HOWTO mount DSLR + lens for astrophotography?

[SimM]

I want to start astrophotography as simply as possible using an HEQ5 equatorial mount. From reading about imaging, the complexity increases with longer focal lengths (so walk before run etc.):

1. DSLR camera and wide/normal lens (easiest)
2. DSLR with small/long telephoto
3. Small refractor with 1 1/4" T mount
4. SCT C8 with T Mount + camera mount e.g. for Canon/Nikon
5. SCT with afocal mounted camera lens (hardest)

To start with option 1 and later progress to option 2, I looked at the Celestron Piggyback Camera Mount for SCTs (93609) - what could be simpler?

can see some problems with this "solution":

• Mechanism is plastic - so prone to breaking
• Camera or lens mount point may not be perfectly balanced
• Put's lot of stress on the mounting holes and/or knocks collimation out
• Carrying the weight of an SCT OTA (10kg + not using it) for a camera/lens weighing just under 2 Kg

So wouldn't a better alternative be to remove the OTA and use a 2nd vixen dovetail to attach the lens mount point or the camera mounting point or lens mount point? To balance the declination a small counterweight would be required e.g. probably less than one of the 5Kg supplied weights.

Option 1 and 2 is (for me) most likely to succeed whereas I can imagine that options 4 and 5 and very difficult to get right?

Simon

[JamesF]

The simplest option is probably as you say to fit the camera to a new dovetail and put the camera on the mount using the dovetail.  Or where the lens is suitable, use a mount for the lens fixed to a dovetail.  If fitting the camera direct to the dovetail is awkward, putting the camera on a ball head and then putting the ball head on the dovetail would probably work.  You might find that the extended counterweight bar is sufficient by itself to balance the camera, but some people have used wrist or ankle weights for counterweights when they didn't need very much.  When on holiday I have even used three cans of coke (other sickly fizzy drinks are available) taped together around the counterweight bar

It is possible to get mounting brackets to fit to the larger SCTs that allow further accessories to be mounted on them (large finder scopes, for instance).  That would make it possible to piggy-back the camera on the SCT, but quite possibly might entail you having to drill holes in the SCT to fit the brackets.  You're probably pushing the limits of the mount for imaging if you have the SCT, camera and other gubbins all mounted up at the same time however, whereas with the camera-only option you leave yourself with the opportunity to add a guide scope and other toys at a later date without really stressing the mount at all.

If you get to the stage of wanting to put the camera on the back of a telescope, you'll probably find that most will allow you to connect it using a T2 adapter rather than using a 1.25" fitting.  The former is likely to give a much more rigid connection.

James

[SimM]

Thanks James. I talked through the options with David Hines today. They think the plastic camera mount is quite strong, won't affect the collimation and would be OK. I think they are too optimistic... my original idea was a dovetail e.g. using a strong construction.

My choices really are firstly to use a 21cm Sky-Watcher Vixen dovetail that I have, and mount the lens bracket to the dovetail using either a tripod bolt or take the lens bracket apart and mount it directly to the dovetail. This is an alternative to the OTA being mounted.

The DSLR is small/lightweight and the lens is long/heavy - so the original balance point (on film camera) is upset. For this reason I don't think a lightweight ball head would work. I also want to keep the Len facing forward to avoid "cone" errors e.g. inline with the declination.

Another choice is to buy a longer 33cm Vixen dovetail e.g. the same as comes with the EdgeHD 800 (which I get tomorrow - pretty exciting stuff). This can be mounted on the other side of the OTA. The 800 can be supplied with Vixen or Losmandy that fix at either end of the OTA and so are considered a secure mount. I have one with a Vixen dovetail. It has been suggested that it's only possible to fit a Vixen on the top as the bolt holes are different - I would be surprised if this were true. Even so if I get a Vixen bar, I can then attach the camera/lens and later a small scope etc. This allows the OTA to be retained.

I agree that the 2" SCT is preferred way of connecting with the back - I may try this but for now I think it's not an easy solution to get right.

Using the dovetails in two ways is complicated because using the small dovetail to support only the camera/lens without the OTA allows it to be balanced moving the dovetail back/forwards. With the camera/lens mounted on the dovetail on top of the OTA gives no adjustment - so it either means selecting the right point to attach it (for different camera/lens) or moving the whole OTA to get a balance. Either options may require some additional support under the lens to add support or for the bracket to be moved backwards. There are camera sliders with camera bolts available - they do the job but are about £80+

The book I'm reading suggests mastering a small lens, moving to a bigger lens and then a small refractor before considering the SCT. Otherwise there are too many things to get right... absolute polar alignment, drift, guiding etc.

If I give the impression that everything has to be perfect or that I'm "nuts" about imaging, that's not quite how it is. I'm interested but not (so far) hooked on imaging. At the movement my interest is visual observing. My SCT weighs about 6.5Kg so well within the spec for my mount which is 15Kg for visual (11Kg for imaging).

Simon

PS with all the "shiny" stuff that I've bought, I really need to "tone it down". I think I might invest in some beat up cans of coke or a smelly ankle weight so these suggestions of yours are apt! I'm going to be upgrading my existing "cra**y" eyepieces for something a bit better with long eye relief - so it will be a while before I contemplate (and reject) doing too much imaging stuff. 

Edited January 15, 2020 by SimM

[rickwayne]

If you're not running off mains power, another counterweight suggestion is simply batteries.

A dedicated dovetail for your camera+lens is a great way to start -- that way it's easy to swap out when you want to do visual, and as you point out it's easier to balance. You also get the benefit of your mount's electronics for finding targets. If you edge deeper into the deep-sky waters, you can get a USB-to-serial adapter and control the mount with a computer, possibly the camera too (depending on what DSLR you have). That opens up a whole new range of possibilities, such as using plate solving to radically simplify pointing at targets too dim to make out visually.