Help required for beginner who wishes to use 8" telescope for terrestrial observations

[Interactive]

Sorry to come straight to the point but action on my part is required immediately.

I would like to have a telescope which could be used for astrophotography, particularly of the planets, and hopefully of nebulae as well, but my urgent requirement is to have a very powerful lens for a camera to make a terrestrial observation of targets at about  1km, 2 km, 4km, and 8km distances, each of which will have an angular size of about 0.01 degrees. As I explain below I believe that if I want to have the targets form a 100 pixel high image in a photograph this requires a telescope/camera combination with a focal length of about 3.8m.

I believe I can get a camera/telescope combination to achieve this focal length with a Meade 8" SCT as shown below in the pictures and here  https://www.ebay.co.uk/itm/8-Meade-SCT-Telescope-Motorised-EQ5-Mount/303633495209?hash=item46b1f774a9:g:cw0AAOSwMmRfHGE2#viTabs_0

The telescope, which has a focal length of 2m is not sufficient by itself, and would be acting as a prime lens i.e. the camera replaces the eyepiece of telescope and would be at the primary focus of the telescope,  so I would need a 2x adaptor between the scope and the camera to get the focal length up to 4m. Is this a viable option?

I would like to make  a number of terrestrial observations with his telescope in addition to the one described, perhaps nature observations, and I think I read on some website that this sort of telescope would NOT be suitable of nature observation, could anyone tell me why? Is it

1) The default 2000mm focal length is too high? So everything is magnified too much? Can one not put a reducing eyepiece on it to halve the focal length?

2) The f/10 focal ratio is too small?

3) It is too bulky/heavy/ difficult to handle

4) Is there a focusing issue? Can this telescope be focused at the distances I described above and could I focus on objects as close as 100m?

Additionally, the telescope I am considering comes with a motorised equatorial mount, does this make it difficult to use for nature observations where the telescope would be near horizontal and where I would not be using the motor?

Is there a better way to achieve the terrestrial observation with a different method, perhaps just using a horrible Canon P1000 camera which has a focal length of 3000mm?

Thanks in advance for any assistance, and sorry for the urgency in my questions.

 

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Here is my calculation for determining a required focal length of the telescope for target of 5 feet high at 5.5 miles and I wish the target to form at least 100 pixels in an image taken by a camera.

Specifically there will be a 5 feet high target at 5.5 miles (8800m), so size of target in metres is 5*12*2.54/100 which needs to be divided by distance to the target to get the angular size in radians, and finally multiply by 180/Pi to get degrees, so that gives
angular size = 60*2.54/100 / 8800 * 180 / Pi = 0.01 degrees
The above calculation does not feature in my calculation below but the angular size I am dealing with is useful to consider.

I wish the target to form at least 100 pixels pixel height in an image, and so if using a full size 24mm high sensor in a Canon DSLR camera with the ability to output images of 3700 pixels in height, I believe that gives a magnification (the ratio of image height to target height) of magnification
mag = 100 / SensorPixels * SensorY / TargetHeight ,
where
100 = desired pixel height of target in final image
SensorPixels = 3700, the number of pixels vertically in the sensor
SensorY = 24mm, the size of sensor in mm vertically
TargetHeight = 1.5m

This gives a magnification
mag = 0.0004
or 1/2312

Using the lens maker equation 1/f= 1/u+1/v,
where
f = focal length
u = image distance,
v = object distance,
and using that magnification is the ratio of the u and v distances, and v = 8800m, therefore knowing that the magnification is mag = u/v, so u = v * mag
1/f = 1/(v * mag)+1/v
that gives the focal length f
f = 1/ ( 1/(v * mag)+1/v)
putting in the numbers
= 1/ ( 1/(8800 * 0.0004)+1/8800)
= 3.8m

 

Edited August 1, 2020 by Interactive

[Cosmic Geoff]

I think, with respect to the terrestrial use, you are proposing to do something which will work on paper, but not in the real world with a real atmosphere between telescope and target.   In theory, you could fit a Barlow lens to the Meade and get a 4m focal length. However, I have performed imaging of low altitude planets with an 8" SCT and found that the atmosphere is the limiting factor, and that adding a x2 Barlow to the imaging train is not helpful.    Many amateur astronomers find that there are nights in which reducing the aperture of their telescope improves the image.  In daylight, the warmth of the Sun will make matters worse than at night.   Single images will, I anticipate, be completely useless for your scheme, and you should consider the 'lucky imaging' video method used by planetary astro imagers to reduce the effects of atmospheric shimmer and distortion.

