the focal length of the telescope

[hubble space telescope]

hello

im thinking maybe to buy a new telescope for astrophotography for deep sky object.

i already have the cpc 1100 gps xlt schmidt cassegrain telescope of celestron with 2800 focal length and people said to me that this telescope is not good for strophotography for deep sky object, because of its big focal length, but i see on the internet Ritchey-Chretien Astrograph telescope with 2000 focal length, so i dont understand why this telescope good for astrophotography with big focal length and my schmidt cassegrain telescope with 2800 focal length is not good. the difference between the focal length are not so big, so why the Ritchey-Chretien Astrograph telescope is good for astrophotography and my schmidt cassegrain telescope is not so good, despite the small difference between the focal length.

thank you!           

[pete_l]

Hello and welcome!

Your telescope is of a type called a "fork mount". This means that the tube (the "telescope") moves in 2 directions - either parallel to the ground: left and right or up and down. With those two motions, it can point at any part of the sky. And with its computer and motors it can follow the path of any object as it crosses the sky during the night.

But there is a problem. When a deep sky object (or the Moon or a planet) rises, it has a specific orientation: the face of the Moon is a good example. But as it crosses the sky, it also rotates. An equatorial mount is set up at an angle so that it will naturally follow this rotation of objects across the sky. But a fork-mounted telescope cannot do this on its own.

You would need to set your telescope up at an angle - just like equatorial mounts are. The simplest way to do this is to buy a "wedge". Don't try to shorten some of the legs on the tripod, the telescope will fall over.

 

As for the focal length, you can easily image some objects at  that focal length, but the longer the focal length, the greater the apparent magnification. That makes objects appear larger in your photos. But it also magnifies any inaccuracies in tracking those objects across the sky. No mechanical system is perfect - they all have variations, manufacturing inaccuracies, vibrations and movement. The greater the magnification the larger these will be. More expensive mounts and ones designed specifically for imaging, are built to finer engineering tolerances and therefore have fewer problems.

It isn't the focal length itself that causes issues. it is the combination of a long focal length with a mass-produced mount that was not designed with the accuracy needed for imaging. With a wedge and a focal reducer (to make the focal length shorter) these problems can be reduced to some extent. There will be a cost and some inevitable compromises. But you can image with your telescope - but you will have to choose your targets carefully and spend a lot of time setting it up and aligning it.

[vlaiv]

It's not the focal length per se, but how it plays with other aspects of imaging rig that determines if scope is "better" or "worse" for AP.

Focal length is important part of aperture / resolution equation, depending on sensor used, it dictates resolution of imaging (arcsec / pixel scale). This has impact on mount / guiding precision and also is affected by seeing. You need good skies and good mount to go below 1"/pixel scale. It is also impacted by aperture of the scope - you want as much light collecting area as possible for given resolution / scale.

It also plays part in useful imaging field size - this depends on sensor size, but also on optical characteristics of telescope - will the off axis aberrations inherent in any scope design show, and to which extent on your images (coma, astigmatism, field curvature, etc ...)

So you need a good balance of all of the above in your scope / camera / mount combination.

So if you want to use scope with big FL, you will need camera with big pixel size to achieve meaningful resolution. For 2800mm, if you aim for example 1"/pixel or a bit less, you will need camera with pixel size of around 14um, or more realistically something around 7um and then bin it 2x2. Cameras with ~7um pixels tend to have large chips (simply because of resolution x pixel size = size of chip), so you need to ensure that scope has decent star shapes over large chip.

[ollypenrice]

There is nothing wrong with a long focal length but there are some things to remember.

1) The camera's pixel size needs to be matched to that long focal length. Here is a free calculator to find your image scale in arcseconds per pixel. http://www.12dstring.me.uk/fovcalc.php

The small pixels of modern DSLRs are simply too small to work well at this focal length. A monochrome CCD can be binned 2X2 and may have large pixels anyway, making it possible to get a workable scale. (A Canon 1100D would be working at 0.38"per pixel at F10 or 0.61" per pixel at F6.3.) The F10 pixel scale is never going to work. 0.61"PP might just about be possible but we come to points 2 and 3...

2) The local seeing (stability of the image as allowed by the atmosphere) needs to be excellent.

3) The guiding needs to be excellent. (The RMS value in arcseconds needs to be about half the pixel scale. An absolutlely excellent mount can often keep to an RMS of about 1/3 arcsecond but don't count on getting that out of a budget mount. We have two Mesus here which can manage that. I personally would not try to image at that focal length on a Celestron/Meade fork mount even with wedge.

If guiding and seeing don't let you resolve to the pixel scale of the system you still get a picture but it won't have more real detail than you'd get at a shorter focal length and a shorter FL would give you a wider field of view.

The standard Meade and Celestron SCTs (rather than the Edge/ACF) don't give very good star shapes over larger chips even with the reducer-flattener.

