, as the lens elements are further from the sensor than with a TC, therefore you might be looking at 2.5x or even 3x and the consumate further increase in exposure time.