Dob to SCT - what to expect ?

[BRUN]

Ive decided to sell my dob to get something more portable, im hoping to get a Nexstar 8 with star sense, but as the dob was my first scope ive no experience of anything else

Due to price, ill have to drop down from a 10" to an 8", but apart from the aperature what other differences should i expect, ill only ever be observing, planets interest me most, i dont plan on doing any photography

Im also considering going to an EQ mount as they seem cheaper and able to handle heavier OTA's, how long does it take to set one up

Thanks

[nightfisher]

Some people adore the 8" sct, i myself dont care for them as i find a Maksutov better on contrast, each to there own.

on planets i dont think you will trade a big drop going down to 8", re an EQ mount....manual or goto?

[Stu]

First thing is the longer focal ratio. SCTs are generally f10 so you will have a longer focal length, 2032mm on an 8" vs perhaps 1200mm on your 10"? This means a narrower field of view and each eyepiece you have will give a much higher mag.

Cool down time will be longer too I should think, it can take a long time for an SCT to acclimatise and for tube currents to stop. Some form of active cooling is worth considering, a CatCooler for instance can really help.

Dew will also be more of a problem. The front corrector plate means you will definitely need a few shield and quite possibly a few heater too, depending upon what your conditions are like. I live near a lot of water so few is often bad.

The benefits are obviously the more compact and portable tube which mean it is easier to get to dark sites. For visual planetary use something like an EQ5 with dual axis motors would be useful to provide tracking, although an AltAz such as a Mini Ercole would also work well, but without providing tracking.

I found a C8 to be surprisingly effective on DSOs at a dark site despite its narrower field, most DSOs will fit in the fov with a widefield long focal length eyepiece.

[BRUN]

8 minutes ago, nightfisher said:

Some people adore the 8" sct, i myself dont care for them as i find a Maksutov better on contrast, each to there own.

on planets i dont think you will trade a big drop going down to 8", re an EQ mount....manual or goto?

it would definitely be goto, id be lost without it

[nightfisher]

right, set up time should be 5 to 10 mins including doing an align

[alanjgreen]

Look for a second hand cpc800, the mount is rock solid with dual mounting bars. The 8se single side mount can be wind affected, the CPC is not.

i would not go EQ, stick with AZ, you know where the eyepiece will be with AZ.

buy a nice chair/stool, and you can sit and view for hours with the CPC AZ setup.

it separates in two parts and weight is not a problem.

ive owned both cpc800 and cpc1100. They are great scopes. You will need a decent capacity battery and a dew shield as a minimum.

i found that focusing was easy with the 800 but I needed to upgrade to two speed focuser with the 1100.

collimating is only needed rarely with an sct so will be much easier than the dob.

i had starsense on my cpc and it worked well. 

Alan

[BRUN]

Thanks @Stu

id hope cool down time would be negliagle as is likely to be in my garage which is nearly as cold as outside

your comments on FOV etc, does that mean id need to look at specific EP's ? I was hoping to build up some ES 82's. ive got a 24mm 68 degree at the moment and this was my first 'proper' EP

[BRUN]

10 minutes ago, nightfisher said:

right, set up time should be 5 to 10 mins including doing an align

thanks, thats not too bad really i suppose, if i can get something easier though, i will lol

[John]

I went the other way a few years ago. SCT's are compact though - I think the Nexstar 8 is about as compact and portable as an 8" scope can be.

With the positions of the planets currently and over the next couple of years I'd not want to specialise in them but an SCT or refractor can seem to get to low targets more easily than dobs can.

Your current eyepieces should be fine but most of the time about 8mm will be the shortest focal length you need.

Don't forget collimation - SCT's hold it well but it's imprtant that they are accurately collimated to get good planetary performance.

Plan to use a dew shield and maybe an heated dew band right from the off - the glass at the top end of SCT's and mak's is a "dew magnet".

 

[Stu]

Just now, BRUN said:

Thanks @Stu

id hope cool down time would be negliagle as is likely to be in my garage which is nearly as cold as outside

your comments on FOV etc, does that mean id need to look at specific EP's ? I was hoping to build up some ES 82's. ive got a 24mm 68 degree at the moment and this was my first 'proper' EP

Storing in a cool dry garage will be ideal, cooldown time should be minimised.

The 24mm 68 degree will be quite nice but I found say a 30mm 82 degree will give a significantly wider field, if you have  2" diagonal and visual back. There may be some vignetting due to the baffle tube aperture but I find it generally a non issue visually.

The smaller circle in this chart is the 24mm 68, the larger is a 30mm 82.

[BRUN]

thanks @alanjgreen ive actually spotted a CPC925 which im interested in, i suppose if i added starsense to that id be good to go

[nightfisher]

Yes a dew shield is a must on an SCT, i use a dew shield on the Mak and if i am careful i can get a good session with out to much problem

 

[ollypenrice]

I agree with Stu and with those who suggest Alt Az rather than EQ. EQ is a pest for visual observing, you have the meridian flip to complicate things, odd EP positions, a big variation in EP height, the need to polar align - and all this for a feature you don't need in visual, which is no field rotation.

