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Celestron 76mm FirstScope as a Finder Scope?

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#1 highertheflyer

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Posted 12 October 2010 - 11:31 PM

Been working on a 6" F/8 optical tube, with a Meade EQ post mount, and wish to use as a star party scope.
The mount has a 115V synchronous R/A drive and I'm attaching digital setting circles with a Sky Commander.
And here's a cute little Celestron 76mm FirstScope in a 50th year anniversary model.

Here's the link.
http://www.celestron....php?ProdID=568

It's just shy of an F/4 and wondered if anyone has taken the time to star test this little fellow?

I think it would make a delightfully lightweight little finder scope mounted with 3" rings on top of the 6" newt.
So has anyone here on this forum purchased one, and can advise if the optics are useful as a Star Party finder scope?
Thanks,
Jim

#2 avarakin

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Posted 13 October 2010 - 12:17 AM

I am thinking about the same thing.
Advantages are:
1. same image orientation as main scope
2. should have no dew issues
3. may also work as guiding scope and maybe even short FL imaging scope. It would require coma corrector, thus 2" focuser. It would look weird: 2" focuser on 3" scope.

There are also similar scopes from Orion I am thinking about: 114mm StarBlast and 100mm SkyScanner, both around f4 and very light. The only problem is they are little bit costly for just a finder...

Alex

#3 seryddwr

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Posted 13 October 2010 - 12:37 AM


1. same image orientation as main scope
2. should have no dew issues
3. may also work as guiding scope


My thinking exactly.

There are also similar scopes from Orion I am thinking about: 114mm StarBlast and 100mm SkyScanner, both around f4 and very light. The only problem is they are little bit costly for just a finder...


I think that I'll use a 114mm StarBlast as the finder for my 20". It weighs the same as the short tube 80 that I was going to use (Junior). Also, the 114mm appears to have a fully collimatable primary cell, the 100mm and the FirstScope do not. I saw this by looking at the PDF spec sheets for each and closely examining the pictures.

I'm working on rebuilding a FirstScope. The reason being that I couldn't collimate it properly. You can see my progress here. I'm just using mine for a finder scope, so I'm not worried about coma.

#4 Jon Isaacs

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Posted 13 October 2010 - 11:30 AM

Hi:

I made a finder out of the predecessor to the First Scope 76, the Tasco Rocket Scope. It was a mini-strut scope to fit my 16 inch Dobstuff Strut scope and mighty cute but now I am using a standard 9x50 finder.

I guess on axis the small newt might be brighter but they are pretty messy off-axis and typically the secondary is a compromise so you don't have the well illuminated, well corrected widefield of view that is most helpful in a finder. Not a good choice for spotting "non-stellar" objects.

Jon

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#5 Sarkikos

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Posted 13 October 2010 - 12:02 PM

Jim,

I wouldn't use the 76mm FirstScope as a finder on a bigger Newt, for these reasons:

(1) Same orientation as bigger scope. I think the finder should have the same orientation as the star atlas that you are using. Since these are correct image, I think the finder should be correct image also. I use a 70mm refractor with a RACI diagonal as the finder on my 10" Newt. It agrees with my star atlases, lunar atlases, and finder charts, making my observing sessions much easier and more productive.

(2) Spherical primary. You not only have coma and field curvature to deal with, but spherical aberration.

(3) Clear aperature is probably only about 50mm, not 76mm. The FirstScope has an undersized diagonal. That's to be expected, I suppose, in this size Newt with short f/l and spherical primary. But the result is a reduction in clear aperture to about 50mm, from what I have seen just looking in the focuser.

I think the best use for the FirstScope is as a hand-held grab-n-go scope. Put a GLP on it and you have a neat little point-and-look scope for quick walks outside, or for longer sit-downs to refresh your memory of the constellations and showcase objects. Unlike binos, you can look straight up at the zenith - where the sky is clearer - and not strain your neck. If you have a picnic table or some such, bring the scope out on its mini-Dob base and set it on the table. But I wouldn't use it as a finder on my bigger scopes. That's what little refractors with RACI diagnoals are for.

Mike

#6 seryddwr

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Posted 13 October 2010 - 01:28 PM

But the image in a newtonian *is* correct, just upside down. So turn the atlas over...