I have never heard of an 8" spotting scope, only a 5" (the Celestron C5) I wonder why that is?

re the mount you cite, it is not ideal for terrestial use, but you would be able to swivel the Meade's diagonal to get an upright image.  A SCT with its large focal range should be able to focus on relatively close objects.  But really you want a smaller telescope on a manual alt-azimuth mount.

[Peter Drew]

A 8" SCT would be your best bet but it would have the seeing restrictions terrestrially as explained by Cosmic Geoff.  I have, in the past provided 8" SCT's specifically for long range terrestrial use, it's quite doable provided that the atmosphere is suitably steady.  We often, for fun, use our 16" terrestrially and can recognise people at a range of 10k.

The air is usually fairly steady early morning and evening.  You would ideally need an alt-azimuth mount or an equatorial that could be configured to alt-azimuth.  Slow motions, either manual or powered would be essential.      🙂

[Interactive]

Comsic Geoff,

Thank you so much for your reply.

I am familiar with the issues of looking over long distances such as 5.5 miles, or 8km, horizontally, and you are absolutely correct that the atmosphere will do terrible things for much of the time, and perhaps I hadn't made clear that I was willing to put up with lots of horrible distortion of the image at the extreme distances, I just needed to be able to identify the target. I suspect that your low altitude observations of planets would be much worse because you would be looking through far more atmosphere than 5.5 miles and you would be doing so through turbulence etc.

I think you are right about the mount, you have made me realise that fiddling with an equatorial mount for terrestrial observations would be difficult. I had vaguely thought about the issues, but as I am not familiar with telescopes etc and eq mounts I hadn't considered all the issues and I can see your points. 

I think I will go for a Celestron SE6 SCT with a 1500mm focal length with an altazimuth mount to achieve what I want, and use the adaptor, the Barlow you mentioned,  to get near the 4m I was aiming for.

So thank you so much for enlightening me.

Edited August 1, 2020 by Interactive

[Interactive]

14 minutes ago, Peter Drew said:

A 8" SCT would be your best bet but it would have the seeing restrictions terrestrially as explained by Cosmic Geoff.  I have, in the past provided 8" SCT's specifically for long range terrestrial use, it's quite doable provided that the atmosphere is suitably steady.  We often, for fun, use our 16" terrestrially and can recognise people at a range of 10k.

The air is usually fairly steady early morning and evening.  You would ideally need an alt-azimuth mount or an equatorial that could be configured to alt-azimuth.  Slow motions, either manual or powered would be essential.      🙂

Thank you so much for your answer.

Yes I hadn't fully appreciate the need for a SLOW motion to manipulate the telescope, because of course I haven't handled high magnification optics before. But I now appreciate it after having watched some videos of hi-magnification telescopes in action.

Wow! So you can do 10k terrestrial observations in good conditions, fabulous. Sometimes the atmosphere can cooperate. That does sound like lots of fun. Presumably doing birdwatching at 10k for flying birds might be a bit tricky, but then I suppose if they don't move across the field of view, then you could.

[RT65CB-SWL]

Hi @Interactive and welcome to SGL. 

Below few images of my alt-az mounts for you to mull/think over...
 <---(left) AOK-AYO and (right) Tele-Optic Giro GR-DXll 

 <--- (Left) Celestron C6/SCT-xlt and (right) Meade ETX105

I also have this alt-az mount... 

Above is a Universal Astronomics DwarfStar... apparently it can hold a maximum payload  of 8kgs., (though I have not attempted this).

 

And if you have/anyone has not seen this...

 

[Interactive]

Wow! Those mounts are gorgeous. Really gorgeous! Beautifully elegant yet robust.

I don't think any of them are motorised, is that a conscious decision? I appreciate that motorised altazimuth mounts aren't terrible useful in astrophotgraphy because of field rotation.

[Tenor Viol]

I suspect one of the reasons people say 'avoid' is the inverted image you get with an astronomical telescope.

[RT65CB-SWL]

2 hours ago, Interactive said:

Wow! Those mounts are gorgeous. Really gorgeous! Beautifully elegant yet robust.

I don't think any of them are motorised, is that a conscious decision? I appreciate that motorised altazimuth mounts aren't terrible useful in astrophotgraphy because of field rotation.

Yes, it was a conscious decision! - I live in a small apartment and storage space, power outlets, etc., is limited.
The plus side is that in the event of a power outage, I can still use my 'scopes.

Two versions of the AOK-AYO mount are available using encoders and 'Argo Navis'TM digital circle system... link here: http://www.aokswiss.ch/ayo/ 

It is also possible to retro-fit the Giro mount with encoders and 'Argo Navis'TM as well - though I am not sure how up to date this link is...
http://www.wildcard-innovations.com/kits/pn-gr2-bspec_describe.html 

Edited August 1, 2020 by Philip R