Skymax 90 + DLSR, can't reach focus.

[TractionMan]

The other day I decided to try to hook up my DLSR to my grab & go OTA, the Skywatcher Skymax 90, using the T2 ring that I bought a while ago. But I just couldn't reach focus with this combo (not a problem with my main scope though). Do I need an extension tube like this one: http://www.firstligh...nsion-tube.html ? Or anything else to solve the problem? Thought I'd use this scope both for daylight "digiscoping" as well as some basic white light solar imaging.

[pixelsaurus]

Before you buy an extension tube, you need to find the focus point of the scope. It may be that the image plane is not far enough along the drawtube for you to use a DSLR . The camera mount to sensor distance is usually around 45mm (depends on the camera make http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html ) and you need to factor this in. You only need an extension tube if you have to extend the drawtube distance. If the image plane falls short, you will have to optically transfer the image plane further down the drawtube or physically modify the focusser. I have the same problem with my 150mm Newtonian. Currently I can't put my DSLR at prime focus but have to use eyepiece projection.

[TractionMan]

Thanks for the reply. Much appreciated! From the link you provided I found out that the camera mount to sensor distance is 44.6mm (Minolta AF/Sony Alpha). But how do I determine the focus point of the telescope? When or if I can figure that out, how do I know which extension tube I need, lenght wise that is?

[JamesF]

It's more complicated than that with a Mak. Effectively you move the image plane to be where you need it (ie on the camera sensor) rather than moving the camera sensor to meet the fixed image plane as you would with a 'frac or newt.

I don't know about the 90, but I took this picture today to show someone else how I connect up my 450D and 127 Mak. The EOS<->T ring converter goes straight on the visual back. Does that help at all?

James

[TractionMan]

More complicated you say? Doesn't sound too good. Well I connected my DSLR to the Skymax 90 just as you've done with your setup. So in that sense you've helped me to confirm that I've done that part correctly at least.

[JamesF]

It's more complicated in the sense that with a 'frac or newt the primary is fixed and that fixes the position of the focal plane and with a Mak it isn't. So, to get an eyepiece or camera in focus with a 'frac or newt you move the eyepiece or camera back and forth until it reaches the focal plane. With a Mak you have the eyepiece or camera in a fixed position and you move the primary mirror back and forth, changing the position of the focal plane until it reaches the camera or eyepiece. But because the effective focal length of optical systems with multiple elements is dependent upon the distance between them and by moving the primary mirror you're changing the distance between the corrector and primary and between the primary and secondary, the focal length of the entire system will also change slightly.

If you've run the focuser all the way in both directions and can't find focus with the same arrangement as mine then perhaps the focal point can't be moved that close to the end of the telescope with the 90 Mak. Unless someone else with the same scope can suggest what sort of extension will work, I'd go for something that approximates the optical length of a diagonal, perhaps with a few short extensions for fine tuning. For example, I'd be tempted to get the entire set of the extensions you linked to. (It's probably also sensible to bear in mind that whilst I can achieve focus as in the image above, someone else with the same scope says they can't, so there may be differences in the same model of scope over time.)

There is probably a range of distances from the back of the scope in which you can get the camera to reach focus. What happens with my 127 though is that the further the camera is from the scope the larger the image and it doesn't take much for the Sun or Moon to become larger than the camera sensor, so you probably want to keep the extension as short as you can.

James

[TractionMan]

Once again thank you very much for trying to help me. Very good explanation regarding the focal point of the MAK vs Newtonian and Refractors. Always great to learn something new.

I have indeed run to focuser to and from both "end positions" without being able to reach focus on the sensor. Not sure if this is a good idea or not, but, I thought I could perhaps make a few stiff cardboard tubes of varying length (same as real extension tubes) just to try to find which tube(s) to order later. Although that might perhaps be a bit awkward. On the other hand, your suggestion about getting a tube of the approximate of the diagonal might be better though.

If this doesn't work, I guess I could always try to find an eyepiece with T2 threads and go down the afocal route...

[JamesF]

A cardboard tube or a piece of drain pipe should work ok for testing I'd have thought, but make sure you fix the camera to something just in case it falls. Diagonals vary in the length of the optical path, but with a 1.25" diagonal I'd expect it to be somewhere around 70 to 75mm. It's longer than you'd think. I'd probably start with something at about 50mm long and see if it will reach focus. If it does then I'd be tempted to repeatedly shorten it by 5mm and see if you can still reach focus. That way you should find something close to the minimum possible extension length, which is really the most desirable.

Using afocal imaging would almost certainly solve the problem, but can be tricky to get the camera, eyepiece and diagonal perfectly aligned with the optical axis. That means you can end up with part of the image out of focus. Getting everything to work at prime focus would definitely be a better solution if you can do it.

James

[jgs001]

A couple of things to try first... put an ep in the tube directly, can you get focus, then add a diagonal, and again, can you get focus. From my experience with a Skymax 102, I'd expect you to be able to achieve focus at both points. I think at that point, you should have sufficient range of travel in the focus mechanism for the camera... Try it during the day, and go slowly on the focuser, you can shoot through the focus point pretty quickly.

[TractionMan]

So my weekend project will be to make cardboard tubes and try to figure out the correct length I need to achieve focus.

Will try to put an EP in the tube instead of the diagonal as suggested by John (jgs001) as well to see what happens. Thanks for the tip!

[TractionMan]

Update! I finally got around to try to solve the problem a couple of weeks ago. Tried various lengths of cardboard tubes and discovered that I needed about 2cm extension to reach focus with this combo. So last week I got a proper T2 extension tube and it works just fine. Have only tried it during the day so far though.

[catalinc]

On 04/09/2012 at 01:21, JamesF said:

It's more complicated than that with a Mak. Effectively you move the image plane to be where you need it (ie on the camera sensor) rather than moving the camera sensor to meet the fixed image plane as you would with a 'frac or newt.

I don't know about the 90, but I took this picture today to show someone else how I connect up my 450D and 127 Mak. The EOS<->T ring converter goes straight on the visual back. Does that help at all?

James

Hello,

May I ask you, please, for some details about the motorization you added for the focuser?

I have a SkyMax 102 and I'm interested to do something similar for it.

Thanks in advance.

Catalin

[JamesF]

Ask away.  What details would you like?  The thread may be almost seven years old, but I'm still using pretty much the same setup on that OTA

James

[gonzalingui]

On 02/03/2013 at 12:58, TractionMan said:

Update! I finally got around to try to solve the problem a couple of weeks ago. Tried various lengths of cardboard tubes and discovered that I needed about 2cm extension to reach focus with this combo. So last week I got a proper T2 extension tube and it works just fine. Have only tried it during the day so far though.

Hey, sorry I'm bothering you on this old topic, but there is little information on the web about the SkyMax 90 & DSLR.

I've just bought that telescope (a newer version I suppose) and yesterday I've received the T2 extension tube with the camera adapter.

Moving the focus point of the telescope I can see the camera sensor is in the focus range, but in a weird way: Seeing trough the camera's viewfinder I can set the image on focus, but when taking the photo, the image isn't on focus. Did that happen to you?

Also, do you have any picture of the setup at that time when you reached a solution?

I hope that you read this and can help me. This is my first telescope, and I'm just starting with astrophotography.

Cheers 🥂