Eos 1100d and aperture while using a barlow

[Gazer2012]

Hey guys im fairly new to astrophotography, i have a canon eos 1100d and a skywatcher 130p. I have the t ring and barlow adapter and have been somewhat successful with taking pictures of the moon. The way i understand it is i have to use the barlow as there is not enough inward travel on the focuser to achieve prime focus.

Heres where my problem is

I know its not ideal to use a barlow to take pictures of dso`s but when i set my canon to the Manual in order to control the shutter speed the aperture seems to be locked at 00, when i try to change the setting i get a message saying that i cant with no lens attached. With it set to this any pictures i take just appear black, even while having the scope pointed at the moon. I assume im doing something wrong otherwise how else will i be able to to take long exposures unless i attach a proper lens and do piggy back style. I have heard people know how to lock the aperture so if anyone can give a noob some help id greatly appreciate it.

Thanks in advance

[jgs001]

The reason the aperture value on the camera shows 00, is there is no electrical connection between the body and the telescope/barlow, which you would get with a normal camera lens. So this is all the camera can display. If the image is blank, then you're shutter speed is too fast.

The aperture value displayed, is not actually the aperture, but the focal ratio of the lens... defined as the focal length/aperture... The aperture of the scope is fixed at 130mm, the focal length is 650mm, therefore the scope is f/5 (you would see f/5 on a normal camera lens). When you add a 2x barlow, you effectively double the focal length... so 650, becomes 1300, to get the focal ratio 1300/130 = f/10. When you double the focal ratio, you actually need 4x the exposure to get a comparably exposed image, so you need to decrease the shutter speed accordingly. Keep the ISO at 100, and try a shutter speed of about 1/50s... check the display after shooting (you'll need the histogram display turned on) and check for blown highlights (any over exposed bits will blink) and adjust the shutter speed accordingly... too bright, make it faster, too dark, make it slower. I don't believe the 1100d has a spot metering mode (method to determine the correct exposure settings) which I would recommend for the moon, so you're best off going for a trial and error method.

I hope that makes sense..

[Gazer2012]

I forgot to mention the 130 i have is the 130m which has a focal length of 900mm (f6.92) according to the website, so if my barlow is a 2x (Its the celestron t adapter but it doesnt say what mag it is. So assuming its a 2x the total focal length would be 1800mm. It may be of note that while i was trying last night even when i had my scope pointed at the moon i tried the live view but the screen was just black, is this normal if the aperture setting says 00 and do i just need to try and photograph using diffrent shutter speeds and see what happens. Obviously i would normally be trying to get long exposures of the moon but while i was trying to get any picture at all i thought a bright target would be best.

Thanks again

[Tinker1947]

If its the travel you need to use the barlow for, try taking the lens out so your just using the tube as a spacer, for DSO's you will need something like 30 second exposures or maybe more....one of these will sort out the exposure times and how many imagines, for stacking,,,

http://www.amazon.co.uk/Timer-Remote-Control-RS-60E3-Canon/dp/B003WD0IKO/ref=sr_1_3?ie=UTF8&qid=1340959957&sr=8-3

[jgs001]

I forgot to mention the 130 i have is the 130m which has a focal length of 900mm (f6.92) according to the website, so if my barlow is a 2x (Its the celestron t adapter but it doesnt say what mag it is. So assuming its a 2x the total focal length would be 1800mm. It may be of note that while i was trying last night even when i had my scope pointed at the moon i tried the live view but the screen was just black, is this normal if the aperture setting says 00 and do i just need to try and photograph using diffrent shutter speeds and see what happens. Obviously i would normally be trying to get long exposures of the moon but while i was trying to get any picture at all i thought a bright target would be best.

Thanks again

That's ok, the principle is the same, the only difference is, that the change in focal ratio, means you'll need a longer exposure. For the moon, you really don't want long exposures, as you'll just get a white blob. There's one of two things... either, your shutter speed was too fast... liveview will attempt to give you an approximate view of the resulting image, so changing the shutter speed will help. Or, the FOV at 1800mm is too narrow to easily get on target, and you were actually missing the moon. As the focal length increases, so the FOV decreases.

A couple of questions..

1) What shutter speed did you have set ?

2) Is your finder proper aligned with the scope ?

[Gazer2012]

Truth be told last night i was using a friends celestron 114 powerseeker, i did have an inkling his finderscope wasnt 100% but as its a similar scope to my 130m which is accuratly aligned and i was at his house i happened to have my camera to hand, as for shutter speeds i tried 1/4000 when i was getting some pretty awsome shots of the moon. We pointed the scope at cassiopea and tried to do a 30 second exposure as i said before, the image just came back completly black. Maybe it might be diffrent with the 130 but i will play with that when i get home as work is calling.

Thanks

[jgs001]

Ah... what we've been talking about is for the moon.. the settings change completely for DSO's... You want an ISO of 800 ish... a lot will depend on the noise handling of the camera, I think the 1100d is a DIGIC 4 so you can probably push that higher, you'll also need to have tracking running at telescope like focal lengths... and using a barlow is going to make it even harder, as, the exposure time increases 4x for each 2x focal length increase for an equivalent exposure. So 30s at 900mm, will need 120s at 1800mm on the same scope, and that will require very accurate tracking to achieve that.

If you're wanting to get more seriously into imaging, then I'd suggest getting a copy of http://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html

[iancandler]

As said unscrew the barlows lens element from the end, that way you can use it as a short extension tube to enable you to achieve focus. I would also look at getting a screw on focal reducer if the barlow is a 1.25" one, you can pick a 0.5x one from most astromomy retailers.

You should be able to push your camera to ISO 1600 before noise becomes a real issue that you cant clean up in post processing, also shoot RAW if you can, doing so gives you a lot more control over the image in post processing.

[jgs001]

I thought the 130 reflector didn't have enough inward focus travel, as opposed to needing an extension to reach focus.

[Gazer2012]

So how exactly does the focus reducer work, i have the t ring on the camera then u screw the t adaptor with barlow into that, and that goes where the eyepiece should be, where does the focal reducer fit in to that?

[iancandler]

Ah John has just pointed out something I wasn't aware off, it seems your using the barlow to achieve focus, to use a reducer you would screw it to the nose piece in place of the barlow, but in doing so you may not as john pointed out have enough inward focus to achieve focus.

[jgs001]

I think that you'd need a coma corrector, rather than a focal reducer, but you need to get focus sorted before worrying about that.