Schmidt Cassegrain, AP (latitude), collimation...

[Perene]

Guys, I have some questions about the Schmidt Cassegrain telescopes...

Do they all need collimation after a while? How much time do we have to wait before doing? I can't find any tutorial explaining this process. How is it compared to newtonian telescopes?

Now about astrophotography, I heard some people complaining about their cities, latitude, the SC could not do any AP if it was below 25º, something like that... What is this problem, exactly? It seems to be related with the mount.

And how much better this kind of telescope would be compared to a dobsonian 254mm? Specially if you are planning to do AP?

[CodnorPaul]

All SCTs will require collimation after some time but they do hold it very well so it is very rare it needs to be done.  Additionally despite the rumours it is very simple to do as you only have to align one mirror, and if you replace the screws with Bobs Knobs so you don't have to point a screwdriver near the corrector plate it makes it even easier.  I have only had to do mine once since I had it about 18 months ago, and that was because I installed Bobs Knobs.  Just make small turns and keep looking through the EP, then make sure it is tight enough but not overtight, and job done - I check it every time I set up.

Re AP at below 25 degrees, well that it not the OTAs fauly - likely the mount as you say.  Is it better than a dob for AP - yes - but that is mainly due to the mount.  A large reflector on an EQ mount for AP is a large animal and will be prone to being affected by wind and flexure.  A large SCT on an EQ mount is much easier to handle but has a longer focal length which needs to be taken into account.

[Perene]

I found a link about the collimation process:

http://astro-tom.com/telescopes/schmidt_cassegrain.htm

About the latitude thing, it seems the mount is not up to the task depending on your latitude. 

A possible buyer from a product said on the comment section it is on the latitude 12º 59' 0" s / 38º 31' 0" and the store replied the equatorial mount for the MEADE LX90 ACF 203mm is not working (inclining?) for his latitude. They said to be "working on it", whatever that means.

Is there a workaround in such cases? I thought all mounts were supposed to be working no matter where you are.

[Davey-T]

Had my SCT for years and it's never needed collimating, some mounts don't have enough downward movement in the dec adjustment to polar align from low latitude (near the equator) don't know if you can just lower the front, north facing tripod leg.

Dave

[Northern Soul man]

Hi Perene, a very warm welcome to the SGL, I have a 280 mm SCT, but mainly used for visual work on the rock solid dual fork mount - the CPC 1100 - I must say that its a very capable telescope for visual, due to its long focal length it works very well on the Moon/Planets - I've done a little web cam imaging with the scope using a Phillips Toucam web cam and had some nice results with it - the dual fork mount is rock steady and ideal for imaging the Planets, but if you want to do some deep sky Photography, the long focal length will not work very well on its current Alt Az mount.  Alignment and tracking are very good, placing each target in the FOV of a low power EP with tracking once aligned on the Planets accurate and central in the FOV.  

I've settled on mainly visual Astronomy with a little web cam imaging with a x2 barlow and processing the frames from a 2 or 3 minute video in Registax software - that's about as much as I have achieved with such a small amount of money - if you want to get into long exposure deep sky imaging - you will have to spend a lot of money with dedicated cameras/CCD's - a very expensive mount will be needed to achieve longer exposure times and a lot of time spent at the computer processing the images. Trying to achieve "round" stars in all corners of the image, for some imagers, really does take a lot of time and effort as well as having to "cut" through the light pollution.

Paul.

[Owmuchonomy]

First, collimating. As Dave says, SCTs are very stable. Mine is 6 years old and is still perfectly collimated. Secondly, for AP of DSOs you should consider a quality EQ mount. To be honest, an SCT is best suited to solar system due to the FL/FOV. Check my Flickr link for examples of AP with a 9.25 SCT (and a 102 frac).

[Perene]

Wait, wait, stop the presses: I thought all Schmidt Cassegrains already had the proper mount for DSOs/AP!

So basically you are saying the mount that comes with a LX90 ACF for example is not the same as the SkyWatcher Az-Eq6 GoTo?

I need to detach the OTA from the mount that comes with it and attach in the EQ6 or perhaps even the EQ8? Doing that not only for AP (DSOs) but to solve all latitude issues?

Is this what you are saying it needs to be done?

This telescope with its mount is very expensive already.

Even more expensive than the EQ6 mount.

In here, a 254 (10) Dobsonian costs US$ 989.

The LX90 ACF (203), US$ 3.585.

The EQ6, US$ 2.423. EQ8, US$ 4.777. 

My goal was to do AP with DSOs, not only planets and the Moon.

[ChrisLX200]

Collimation is stable in SCTs unless you transport it around in which case you need to check it frequently (check it - not necessarily adjust it). It's best to collimate it with the adjustment screws turned in as far as possible - yet still able to offer the necessary adjustment. That gives the secondary the most support. Bouncing it around in the back of a truck tends to knock collimation out...

For imaging the SCT suffers from a variety of issues - especially if using the fork mount. It isn't the best tracking of mounts (as supplied with mass produced SCTs anyway), and it requires a wedge to point the polar axis at the pole in order to avoid field rotation. Accurately polar aligning a fork-mounted SCT is a challenge in itself. Your average SCT also has a long focal length and this immediately places high demands on whatever mount is used, be it a GEM or a fork, so you will be requiring top-class tracking performance from a mount that was not designed for it. Yes it can be done, but it is a struggle with plenty of pitfalls along the way. Lots of bits can move within the SCT's optical tube in unexpected ways that can ruin an image even when it is mounted on the best of GEM mounts. I removed my 10" SCT from its fork and mounted the OTA on my GEM, it's a bit easier to control but the OTA still needed extensive mods to improve stability. It would definitely not be my first choice for a DSO imaging scope unless I needed both the aperture and long focal length.

