Prime focus photography on Celestron C8n

[Daniel]

Hi all, well after sorting out my GOTO woes (a fully charged power tank did the trick) I finally got round to trying some prime focus photography, and tonight for the first time, I've managed to capture nebulosity, which was just incredible, even though the image was pretty crappy to be honest.

One problem i've encountered however is that my Canon EOS 40D seems unable to focus on anything unless I use a 2X barlow, meaning that Im unable to fit much into the viewfinder and my images are coming out quite blurry, is there any way around this?

Anyway, apart from my slightly blurry images and the fact I can't really see anything but the brightest objects in the viewfinder, Im hooked, ordered an electric focuser today which should be with me by the morning.

Oh and one last thing, any tips on what to do about condensation? my scope looked like it had been in a sauna after tonights session!

[MarkS]

It's a very common problem with Newtonians - I had exactly the same problem on my Celestron SLT130 - there's not enough inward travel on the focuser to bring the image to focus on the CCD.  I never really found a satisfactory solution - I used a barlow like you have.

[MarkS]

Blur could be many things. What shutter speed were you using?  What were you imaging?

Possible causes:
1) Camera is not at focus.  Was it initially focused properly?  Did the focuser subsequently hold it very firm?
2) The camera shutter (mainly mirror flop) can cause the whole telescope to shake - especially at high magnifications.  This is not noticeable at very fast or very slow exposures.  If your camera has a mirror lockup mode then certainly use it.  Are you using a remote control or self timer?  This prevents human induced vibrations!
3) Mount tracking errors.  This will affect long exposures, turning stars into streaks.
4) Atmospheric turbulence (technically called "seeing").  This turns stars and planetary features very soft especially for longer exposures.

You asked about condensation.  You had condensation?  I had an iced up scope last night!  Condensation is quite usual and is no problem - dry your scope out when you get indoors.  However,  mist on you mirrors will degrade the image - this is not so common with Newtonians.  If it is, you need to use a hairdryer every so often or buy a dedicated telescope dew heater kit. 

[JohnP]

Daniel, Inward focus travel can be an issue - Is there anything you can do to get the CCD chip closer to the secondary - maybe there is a different adapter you can use etc to mount camera to scope that is lower profile... How far off is the focus...? If you can't do anything the only option is a Barlow which as you have already discovered changes FOV & also makes scope slower which means exposures need to be longer or I think you can move the primary mirror up the tube (i.e. closer to secondary) - but this is a bit daunting on a new scope etc...

John

[Ian]

just a note of caution about moving the primary on a newt. The secondary should be sized to give full illumination (lets not get into a definition about what that is) at the focus and no bigger. If you move the primary mirror up the tube you are obviously moving it closer to secondary. The cone of light from the mirror is larger in cross section at that point, consequently the secondary, if it was the right size before is, is now too small and this will cause vignetting.

But, moving the mirror an inch or two is unlikely to make a noticeable difference. Just don't go moving it a foot or two

Another approach, which is the one I took was to change the focusser. My newt was equipped with a 1.25" chinese focusser, complete with molasses for lubricant which I replaced with an JMI low profile Crayford. For normal observing I have to use an extension tube, but if I want to use my camera or binoviewers on it, I gain 2.5" of in travel just by removing the extension tube. It also gave me an opportunity to get a motorised 2" focusser.

You pays your money...

[JohnP]

Yep - thks for pointing that out Ian - I was only thinking a 1/4 or 1/2 inch at most... like you say anymore than that & you'll have to think about getting a different sized secondary. In fact depending on how far off focus is & the design of the collimation screws for the primary he may be able to recolloimate the scope & move the mirror up 1/4 inch or so with what is already there.... Just a thought...

John

[Mike]

4) .....You had condensation?  I had an iced up scope last night!  Condensation is quite usual and is no problem - dry your scope out when you get indoors.  ....

Make sure you don't move your telescope from a warm room to the cold outdoors, especialy if it's a night with high humidity. If at all possible keep your gear in a garage or shed were it can be kept at pretty much ambient temperature all year round. Cooling a scope down to the same temperature as it is outside can take many hours if it is kept in a warm house. However, a Newt should only take 30-60 mins max.

What I do is if I see it is going to be a clear night for imaging is I take the gear outside in their boxes as soon as I get home from work, and leave them there for an hour or so whilst I get changed, have dinner, etc. and by the time I go outside to image they have cooled down to ambient.

[Daniel]

Firstly, wow, thanks for all the advice guy's, looks like i'll be stuck with using the barlow for now, since i don't like the idea of playing round with the mirrors just yet, maybe after i've collimated a few times, i'll be brave enough too (which admitedly is something i probably need to do soon)

I had a little ice last night, fortunately not on my scope but on the counterweights

I installed my electric focuser this morning and i must say it seems really good, haven't had the chance to road test it yet, but the fact that when i turn the speed down to slowest i can't tell the knob is turning with the naked eye must be a good sign. so now hopefully i can focus in on things a little better without the subject juddering in and out of shot when i do.

also I've been reading up on different ways of focusing and it looks like i may be able to use my camera's live view function to get quite a precise focus which i shall try as soon as the weathers with us again.

Anyway, again, thanks for all the advice


Daniel
:O)