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Astrophotography with DSLR - focussing problems.

Tinlad

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Tinlad

Tinlad

Posted
Posted

Hi, first post here.

Recently I (well, my dad) have bought a Sky-watcher Explorer 130PM (130mm Newtonian). We're both new to astronomy and been having great fun so far on the few clear nights we've had. I'm a second year Physics with Theoretical Astrophysics student, so I've finally been able to but some of what I've learned into practice. :(

I'll get to the point now.

I've got a Canon 350D DSLR (Digital Rebel XT in the US) and saw that it's possible to mount this to the 'scope. The telescope has the 42mm T-mount thread integrated into the focuser, so all I've had to buy is the Canon EOS adapter. I bought this one: Link

It fits OK, but I cannot focus on the sky. The focuser doesn't go far enough 'in'. I've seen that extension tubes are available, but wouldn't they be used when the focuser couldn't go far enough 'out' to achieve focus? I'm a bit confused. Any help would be appreciated. Sorry if this has been asked many times before.

Cheers,

Guy

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Paxo

Paxo

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Guy,

The usual trick when you do not have enough 'in-travel' is to use a barlow lens, this allows focus to be reached. It unfortunately means there is extra glass and magnification brought into the equation. I hope this is of help, others may be able to ofer other solutions.

Cheers.

Steve..

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jimlennie

jimlennie

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Thanks for the reply Steve.

The 'scope came with a barlow, so I'll probably buy a T-mount adapter and give it a go.

Unless, as you said, anyone else has another solution?

Cheers.

Guy,

Another option is to fit your scope with a low profile focusser which should allow your camera to achieve focus as it will bring the chip on the camera closer to the diagonal mirror. Another, more drastic, method would be to move the primary mirror up the tube towards the diagonal by the amount of extra inwards travel you need. If you only need say 12mm or so then you wont lose any light transmitted to the diagonal from the primary mirror due to the now shallower light cone ( field illumination ).

Hope this helps, I'm new to this photography malarkey as well !!

Jim

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Sam

Sam

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Guy,

You have another option and that's to move the primary mirror forward a few centimetres. It's not for the faint hearted but very much possible - moving it just a small amount will not mess up the optics either. I moved my mirror on my 8" reflector up about 3cm and haven't looked back.

Sam

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Tinlad

Tinlad

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Thanks for the suggestions everyone. Some of them sound a little bit scary to me, but I'll look into them. :(

@thing: Yes, that's the Mandelbrot set. My avatar was generated by a primitive little PHP script I wrote, but I've also written a C program that can produce huge, smooth shaded images. It's still pretty primitive, but the vast speed increase over PHP makes it capable of more. I think the Mandelbrot set is pretty amazing (I'd probably go as far as saying it's beautiful).

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Tinlad

Tinlad

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Jim: I'm trying to use the prime-focus method, so there is no lens on my camera to manually focus! When I said 'focus on the sky', I meant that I couldn't focus on any distant object.

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Kaptain Klevtsov

Kaptain Klevtsov

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If you have the Skywatcher 2X Barlow that comes with most Skywatcher 'scopes, that already has a T thread on the back I think. Screw the T ring on that and you should be in business.

Plan B is to move the primary mirror up the tube by using the collimating screws, but make sure you know how to collimate it as it'll need doing when you're finished. When I moved mine I took the precaution of unscrewing the screws another two turns to make sure that they weren't about to fall out at the end of the thread, then put them back again to make sure they were secure when I'd finished.

Kaptain Klevtsov

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  • 2 weeks later...
Vega

Vega

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I believe the 1.25" eyepiece holder doubles up as a low profile adaptor that screws in to the Canon t-ring. I'm not sure if the 130PM comes with this kind of eyepiece holder but here's some snaps of mine...

slr%20connection1.jpg

You unscrew half of it like this...

slr%20connection2.jpg

The adaptor part is threaded to fit T-rings :( . Screw it to your Canon T-ring and it should look like this...

slr%20connection3.jpg

Here's it attached to my scope (sorry had no time, took the photo while scope tube stored in shed ;) )...

slr%20connection4.jpg

Hope this helps

Matt

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Vega

Vega

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You're dead right and its something which is troubling me at the moment Ant, I've been surfing the web for filter threaded low profile adaptors. I've even been looking for low profile, double threaded coma correctors which would solve two problems in one! No joy so far

Matt

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Vega

Vega

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Right.... where's my hack saw... NO ... where's my 240v jig-saw ;):(

Thanks Trev. I seem to remember that thread now you mention it. I do have one of those adaptors ready to be butcher'd

Matt

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Vega

Vega

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An update... I have done my butchering and so far so good. With the rain over the weekend, I just had to do something in the garage. The hack saw & jig saw combination seemed to work well on slicing off the threaded end of my SLR adaptor. I filed it down a bit ready for sticking on to the low profile adaptor. Unfortunatly they are the same diameter so I couldn't neatly slide one in to the other. However, I used some of that metal fusing compound stuff (with two tubes of gunk that you mix together for hardening). That should do the trick. I havent had the chance to test it 'proper' yet. The adaptor wont drop right down in to the scope hole due to an obstruction lower down but it goes far enough for the grub screws to lock it tight. In hindsight I could of hacked a little more off the threaded peice to make it more low profile. I should be able to achieve focus as it still holds the camera close to the secondary (but not as close as before). I can always move the primary mirror up a few mm via the collimation screws if needed.

Matt

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high25

high25

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Just a note : if u go with a low profile focuser (crayford type) to acheive more infocus you'll probably need to buy an extension tube to solve the lack of back focus for visual work, i had the same problem when using my Moonlite crayford. However its no big deal and focus extension tubes are plentifull and inexpensive.

Andy.

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Beyond_Vision

Beyond_Vision

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You have two choices as far as I can see.

1. A low profile focuser - FLO does some nice ones.

2. An internal filter (fits inside the camera), here a link to a CLS clip filter for the canon http://stargazerslounge.com/index.php/topic,23805.0.html

Ant

Heres a link to a review I did on the clip filter.

http://stargazerslounge.com/index.php/topic,24063.0.html

Regards

Kevin

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