EAA with a Guidescope?

[PeterC65]

With the Milky Way rising above the horizon in August, and since I enjoy widefield views of the sky, I've been thinking about how I might get a wider field of view when I use EAA, and that's got me thinking about using a guidescope. The one I have in mind is the William Optics 32mm Uniguide which has a focal length of 120mm and so should let me see 5.3° with my Uranus-C camera, but it's a guidscope and an achromat, unlike my 'proper' refractor (432mm Apo).

Has anyone tried using a achromatic guidescope for EAA?

Askar make a nice little 135mm apochromat, the FMA135, but it's as expensive as many scopes and would give me a narrower field of view than the Uniguide.

As things stand, I'm planning to use a DSLR with its telephoto lens when the Milky Way finally appears, but its the family camera and I worry about the excessive shutter use that EAA involves.

 

[Stephenstargazer]

I have a camera for EEA with a relatively small sensor. Can use it with ridiculously cheap C type lenses and recently acquired a Carl Zeiss Jena 135mm lens in M42 for £33 . These will give me wide fields and work at f/3.5 or less. Cheaper than these doublet guidescopes and proper camera lens quality. Not expensive to try! You may also be able to adapt your camera lenses to fit the astro camera?

I have used them for daytime testing, to sort focus point, but not done a proper sky test yet - whilst solving other problens!

(To work out fields of view use Sky Safari scope display or do the maths with the sensor sizes)

 

Edited May 10, 2023 by Stephenstargazer

[PeterC65]

I've been investigating the use of a camera lens since making this post and it seems to be possible using an adapter like this.

My 'proper' refractor is 432mm so with the x0.6 reducer that gives me an effective focal length of 259mm. So to significantly increase the field of view I need to get down to 130mm or less. A 120mm guidescope would be OK but a camera lens, particularly a zoom, would be even better. My existing Canon 18mm - 55mm lens would give me a field of view of between 11.6° and 34.4° with the Uranus-C camera.

I've read a comment that aperture control can be a problem with a camera lens. The focus and zoom can be controlled manually but I understand that the aperture is controlled electronically from the camera body which is not possible when the lens is used disconnected. Have others used a camera lens with an astro camera for EAA?
 

[Stephenstargazer]

Many people take widefield astro photos with a camera lens. So there is no inherent problem. It is generally reckoned that fixed lens are preferred and it would be simpler to use a manual lens, so that it can be stopped down. A 2nd hand T2 or M42 is probably easiest to connect. Camera sites have cheaper adapters.

You my find some good advice here. Ian has tried most things and researches the ideas  well.  https://www.ianmorison.com/

Edited May 11, 2023 by Stephenstargazer

[Cosmic Geoff]

Earlier this year I used a 50mm aperture f4 finder/guider to image that bright comet that received a lot of attention.  (C/2022 E3 ZTF). I used it with an ASI224MC.

Mainly I wanted to ensure I got the whole comet in the field.  Yes, it did work, but I got better images with a 102mm f5 refractor.

OTOH, if I want to take a widefield shot again I will know what to do.

 

[Elp]

Not EAA but AP I've used a variety of lenses old and new. Fixed primes are the best, preferably with ED or APO lenses, otherwise you tend to get bad coma/abberations toward the edges or your stars don't focus fully to solid points of light, the larger the camera sensor the worse it can be. I've found a lot of zoom lenses also cannot focus to infinity but it's not impossible. Sometimes with auto lenses you can attach it to a DSLR body, set the aperture, then disconnect, but it's far easier to use step down rings on the front to close the aperture, having a large f ratio aperture to start with helps (f1.4 to f2) so they can be stopped down to F3 or so, stopping down to F5.6 or higher the lens starts to get a bit slow, remember they are also aperture limited due to their physical size which affects light gathering capability. One of the best lenses out there is the Samyang 135mm F2, it's actually one of the best pieces of equipment full stop.

Edited May 12, 2023 by Elp

[Ratlet]

Go for some vintage glass.  135mm smc super takumar 135mm did this in 20minutes with a duo band filter.  Autostretch and colour adjustment only.  Not true eaa.  I can send you one of the 1 minute subs from the stack if you want a better idea.

Lens cost £15.  Adapter from m42x1 to canon and then a canon to m42x0.75 to get onto the camera.  That cost about £70.

This was with a imx533 sensor so your Uranus one half side of this image.

Edited May 13, 2023 by Ratlet

[PeterC65]

After reading the comments and doing some more research I'm erring towards getting a Canon DLSR lens adaptor (this one) and using camera lenses with my Uranus-C camera for widefield EAA. The adaptor is meant for Altair Hypercam's but they have the same back focus as my Uranus-C and the same M42 attachment so the adapter should work fine.

I'm not a photographer so the world of camera lenses is new to me. There seems to be more camera lenses than there are eyepieces for visual, and the options available are even more confusing!

