Final advice needed for Celestron Nexstar 102 please

[Welrod50]

Folks, after much deliberating, research and bank balance consideration, I have decided to purchase a Celestron Nexstar 102 SLT GO-TO refractor. I have posted previously on some considerations/potential choices, but have now made up my mind and am asking a couple of Q's on a fresh post to hopefully gain a bit of renewed interest.

Having considered an EQ5 mount for my SW 130 Newt, a Nexstar 4se (with EQ wedge) and a number of other options somewhere inbetween I have finally decided to commit to the 102 SLT. It is portable, GO-TO, will track objects with a reasonable degree of accuracy and also wont cost the earth. (None of these apply to the SW 130 Newt). I intend to use it for planetary viewing (with Tal x2 barlow and SW 8-24mm zoom EP) and hopefully, as it will go in the car without causing me an injury, will be travelling to a couple of dark sky sites near me, hence I am happy to lose out on focal length (but gain an f6 optic as opposed to the 130's f7) as DSO's will be more accessible, a two fold advantage given that I will actually be somewhere other than the back garden!

It will also be used for AP too, with an unmodded Pentax Kx on the back. Therefore I am after an answer to the following please:

1. Will a Baader Semi APO filter reduce CA without affecting exposure times on the scope/camera?

2. Will the Kx focus ok using a 90 degree erect image diagonal, or will it be better to attach it directly to the OTA? Might this be preferable to using the EP as I do now?

3. Is there any merit in using a focal reducer? I understand that the Celestron isn't too slow already, but would this be of any benefit for AP on this particular 'scope for DSO work?

4. I understand that for serious AP, an EQ is viewed as a 'must have'. However, if I use 20-30 second subs and stack with DSS, will this software compensate for field rotation enough to allow me to use 20 to 30 images, or will it not cope with this degree of star rotation??

This last Q is just really to give me a head start, as with the Kx I'm pretty happy I could use shorter exposures/higher ISO if needed, or longer exposures with fewer subs.

Any ideas everyone?? Please forgive my apparent lack of warmth towards the EQ5, but I do want to be able to take the kit out in the car, and whilst it is possible I suppose with a mount that would be no bigger than the one I have now, the OTA and the need to mess about fixing it all up, and worrying about collimation when the 'scopes been in the car are off-putting.

Thank you and clear skies!!!

Scott.

[jgs001]

Scott... Hmm... tricky... the little NexStar SLT mount is pretty light and portable so would fulfill the grab and go aspects nicely. The views will be pretty nice I think.

Now... for AP...To be honest, the EQ mount you already have is probably going to be better for AP than the NexStar. The NexStar, being an AltAz mount, tracks the sky in steps (up/right) etc, rather than in a smooth arc like the EQ mount. That doesn't mean it won't work, but you will struggle with exposure times... Going from a max of 2 minutes low in the east/west, and decreasing down to 30 to 40 seconds in the North/south and overhead.

1) A Semi APO will help, but the brighter stars will still show CA

2) You don't want to be using diagonals for imaging, as they can introduce problems, you want to connect the camera direct to the back of the scope.

3) A focal reducer will provide a 'faster' system, allowing for more light in the same exposure, so yes, that would help for AltAz

4) DSS will compensate for field rotation effects between exposures. It cannot, however, deal with rotation effects within an exposure.

I hope that helps. I have some images I shot with a NexStar SLT with an ST80 clone on it, with a Semi APO filter I can dig out if you want.

If you're looking for a portable imaging mount, that's great for widefields (although you'll be using camera lenses) take a look at http://www.firstlightoptics.com/astrotrac/astrotrac-tt320x-ag.html as an alternate option, although it's a bit more expensive. The tracking is amazing.

[Capricorn]

The 102 will show CA, it is on an Alt/Az mount so DSO long exposures are not appropriate, 30 seconds is long. Hanging a DSLR off the end is a problem, the scope is still 600mm and you have a weight at the end of the arm, you might be able to rebalance to improve things.

Since a semi-apo filter cuts out light it is bound to effect the exposure times, they also do not remove CA just (hopefully) reduce CA. So you have CA and longer exposures.

hence I am happy to lose out on focal length (but gain an f6 optic as opposed to the 130's f7) as DSO's will be more accessible, a two fold advantage given that I will actually be somewhere other than the back garden!

This is meaningless unless it refers soley to prime focus AP. if you are talking visual then with the same magnification the f/7 will be brighter.

The 102 is f/6.5 not f/6 so you are not gaing very much in terms of speed of scope.

A focal reducer may be no relevance but a field flattener may be necessary, may actually be essential at f/6 witrh DSLR.

If you want to get in to astrophotography then you need a decent EQ mount with dual motors and a small semi-apo refractor at least. You cannot build a semi apo scope from an achromat and filters, and an Alt/Az mount is not an equitorial

[dph1nm]

There was another thread about this scope recently. As I can never work out how to reference previous threads (!), I will roughly repeat what I said before.

I have a Nexstar 102 SLT which I use as my holiday scope for imaging. I added some skywatcher tube rings so that the balance can be adjusted by sliding the tube up much more than the tiny dovetail it comes with allows. I also fitted a skywatcher motor focuser. The rings have the advantage of having a camera thread on them so you can just mount your camera + lens and do wide field shots. Yes, it is an alt-az mount, so with a dslr you are limited to about 20-30s sub-exposures for imaging (this is not due to field rotation but to the accuracy of the tracking!), but it works fine for most Messier objects provided you don't want high resolution.

