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Maybe another stupid question.......


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Hi All,

I have an meade SCT main scope, and have bought a small 66mm refractor to piggyback, the small scope will be used for guiding mainly, but also for wide field imaging.

I am using fixed rings for the guidescope, by that I mean they are skywatcher clamshell type with no adjustment like you would have on the guidescope rings with the three adjustment screws.

So,when fixed onto my ADM rail the guidescope is, or should be perfectly parallel with the main scope.

So because there is around 8 inches between the centre of my main objective and the piggybacked objective, will objects that are in the centre of my main scope after a goto command, be in the centre of the piggybacked scope?

Will the fact that they are looking at areas of sky 8 inches apart make a big difference when objects are millions of miles away. As if you move a scope 8 inches in one direction, it would be miles out.

I need to be able to use the goto's on the main scope to image the object with the small scope.

Hope that makes sense to you, it does to me, but not sure I have written very well.

My main scope (normally f10) will be used with a 6.3 focal reducer, @1260mm focal length, and the small scope is an f5.9 388mm scope.

Look forward to your replys

AB

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The 8 inches will make no difference for the reason you have stated - i.e. you are viewing objects many millions of miles away.  The scopes need to be approximately parallel to view the same targets, but you can easily check this in the daytime on a distant target - a tree or telegraph pole.  The field of view will of course be different for each scope.

Chris 

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As said by Chris it will make no difference.

Consider the situation of polar alignment, you can do that with absolutely no change immaterial of winter or summer, and the change of distance concerned then is the diameter of the Earths orbit, which seems to be about 300 million Km.

So if 300 million Km makes no difference if you scale down to 200mm it really would be unmeasureable.

You are not the first person to ask, think this is about the 4th I have read. At first the question makes sense it is not until you start taking into account the effectively huge change the earth makes second by second, and certainly over a year, when it becomes a bit more apparent that a vertical or sideways change is in effect negliable.

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No questions are stupid, even if someone thinks they are, but not every question to oneself can be fathomed or answered by yourself, and a little encouragement from someone else often helps settles/answer your questions.


You did answer the question, and members have concurred, based on what you described, it  will  make no difference.
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Thanks for the reply

So to get this straight in my head

The two scopes are perfectly parallel (I have checked this)

If I do a goto on say M42 and centre it in the main scope then sync, you are saying it WILL be in the centre of the piggybacked imaging scope, ready for imaging, even though the two scopes are looking at sky 200mm apart??

Have I got that right

Sorry to be a complete idiot, I am just getting it straight in my head.

Don't answer too quickly, as there will more than likely be another stupid question on its way!, Lol

:)

AB

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Probably not. When you say they are parallel, how are you measuring this? 

As said above, the only way to be sure is to use guidescope rings so you can adjust where your scope is pointed.

For guiding, I dont think it matters. I use mine, and I look at completely different stars with the scope compared to what I am imaging, but so long as I can see something, the guiding works. 

Widefield imaging, you will usually be close, and can then adjust your mount to center the smaller scope, unless you want to image on both at the same time?

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It really is finicky getting them lined up, they could be ok, but chances are there not and won't even be in the field of view, when i first set mine up i used M31 which is quite bright and big in a guide scope then moved on to bright stars, once it was set-up doing PA with the Synscan handset its a dream, watch the PC screen and align the chosen star then rinse and repeat on a different star, its the only time i found doing PA a pleasure...

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If they are absolutely parallel, and 200 apart, the pics will be objectively looking at a point 200 mm apart on the target.

Since the best you can do on the closest object (the moon) is about 1 pixel per 50 metres on your sensor, I guess your pics won't be significantly off from each other.

When you get to a star at minimum 5 light years (60 trillion mile approx) I guess 200 mm offset is not going to affect your picture a lot.

There's of  course another thing to think of, The difference in the F numbers will give you a field that is wider or narrower than the other scope.

I don't  doubt this will obviate any percieved offset to the point where only Hubble could spot the difference and only then if your sensor was a 400 terrapixels and hence more than 200 mm radius.

Enjoy your AP and please don't worry too much about being a squidgeon off sometimes.

It's not rocket science..... or is it ? :confused::undecided:

Edited by Carina Lass
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