Is this a daft question??!!

[choochoo_baloo]

Hello all,

I was just reading about the new William Optics 5 element astrograph. It has a focal length of 348mm. Now having starting to gt into astrophotography from normal terrestrial photography; 348mm for my Canon DLSR body is well within the realms of stand SLR lenses (focal length wise). Camera lenses are multi-element devices like this WO astrograph, and such lenses are approaching the aperture of this astrograph.

So........ my question is thus:

Why do wide field deep sky astrophotographers bother with high quality short focal length apochromatic refractors in the first place, instead of using a suitable lens with the DSLR body; both attached to an equatorial mount as per usual?

I hope this question isn't too nooby - it just entered my head earlier today and am keen to find some answers all being well!

[RCG]

I too, would like to hear the answer to this one.

[choochoo_baloo]

I too, would like to hear the answer to this one.

Thanks: makes me feel better!

[Dubhar]

I have no idea(so would be interest to know also), but if i were to guess... The massive difference in price there seams to be. Camera lenses (Im assuming) might also be a little less versatile.

[wxsatuser]

Camera lenses are not designed with AP in mind.

They suffer from all sorts of abberations when imaging and a fast lens will need stopping to get anywhere near a decent astrograph.

I have been using a Canon EF 200mm f2.8 L and it's not so good at f2.8 but gets reasonable at f4 and very good at f5.6.

At f5.6 the diffraction spikes are well in your face, some like them others hate them.

You can of course cropout the field edge rubbish but then that defeats the widefield we would be after.

If you already have camera lenses and use them for daylight photography, use them for AP as well, but know the limitations.

For pure AP the astrograph would be the way to go, cost wise and for image quality.

[baggywrinkle]

Also to do with light grabbing power. Large mirror/lens will outperform a camera lens in terms of grabbing light.

[Robny]

I have been using a Canon EF 200mm f2.8 L and it's not so good at f2.8 but gets reasonable at f4 and very good at f5.6.

At f5.6 the diffraction spikes are well in your face, some like them others hate them.

I thought diffracrion spikes came from the newts secondary mirror support vanes, where would the come from with a camera lens?

Rob

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[wxsatuser]

From the diaphragm.

EF 200mm L has 8 blades

[Erekose]

The standard lens supplied with most camears is not as good as a telescope, the intention of a camera lens is to produce an image during what is daytime. It is a lot brigheter and this additional light can "mask" problems.

To get close you would need to buy a good lens and they can cost a few thousand pounds. A Sigma 300mm APO at f/2.8 costs £2900 which you would have to stop down a bit for maximum sharpness.

That is only a 300mm and a good apo refractor, say a William Opticsa again will cost around £1000, the GT-81 (GTF-81 ?) or the Star-71 if available here in the UK. The Sigma 500mm f/4.5 is £4800. Maybe to reason people use telescopes is simply cost as a good camera lens is say 5x the cost of a scope.

I see you have a location of Cambridge and Somerset, assume Somerset is home, your avatar is Robinson, if you can make it to the NLO Observatory for the SWAF then do so as they have a group of imagers there and you can spend the day getting information from them. I dropped in there about 3 years ago and they were useful to talk to.

I will point out that no William Optic's telescope tends to be "lightweight", and a Skywatcher EQ3-2 will not be stable enough to do justice to the possibilities of the scope. Possibly look, wait, for a used scope to come along. Imaging can get to be a very costly activity and perhaps picking up items as they appear. The approach being acquire some good used items while at university to learn with and if it is still your intention after that then look to upgrade the equipment to perform to higher expectations.

Does the university Astronomy department not have a mount or scope that you could utilise ? I would have expected them to have a suitable mount at least as I see they hold public talks there with observing.

[Moonshane]

it's rare that something is cheaper, better and lighter than other alternatives but it seems to be the case here!

[Robny]

From the diaphragm.

EF 200mm L has 8 blades

Thanks for sharing that, I never knew

Rob

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[ollypenrice]

We shouldn't throw out camera lenses from the story. If you have a very good one then it will beat a small WO but not beat a Takahashi. The problem with camera lenses is that they are multi functional and have many elements to allow for a wide focal range and controllable depth of field (which astronomers never want.)  All this costs money and does involve extra glass.

You don't necessarily have to stop the good ones down and, if you do, you can use a front aperture mask and avoid spikes from the diapragm. Peter (Psychobilly) has demonstrated that the Canon 200L (I think) can work wide open if you meticulously focus on the 1/3 lines.

Some world class widefield imagers uses lenses but they use expensive ones often costing more than comparable telescopes. All very fast F ratios have terribly narrow depth of field and focus is ultra critical.

You can also do reasonably well with modest lenses like the Samyang 85 but it will need stopping down. This is a 6 pane Orion mosaic taken with this affordable lens and a CCD camera.

Olly

[choochoo_baloo]

Sorry what does stopping down mean RE DSLR lenses?

[ronin]

Manually, or otherwise, reducing the aperture from say f/4.5  to f/6.

Basically going from a wide aperture to a smaller one.

It tends to increase the sharpness of the image.

Also increases the depth of focus - seen a nice sharp image of a subject with the background out of focus? That is a wide aperture shot.

That is the problem with modern cameras, users just aim them.

[choochoo_baloo]

Thanks all for the info. 

it's rare that something is cheaper, better and lighter than other alternatives but it seems to be the case here!

This sentence seems to accurately sum up the opinions expressed here! I shall indeed stick to a William Optics short focal length APO refractor.

[Scott]

There are plenty of times when a camera lens is the only alternative (excluding mosaic). Some of the nicest images I'veever seen come from lenses 10-20mm long .