I have some images here: http://www.dur.ac.uk...o/slt_canon.php

Look for the ones labelled SLT, these were taken on the alt-az mount (the other are with the tube on an HEQ5). I use a LP filter but not a CA filter.

NigelM

[Welrod50]

There was another thread about this scope recently. As I can never work out how to reference previous threads (!), I will roughly repeat what I said before.

I have a Nexstar 102 SLT which I use as my holiday scope for imaging. I added some skywatcher tube rings so that the balance can be adjusted by sliding the tube up much more than the tiny dovetail it comes with allows. I also fitted a skywatcher motor focuser. The rings have the advantage of having a camera thread on them so you can just mount your camera + lens and do wide field shots. Yes, it is an alt-az mount, so with a dslr you are limited to about 20-30s sub-exposures for imaging (this is not due to field rotation but to the accuracy of the tracking!), but it works fine for most Messier objects provided you don't want high resolution.

I have some images here: http://www.dur.ac.uk...o/slt_canon.php

Look for the ones labelled SLT, these were taken on the alt-az mount (the other are with the tube on an HEQ5). I use a LP filter but not a CA filter.

NigelM

Thanks guys,

Firstly John, I didn't explain myself very well originally... the mount I currently have is a manual EQ2, sturdy enough but not capable of auto tracking. As for the advice on DSS, thanks that has put my mind at ease somewhat, as my subs will be 30 seconds or less definitely.

NigelM, yes thanks - I did try to search posts before but could not find what I was after, maybe I did n't look well enough

This is precisely the kind of stuff I am after capturing myself - they are very good!

May I ask, can you put the tube rings onto the dovetail directly? Also prime focus - will the camera fit directly onto the back of the OTA?

Sorry for the silly questions

Thanks again

Scott.

ps - Capricorn, thanks for the input re the APO filter. I know it wont be the same as having an expensive OTA, but if it helps then that's what I'm after really. As for field flattener, what benefit might I expect from it, as I hadn't really thought that I may need one to be fair?

[jgs001]

Scott.. understood... If you're interested in wide field imaging, then you could put a motor drive on the EQ2 and use camera lenses... at short focal lengths, you'll get much longer subs than 30 seconds.

I'd suggest a medium length dovetail and tube rings for the flexibility. You mount the rings to that dovetail, then the dovetail can slide in the mount, and the tube can slide in the rings for better balance... you want to balance slightly tail heavy.

You'll need a T ring and an appropriate push fit for the rear of the tube. I don't know if it's 1.25" or 2" on that scope. If 1.25" you will get vignetting.

[Welrod50]

Scott.. understood... If you're interested in wide field imaging, then you could put a motor drive on the EQ2 and use camera lenses... at short focal lengths, you'll get much longer subs than 30 seconds.

I'd suggest a medium length dovetail and tube rings for the flexibility. You mount the rings to that dovetail, then the dovetail can slide in the mount, and the tube can slide in the rings for better balance... you want to balance slightly tail heavy.

You'll need a T ring and an appropriate push fit for the rear of the tube. I don't know if it's 1.25" or 2" on that scope. If 1.25" you will get vignetting.

Right. Dyu know I did consider upgrading my current mount or even trying a EQ5 GO TO mount, but the weight and lack of transportability put me off. That's why I've gone for the smaller 102 SLT. I'm pretty conversant with star charts now and have located quite a few objects manually since getting my SW 130 Newt. I 'saw' M1 the other night. I knew I was in the right spot and after trying really hard, could just about make out a smudge ( a very small smudge). Therefore the GO-TO capability is really only for basic AP as I quite enjoy the tracking down side to things

The 102 SLT is indeed a 2" focusser with 1.25" adapter. I have an M42 threaded T adapter already. Will this fit a 2" push fit for the OTA or will I need another T adapter do you reckon??

I did think about AP through the eyepiece as I do now, but you have advised against it as it will cause problems. May I ask, what issues this would cause? Just for curiosity really, as I do think that prime focus is the way forward - something I cannot do on my 130 Newt !

Thank you again

Scott.

[jgs001]

The logic makes sense to me. As for 'seeing' M1 that sounds very familiar... I 'saw' it in much the same way .

You'll need one of

http://www.firstlightoptics.com/adaptors/flo-2-inch-t-mount-camera-adapter.html

and almost certainly one of

http://www.firstlightoptics.com/adaptors/two-inch-focus-extension-tubes.html

too. although, it may be that

http://www.firstlightoptics.com/adaptors/skywatcher-2-inch-t-adapter.html

will suffice, as it looks like an extention tube with t thread... Best to check with FLO.

But that should be about all you need. The 2" fittings give enough room to avoid vignetting with an APS-C camera.

As for EP projection... I tried it and found it very very hard... Prime focus, is to my mind, by far and away the best option.

[dph1nm]

The scope focuses my Canon1000D just fine with just a 2" Canon-mount adapter - you don't need an extension tube.

I should have said that I used a medium length Skywatcher dovetail as well as the rings - this whole arrangement slides nicely over the fixed dovetail on the tube. The only slight downside is that you have to bolt on one of the rings after you have slid the new dovetail onto the mount, otherwise the bolts get in the way (and the reverse to remove).

NigelM