ChrisH

Edit: I note you are in Brazil? In which case the wedge angle necessary to polar align is even more of a problem, these fork mounts don't like working at low latitudes much, they work better with the weight pressing down into the base rather than hanging out the side. If you see what I mean...

[Cornelius Varley]

Wait, wait, stop the presses: I thought all Schmidt Cassegrains already had the proper mount for DSOs/AP!

So basically you are saying the mount that comes with a LX90 ACF for example is not the same as the SkyWatcher Az-Eq6 GoTo?

I need to detach the OTA from the mount that comes with it and attach in the EQ6 or perhaps even the EQ8? Doing that not only for AP (DSOs) but to solve all latitude issues?

Is this what you are saying it needs to be done?

This telescope with its mount is very expensive already.

Even more expensive than the EQ6 mount.

In here, a 254 (10) Dobsonian costs US$ 989.

The LX90 ACF (203), US$ 3.585.

The EQ6, US$ 2.423. EQ8, US$ 4.777. 

My goal was to do AP with DSOs, not only planets and the Moon.

The Meade LX90 ACF is supplied with an alt-azimuth fork mount which is unsuitable for DSO imaging. For that you will need to obtain the optional equatorial wedge for the mount which will allow the mount to be correctly polar aligned. 

[Davey-T]

Yes get an EQ6 or even a EQ5 and a SW ED80.

Dave

[Owmuchonomy]

Wait, wait, stop the presses: I thought all Schmidt Cassegrains already had the proper mount for DSOs/AP!

So basically you are saying the mount that comes with a LX90 ACF for example is not the same as the SkyWatcher Az-Eq6 GoTo?

I need to detach the OTA from the mount that comes with it and attach in the EQ6 or perhaps even the EQ8? Doing that not only for AP (DSOs) but to solve all latitude issues?

Is this what you are saying it needs to be done?

This telescope with its mount is very expensive already.

Even more expensive than the EQ6 mount.

My SGL colleagues have beaten me to it to cover off this one.

In here, a 254 (10) Dobsonian costs US$ 989.

The LX90 ACF (203), US$ 3.585.

The EQ6, US$ 2.423. EQ8, US$ 4.777. 

My goal was to do AP with DSOs, not only planets and the Moon.

[Tim]

The wedges for fork mounts are a compromise, and not a great one. In a lot of cases people who have gone down that route often upgrade to a 'proper' EQ mount.

There are packages with SCT and mounts together from most dealers

[Northern Soul man]

Hi Perene, as mentioned above all the different set ups can be bought separately - not sure of the availability where you live, but, to a certain extent you can mix and match to achieve what you want - the SCT telescopes can be bought in 2 parts, the Optical Tube Assembly (OTA) and the Mount.  I think Celestron supply the tube assemblies on their own (not sure if Meade do this), you just have to attach a rail to the underneath of the tube so you can attach it to your mount, the EQ mount will be the best for you if your going to do AP, with the SCT Tube the EP comes out of the back - this helps when moving the EQ mount around to the different parts of the sky - but as said - the SCT Tubes have very long focal lengths - fine for Planet/Moon imaging, but not so good for DSO imaging - you have a very narrow field of view, with a lot of the DSO's just too big - but for imaging with a web cam (a very small chip) tracking must be very accurate, the Alt AZ Mount on my SCT (CPC 1100) works very well for Moon/Planetary imaging and I can usually get 3 - 5 minutes of data to process later.

You need to think which is going to be correct for you - as there are many different types of set up for you to choose from.

Paul.

[Davey-T]

As Paul says, you need to decide what you're most interested in doing, to do everything possible you need a very expensive mount and several different scopes and cameras.

Dave

[Northern Soul man]

Nice 1 Dave - thats probably the best "1 liner" on here

As Paul says, you need to decide what you're most interested in doing, to do everything possible you need a very expensive mount and several different scopes and cameras.

Dave

It needs to be tattooed on the front of every primary mirror of every scope on here mate - of course "BACK TO FRONT AND UPSIDE DOWN!!!!!"

Paul.

[Perene]

Edit: I note you are in Brazil? In which case the wedge angle necessary to polar align is even more of a problem, these fork mounts don't like working at low latitudes much, they work better with the weight pressing down into the base rather than hanging out the side. If you see what I mean...

My coordinates are 8.0475458 S , 34.876962100000014 W.

I heard the SkyWatcher Explorer 200p with the EQ5 mount has the same issue mentioned in this thread, a person asked if there are any issues with the 15º latitude. The store said you need to remove some sort of plate from it and prevent the tripod from bumping by putting a shorter counter-weight, something like that... these were not the exact those words, but I understand some sort of adjustment needs to be done.

Is this a common issue with these mounts? I didn't expect the same response for another model.

[Perene]

About the SkyWatcher Explorer 200p with the EQ5 mount, for the record:

The problem with the low latitudes can be fixed by purchasing an item called... pillar extension tube.

The store explained in 2 videos how can this item fix the problem. I am glad this issue can be solved without any trouble, because it put me off completely from buying a new telescope.