I have deforked an SCT in the past and regretted it. I now have one on an Alt Az fork and love it. EQ mounts are for imaging, in my opinion.

Don't underestimate dew! A passive system like a long camping mat dewsheild won't suffice in the UK. You'll need a heated band on occasion.

Olly

[BRUN]

i have my own battery pack which i made, hopefully that will suffice power wise

and yes for me i think an alt az mount might be better, i know very little about this hobby really, i just want something i can put down and start observing asap, im quite short on time too, i get up early in the mornings so time at night is very limited

[nightfisher]

I fully agree with olly, re the Alt az mount, i have an AZ goto on a pier and it is a joy to use, also will be okay for a bit of Luna and planet imaging

[BRUN]

@Stu would 1.25" ep's be ok ?

i dont think id switch to 2" due to cost, unless cheaper EP's than the ES's would be an option ?

[BRUN]

id also just like to thank everyone for the replies, without this forum id probably never even own a scope, so many helpful replies in such a short space of time

[Cosmic Geoff]

The C8 is a good scope for looking at planets, and also DSO's. Most DSO's will fit in the field of view, especially the fainter galaxies. I can confirm that the C8 SE + Starsense works well and cuts out the annoying set-up stage of leveling the tripod and re-fixing the OTA/mount on it.

The 8" should not be noticeably inferior to a 10" for looking at planets - in many seeing conditions it may be better. BTW if you are interested in the fainter or more awkward to spot planets, or asteroids, then you will want GoTo.

Don't buy an equatorial mount if you are not interested in astrophotography and only interested in visual - use an alt-az and preferably a GoTo as supplied in one of the standard packages. It'll be lighter and easier to manage.  The Nexstar C8 SE is the most compact and portable mid-sized scope outfit you are going to find.   If you can afford it, consider one of the more upmarket versions such as the CPC800 - it might make for a more pleasant user experience. I use a low stool for observing, with the tripod legs fully retracted.

You will need at least a dew shield. I made one out of cardboard, cost £0.00.  I had to collimate my C8 when I got it, but it's been OK since.

[BRUN]

Thanks @Cosmic Geoff

[Knighty2112]

I use normally an 18mm X-Cel  EP in my C8 SCT for most things except planetary and lunar views, where I'll up it to either a 10mm or 6mm occasionally. The 18mm EP gives 112x magnification in the SCT, with a very nice FOV, and with my goto mount I get 95% objects in this FOV  easily when slewing to them. Not had any dewing issues with the scope as I use an astrozap dew shield, but need to get something for my Telrad finder for when the colder nights come in. I find the scope cools down pretty quickly, not as quick as a refractor no, but easily within 30mins normally for me anyway, especially as it is kept in a cool room when stored.

[alanjgreen]

1 hour ago, BRUN said:

thanks @alanjgreen ive actually spotted a CPC925 which im interested in, i suppose if i added starsense to that id be good to go

Watch the weight specs on the 925, it is much heavier than the 800 and almost as heavy as the 1100.

the CPC is well designed and the carrying handles are perfectly placed to make carrying easy.

the panoptic41 mm eyepiece is fantastic in an f10 sct but you need to upgrade standard 1.25" visual back up to a 2" and also upgrade to a 2" diagonal. 

If you buy second hand try to get a 2" visual back thrown in with the deal and a dew shield too!

[Stu]

1 hour ago, BRUN said:

@Stu would 1.25" ep's be ok ?

i dont think id switch to 2" due to cost, unless cheaper EP's than the ES's would be an option ?

1.25" eyepieces will be fine, the limiting factor will be field of view rather than quality. If you look at the image I posted you will see the inner circle which represents the 24mm 68 eyepiece - this will be the largest field you can get with a 1.25" setup in a C8 i.e. 0.8 degrees. With 2" you will be up at 1.2 or 1.3 degrees.

It does depend on what you would like to view, given its focal length, SCTs do tend to favour high power targets and smaller DSOs such as globs or planetary nebulae so a 1.25" would be fine.

[nightfisher]

1.25 is going to be ideal for a C8, you will want a 32mm for wider work 

[Stu]

Just now, nightfisher said:

1.25 is going to be ideal for a C8, you will want a 32mm for wider work 

24mm 68 degree gives the same fov Jules

[Luna-tic]

6 hours ago, BRUN said:

Thanks @Stu

id hope cool down time would be negliagle as is likely to be in my garage which is nearly as cold as outside

your comments on FOV etc, does that mean id need to look at specific EP's ? I was hoping to build up some ES 82's. ive got a 24mm 68 degree at the moment and this was my first 'proper' EP

As for FOV, go ahead and spend an extra $100 for a focal reducer/field flattener. f/10 is then reduced to f/6.3 and FOV opens up about 40%. It screws on the rear cell, and then the visual back fits to it. Back focus change is negligible, but be sure to measure your clearance to the mount if you aim straight up. No issue with a gem, but alt-az may be an issue (not sure for your choice). The reducer is about an inch thick. This gives a lot more viewing choices, wide-field for deep-sky, or remove it for high-magnification planetary. You can see it on my C6 (which I just sold, and bought a C8 Edge)