AGH! I don't get this? Heeelp (please)

[Devilish]

Hi Guys and Girls

I trolled the forums for many days now, I have a slight answer to my question but still not sure, so I figured I'll ask for help

I am about to buy a new DSLR (Nikon D3100) - want to start with AP - starting mostly with Solar and Lunar (I have a solar filter...)

Now my Q is what to do, this confused me - I have a 130 reflector. I understand the basic prinsipal of what I require, T-Ring and T-Mount, but what about a EP or the Camera lens? how do you get magnification if you use no lense? Also if you use no lens, I've noticed that the 'spider vains' for your secondary mirror is clearly visible, won't this be a problem?

If you could answer, I would really appriciate this

Thanks

[RikM]

With a DSLR mounted at prime focus, you don't use an eyepiece or lens, only the camera body. The scope becomes, in effect, the camera lens and has a fixed field of view (magnification is not the right term when we talk about images).

I am not sure the 1.25" focusser on the 130 is strong enough to hang a DSLR on though? And I don't know if you will be able to achive focus as there may not be enough inward travel. I could quite easily be wrong on both counts though.

The spider vanes are not visible in the final images, but they do give rise to the diffraction spikes that I love (yet others hate).

[jgs001]

Your "lens" in this case is the telescope, you're using it as a large prime. I think the 130 is 650mm focal length, so you have a 650mm prime telephoto lens. If you want to increase the magnification, you can either, use a barlow/teleconverter, or use some other technique...

afocal, use both the camera lens and eyepiece

ep projection, use the eyepiece, and camera body only.

[Devilish]

Thanks for the reply guys, I have seen a few peoplewith 130's - not sure though if they have a 1.25" - if the inward travel poses a problem, is there any way I could maybe 'test' this theory?

So the spider vains won't be a problem?

[Dann]

I’m afraid I don’t have the technical explanation, I’m sure someone will be along with that shortly but don’t worry about the spider veins, you won’t see them in your pictures of the moon and sun.

I have a newt and the only indication of the spider in any of my photos are the diffraction spikes on bright stars, which some people find desirable.

From what I understand the field of view is set by the telescopes focal length, a slow scope with a long focal length will give you a narrower field of view but a fast, shorter focal length gives you a much wider field of view.

To get higher magnification you could use a Barlow if you have one between the focuser tube and the camera.

[jgs001]

They are too close to the camera, when you're focused on the moon to show. If you were shooting stars, you'd get diffraction spikes (in images you see, the actual star patterns).

[Devilish]

My scope is F/5 and my focal length is 600mm, the reason I decided to go for the Nikon D3100 is not just for astro, but normal photography as well - I just hope the camera is good?

And thanks again for the help, I'm sure I'll have many other questions with the months to come

[ollypenrice]

I’m afraid I don’t have the technical explanation, I’m sure someone will be along with that shortly but don’t worry about the spider veins, you won’t see them in your pictures of the moon and sun.

I have a newt and the only indication of the spider in any of my photos are the diffraction spikes on bright stars, which some people find desirable.

From what I understand the field of view is set by the telescopes focal length, a slow scope with a long focal length will give you a narrower field of view but a fast, shorter focal length gives you a much wider field of view.

To get higher magnification you could use a Barlow if you have one between the focuser tube and the camera.

A couple of things here. Fast or slow makes no difference to image scale ('magnification'). Only focal length governs image scale.

Fast or slow (ie focal ratio) governs exposure time.

If you increase image scale by using a Barlow you pay for this by slowing the focal ratio down, which would almost never be a good idea. A 2X Barlow would quadruple the size of the object by area on the chip and also quadruple the exposure time. Bad idea!

In discussing f ratio and speed, always think in terms of scopes with the same focal length or you fall into a little minefield called 'the f ratio myth.'

Olly

[Dann]

Ah ok, thanks for a clearer explanation Olly.

[Devilish]

Thanks Olly! Think I'm starting to understand the concept

[Cornelius Varley]

If you have Skywatcher or Celestron Astromaster 130/650 you will probably need a barlow lens to allow the camera to come to focus. Without the barlow the point of focus is too close to the telescope and you can't get the camera close enough. By adding a barlow lens the point of focus is moved further away and the camera will now reach focus.

Peter

[Devilish]

Thanks for the tip Cornelius! I will keep that in mind i have a celestron 2x barlow

[noakesy]

how helpful was that ive never been able to focus my nikon d50 with my 130p

[ollypenrice]

how helpful was that ive never been able to focus my nikon d50 with my 130p

Yes. Ths does come up quite often. The problem with the Barlow is that you do get to focus but at a much reduced focal ratio.

Olly

[jgs001]

For lunar and solar (with the proper objective filters of course) the change in focal ratio f/5 to f/10 is not an issue as the shutter speed is fast enough. I regularly shoot single shot moon and sun at about f/18 from a static camera tripod with my ed80.

[Devilish]

"...with the proper objective filters of course..." and what filters would this be John?

[Shibby]

A good quality, full aperture solar film. Needless to say, don't even think about imaging or observing the sun without one.

As has been said, you will not be able to achieve focus unless you use a barlow (not ideal) or fit a new, low profile focuser. Also, you will soon find the poor control you get over a Nikon becomes a limitation for DSO imaging, which is why Canons are preferred. These are limitations you can overcome, but only with a certain amount of hassle.

Edit: I've just realised that was my 1000th post. I hope it was a useful one!

[Devilish]

Well I have a Baader solar film, safely tucked away Also I do have a 2x Barlow, a low profile focuser could work, just I don't think the guy I always buy from would have any

With DSO what does the Canon have over the Nikon?

(At the moment I'm very limited to DSO Observation even, heavy LP) I thought to start off with Lunar and Solar - ease myself into the hobby

And congratz on your 1000th post!

[barkis]

Just one point I would ask you about. You say you have a Solar Filter, but you didn't say what type. Since you are new to this, and solar work carries some danger, could you put my mind at rest, and say which filter you will be using.

In the interest of your safety, and also the well being of your new camera.

Ron.

[Devilish]

Hi Ron

Good point! I have the baader planetarium solar filter, pretty much a good review in the forums so I decided it would be a good buy, haven't had any problems with it, and everytime I observe I ensure it won't fail, duct tape and cable ties

[RikM]

If you are only looking at lunar / solar, you might well get better results with a webcam rather than a DSLR. The Philips SPC900NC won't reach focus in a 130P without a barlow but that is not such a problem with high frame rate stuff. You will just have to learn to make mosaics but it is posible to get very nice images

[Devilish]

Reason why I'm getting a DSLR is not just for Astro but for normal photography as well - last weekend I gave a half modded Logitech QuickCam a go, wasn't that succesfull, but okay - I don't have the proper adapters lol

[barkis]

Well, you have a big OK on the Baader Filter .

Ron.

[Devilish]

or what about this?

NexImage Solar system imager [C/93712] - R2950 : Telescope Shop, Everything for the Amateur Astronomer

NexImage Solar system imager

[jgs001]

I believe the neximager is the same as the spc900 but more expensive. You can get an spc880 which can be flashed to the 900, for £7.50 at the moment.