I already have a Canon 18-55mm zoom and a Tamron 70-300mm zoom and will try these first, but I expect I may then start to grow a lens collection. Prime focus lenses would work out very expensive if I want to cover a range of field of view but I assume they are better optically and they seem to have much bigger apertures than do zooms. I have heard the Samyang 135mm lens mentioned many times and I see that their lenses are prime focus manual lenses which would be better for EAA as the aperture could be controlled and there is no redundant focus motor or image stabilisation function.

It will be interesting to see how these various lenses perform for EAA. I can see me needing a new case, this time for camera lenses!

 

[Stephenstargazer]

On 13/05/2023 at 07:20, Ratlet said:

Adapter from m42x1 to canon and then a canon to m42x0.75 to get onto the camera.  That cost about £70.

Baader do an M42x1 to M42x.75 adapter, quite small light path ? 3mm. Mine was nearer £20, probably more now! Still need spacers though. S/H M42x1 'macro' spacer tubes cheapest, T2 variable most convenient.

[Ratlet]

4 hours ago, Stephenstargazer said:

Baader do an M42x1 to M42x.75 adapter, quite small light path ? 3mm. Mine was nearer £20, probably more now! Still need spacers though. S/H M42x1 'macro' spacer tubes cheapest, T2 variable most convenient.

Probably a better way to do it.  Some of the vintage glass doesn't quite focus to infinity so being able to play with the spacing would be ideal.

[Stephenstargazer]

For info of those looking at using a camera lens, the distance from lens flange to sensor should be as below (flange focus distance).Any more will not reach infinity.

Canon EOS EF.   44.0 mm

M42 (Pentax,Praktika etc)   45.5 mm

T2 (Tamron et al)     55.0 mm

C mounts.   17.5 mm.      CS.  12.5. mmm

Others you can find online! 

[PeterC65]

A couple of nights back I managed an EAA session with the Canon EOS 1100D camera and a Tamron 70-300mm zoom lens that I already had.

Normally I use the Player One Uranus-C camera and a telescope for EAA so this was a test of both the DSLR and a camera lens for a wider field of view.

The camera was quite frustrating. It needed a longer exposure time than the Uranus-C, 15s instead of 4s, and each frame was taking 15s to download, so I was getting two frame a minute instead of 15. Given the much bigger size of the DLSR and the shutter clicking away all evening, I much prefer the Uranus-C and will probably not use the DSLR again for EAA.

The camera lens test was more positive. There was some star distortion around the edges but it was perfectly acceptable for EAA. Focusing was a challenge as the control was very sensitive. I did manage to make use of a miniature Bahtinov mask but just the tiniest of movement took a star from one side of focus to the other. I expect this would be a feature of any camera lens used for astronomy.

I found myself mostly using the lens at 300mm which is the equivalent of 150mm if I had been using the Uranus-C camera. This gave me a 4.3° field of view which was plenty for any of the bigger objects. I had been thinking that a wider field of view might be useful so I zoomed out to 70mm which gave me an 18.1° field of view, equivalent to 35mm if I had been using the Uranus-C camera. The Milky Way was above the horizon, only just, but I couldn't see anything. Overall I came away thinking that around a 5° field of view, or 130mm with the Uranus-C camera, was all that I needed.

So I'm back to considering guide scopes.

The options seem to be:

• William Optics 32mm Uniguide which is the low cost option but there might be quite a bit of distortion and no way to use the filter wheel.
• Askar FMA135 which is a proper Apo with a built in flattener so the view should be sharp, and as it needs 55mm of back focus I can use it with my filter wheel.
• Canon DLSR lens adaptor with one or more used lenses which would give versatility (I can get an 18-135mm zoom and then there is the Samyang 135mm) but may not be as good optically as the Askar and there is no way to use the filter wheel.

I'm currently favouring the Askar.

 

[Elp]

No filter wheel but I use both types of ZWO eos adaptors with the Canon Samyang 135 and can screw in whatever filters I want to use at the time.

Another option for you is the Skywatcher 50ED guidescope. Having used a 30mm and 50mm guidescope, the aperture increase is recommended for light gathering capability.

[PeterC65]

10 hours ago, Elp said:

No filter wheel but I use both types of ZWO eos adaptors with the Canon Samyang 135 and can screw in whatever filters I want to use at the time.

Another option for you is the Skywatcher 50ED guidescope. Having used a 30mm and 50mm guidescope, the aperture increase is recommended for light gathering capability.

I could use filters with Uniguide and DSLR options but the DSLR option would need 2" filters and I have 1.25", all installed in filter wheels which I find very convenient.

I've been considering aperture. A DSLR zoom lens actually has a smaller aperture than the 30mm guide scopes at 135mm and less, so the only option with a larger aperture is the Samyang 135mm DSLR lens. I did consider 50mm aperture guide scopes but these are all 200mm in focal length and so don't give the field of view that I'm looking for.