Olly

Edit. I crossed with Pete and Vlaiv but we are all saying the same things.

 

[hubble space telescope]

are you suggest that is easier if i will buy a new telescope with german equatorial mount?

[Galen Gilmore]

Just now, hubble space telescope said:

are you suggest that is easier if i will buy a new telescope with german equatorial mount?

*Disclaimer* I'm NOT an expert at this

Well if you can afford to get a shorter focal length telescope and a good GEM, then sure go for it. But as others have said, you can get a wedge and focal reducer to fix the problem to an extent. Although if your serious about imaging then a GEM would be a good purchase in the future.

[hubble space telescope]

and the wedge is really worth it and make a difference it is really good?

[ollypenrice]

I know a lot of people, including myself, who found that the wedge was an expensive road to nowhere. I know another set of people who made the wedge-and-fork work but not so well that they stuck with it for all that long. And then I know an even smaller set of people who work permanently with the fork and wedge. So clearly it is not impossible, but nor is it a trouble free road. I took that road once. Never again. But that's just a personal choice. When I imaged at 2.4 metres focal length it was on a Mesu 200 mount. If I ever do so again, it will be on a Mesu 200 mount!

Olly

[AKB]

10 minutes ago, ollypenrice said:

I took that road once. Never again.

So, in fact, do ALL roads lead (eventually) to a Mesu 200 and the largest refractor(s) with the shortest FL(s) you can imagine? 

[ollypenrice]

24 minutes ago, AKB said:

So, in fact, do ALL roads lead (eventually) to a Mesu 200 and the largest refractor(s) with the shortest FL(s) you can imagine? 

 

No. The Mesu isn't the only good GEM. There are also, at a price, non-GEMs which are much to be desired!

Lots of people do great images with reflectors and I was happy with the M51 above with a Dall-Kirkham reflector. I cannot, however, recommend dealing with the manufacturer of the scope in question. 

Very good images are taken with Celestron Edge and Meade ACF SCTs when the camera has been matched to the optics.

The other way to get into high resolution imaging is to use a long (not short) focal length refractor with a small pixel camera. Below we have about a metre FL (TEC140) working at 0.89"PP (Atik 460.) I think you would beat this with a larger reflector of high quality but it would need to be a good one and well fettled. The devil is in the detail. The ease is in using refractors!

Olly

PS Check out Sara's recent posts regarding the use of a large (by refractor standards) refractor rather than a straightforwardly large reflector. I don't think there's a 'right answer' but a bit of back-and-forth on the question is surely interesting.

[AKB]

6 minutes ago, ollypenrice said:

non-GEMs which are much to be desired!

That's the road I took...

[ollypenrice]

1 minute ago, AKB said:

That's the road I took...

Lucas Mesu initially took that road as well, with a roller drive single arm (fork-like) flipless mount which he no longer offers. I'd rather like to give one a try!

Olly

[Alien 13]

Interesting topic, I would suggest imaging at 1.5 - 2 meters is never going to be easy with any mount the fact that a lot are sold with a fork mount makes the choice kind of biased initially. I would have thought that the fork with wedge would be lovely with short focal lengths.

As a bit of a puzzle is something like a Star adventurer a GEM or a modified fork on a wedge?

Alan

[pete_l]

6 hours ago, hubble space telescope said:

and the wedge is really worth it and make a difference it is really good?

At the risk of dragging this conversation back onto the original topic, I would not say that a wedge and a fork mounted telescope is the best option. It is usable and at $400 is a cheaper option than buying a complete new telescope.

However there are limitations.

First is the alignment process for setting up your telescope so that it tracks the movement of stars correctly. This requires you to point the telescope at the pole, so that the eyepiece (or camera) is in the space between the bottom of the telescope and the circular base. This makes it difficult to have your camera installed when performing this alignment procedure, since there is very little space between the base and the bottom of the telescope,

Second, if you wish to image anything near the southern horizon (assuming you are in the northern hemisphere), the angle that the base of the mount is at meas that it stops the telescope from being lower than a certain point. That limitation will depend on your latitude.

 

And finally, a point that is common to almost all telescope types, no matter what sort of mount you have. It is not possible for the mechanical system that drives the mount to keep it perfectly in sync. with the stars as they move. So you will need a second, smaller, telescope attached to your mount to "track" this movement. Will will require a second camera and a connection to tracking software on your computer. The computer will also have to be connected to the mount to send it commands that keep it pointing exactly at the point you are imaging.

[ollypenrice]

Very good points from Pete, above. When I was using a fork and wedge my camera was small enough to fit in the gap so I'd forgotten that many won't.

I think an off axis guider would be best with an SCT. It will 'see' and correct for any mirror movement and the pixel scale mismatch between a small guidescope and a C11 might be a problem as well.

Olly

[Merlin66]

As Olly and others already know, I have used a series of Meade SCT's (8",10" and 12") on a cut down HD tripod and a rebuilt HD wedge at f10 for spectroscopy.

when properly balanced, the guiding was very good. 

It was only due to the limited access behind the OTA that made me move to a C11 on the NEQ6pro mount.

 

[Rodd]

I was in the same dilemma--I had a 14" Meade (it was an RCX--optically the same as the ACF, so optically it was OK), but it was on a fork mount.   I decided to forgo purchasing a $600 wedge do to some of the reasons mentioned.  Also, the 14" was a bear to set up alone--putting it on a wedge would have been even harder.  And I had never imaged before.  I was looking at a long, arduous road that could very well have resulted me having to change routes anyway.  So I sold the scope and bought a 4" refractor and a GEM.  I am so happy I decided to go that route as an inexperienced imager.  I have since added an 11" Edge but on the same GEM.  Part of the weight problem of the 14" was the fork system--the OTA and forks could not be separated.  Together they weighed allot.  My 11" Edge OTA only weights 28 pounds.   That's the beauty of a GEM--you can switch out OTAs as you please (as long as they do not exceed the mounts capacity).  With a fork mount, your limited to the OTA that came with it (I saw a video a guy made showing how he seperated the fork mount from the OTA and mounted the OTA on a GEM--but it was a HUGE ordeal).  If you get a mount with a 40-50 pound imaging capacity, you can use it for quite a variety of scopes of varying focal lengths and focal ratios.

Rodd

 

[Merlin66]

Rodd,

De-forking a Meade OTA is actually very straight forward.....I've successfully de-forked the 8", 10" and the 12" with no issues. In fact I have a write up somewhere of the procedure.

 

[Rodd]

25 minutes ago, Merlin66 said:

Rodd,

De-forking a Meade OTA is actually very straight forward.....I've successfully de-forked the 8", 10" and the 12" with no issues. In fact I have a write up somewhere of the procedure.

 

Well--for someone who has never imaged, I would hesitate to recommend it.  All the electronics have to be removed and a whole new focusing system has to be installed.  Then you have holes in the OTA to deal with.  I guess what is a huge task for one is a minimal task for another.  I think most would fall into the former category however.  I had the ACF 14" and when I inquired a person that did it showed me a video/write up.  heck, maybe it was you.  Put simply, it was enough to lead me to another route.

Rodd

[Merlin66]

50 minutes ago, Rodd said:

 All the electronics have to be removed and a whole new focusing system has to be installed.

Hmmm.........

Robb,

I'm not sure we are talking about the same telescopes....

On the Meade 8", 10", 12" LX200 SCT's I did there was no electronics involved to be removed and the existing rear cell focus arrangement was untouched. No residual holes in the OTA either.

Ken

 

[Rodd]

17 minutes ago, Merlin66 said:

Hmmm.........

Robb,

I'm not sure we are talking about the same telescopes....

On the Meade 8", 10", 12" LX200 SCT's I did there was no electronics involved to be removed and the existing rear cell focus arrangement was untouched. No residual holes in the OTA either.

Ken

 

The ACF/RCX must be different.  The only way to focus them is electronically with a hand controller (at least mine).  Everything was electronically controlled through the forks (Right fork I think).  Anyway, for good or ill I am glad I did not pursue that adventure. (with teh right tools I am sure it would be fun.)

Rodd

[Merlin66]

OK.

The RCX was something else(??!!!).

Discontinued by Meade after only a couple of years in 2007.

[hubble space telescope]

On 23.7.2017 at 00:02, pete_l said:

At the risk of dragging this conversation back onto the original topic, I would not say that a wedge and a fork mounted telescope is the best option. It is usable and at $400 is a cheaper option than buying a complete new telescope.

However there are limitations.

First is the alignment process for setting up your telescope so that it tracks the movement of stars correctly. This requires you to point the telescope at the pole, so that the eyepiece (or camera) is in the space between the bottom of the telescope and the circular base. This makes it difficult to have your camera installed when performing this alignment procedure, since there is very little space between the base and the bottom of the telescope,

Second, if you wish to image anything near the southern horizon (assuming you are in the northern hemisphere), the angle that the base of the mount is at meas that it stops the telescope from being lower than a certain point. That limitation will depend on your latitude.

 

And finally, a point that is common to almost all telescope types, no matter what sort of mount you have. It is not possible for the mechanical system that drives the mount to keep it perfectly in sync. with the stars as they move. So you will need a second, smaller, telescope attached to your mount to "track" this movement. Will will require a second camera and a connection to tracking software on your computer. The computer will also have to be connected to the mount to send it commands that keep it pointing exactly at the point you are imaging.

you said that the wedge as a limitation for stars and object that close to the horizon, but if i will wait that the star will move from the horizon? of i will start to image it before the horizon (assuming that the time and the weather are good for me)? 

[Merlin66]

Only affects objects very low in the south near the meridian.....

[hubble space telescope]

so this is not a big problem?