#7 Jon Isaacs

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Posted 13 October 2010 - 01:29 PM

The FirstScope has an undersized diagonal.


Are you sure? I had one and I believe I verified that the diagonal was large enough to fully illuminate the center of the field of view.

The Tasco 76mm optics I used required super low profile, I just used a sleeve and a set screw, slip and slide focusing, and let the eyepiece intrude into the light path if necessary.

Jon

#8 seryddwr

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Posted 13 October 2010 - 01:48 PM

I was worried about the diagonal being undersized, too. That's in part why I went with a lower profile focuser.

#9 Sarkikos

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Posted 13 October 2010 - 01:57 PM

Greg,

But the image in a newtonian *is* correct, just upside down. So turn the atlas over...


Wise guy. :smirk: Of course you understand what I meant by "correct image." Any orientation other than one which matches the natural orientation is in some sense incorrect. :grin:

I do not like to read upside down. I do not want to turn the atlas upside down. I shouldn't have to, since we have nice refractors with RACI diagonals nowadays. Why take a step backwards?

Mike

#10 Sarkikos

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Posted 13 October 2010 - 02:08 PM

Jon,

The FirstScope has an undersized diagonal.


Are you sure? I had one and I believe I verified that the diagonal was large enough to fully illuminate the center of the field of view.


Yes, I'm sure. When you look into the focuser, you should be able to see the entire face of the primary reflected in the secondary. That is how all my other Newts are. But for the FirstScope, you have to physically move the diagonal around to see the outer areas of the primary. I'd say 20% or more of the primary is not seen by the diagonal. That's why I say the clear aperture may be only about 50mm or so.

So is the diagonal undersized or is the focuser too tall? Take your pick. You shouldn't have to replace the focuser in order to see the whole primary.

Mike

#11 seryddwr

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Posted 13 October 2010 - 02:13 PM

Wise guy. :smirk:


You caught me. ;)

#12 highertheflyer

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Posted 13 October 2010 - 04:37 PM

I was worried about the diagonal being undersized, too. That's in part why I went with a lower profile focuser.


http://www.celestron....php?ProdID=568

Thanks guys for your thoughts.
I had at one time a 4.5 newt mounted on a 12.5 inch dob and really enjoyed it's use as a finder scope.

Of course, that was long before goto scopes, and star hopping was a bit of a chore, referencing a star chart.
But I'll have a SkyCommander for push-to on this scope.

The best part is that I'm thinking of having this very-light-weight-little-fellow mounted near the primary focuser for that wow-appeal. (smile)
Think the kids would enjoy that!

I'm most impressed with the weight of the tube assembly and the ease one could mount as a finder scope without appreciably disturbing the balance.
I notice no culminating adjustments for the primary, so that may require some finagling.
And there are no ventilation openings for the little primary mirror.

But has anyone with the little scope placed the parameters into the free program called NEWT?
That should help to resolve the issues of an undersized secondary....

And has anyone actually star tested the little fellow?
I think a lot of people would be interested in a review.

So well over a dozen scopes now and don't want to add another unecessarily now, cause I take the chance of my wife finding out...

Thanks for all your responses.
Jim

#13 Jon Isaacs

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Posted 13 October 2010 - 05:31 PM

So is the diagonal undersized or is the focuser too tall? Take your pick. You shouldn't have to replace the focuser in order to see the whole primary.

Mike



I agree. As I recall, I did what I typically do, I figured out where the focal plane of the scope was by focusing on infinity with an eyepiece with a known focal plane location. Then I put the top of the focuser at the focal plane and put a pinhole in the focuser at the focal plane...

Then I looked to see if I could see the edge of the mirror. As I recall, I could see it. I also believe I measured it and checked it with Newt and all was well. Unfortunately I gave that scope away.

Jon

#14 avarakin

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Posted 13 October 2010 - 05:35 PM

How about 114mm Orion StarBlast ($150)? I read some very good reviews of it and it has proper parabolic mirror. It has aluminum tube and is very light - about #3.
Another one is 100mm Orion SkyScanner ($100). Also very light but I have not seen any reviews on it.

Alex

#15 Sarkikos

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Posted 13 October 2010 - 06:21 PM

Jon,

I agree. As I recall, I did what I typically do, I figured out where the focal plane of the scope was by focusing on infinity with an eyepiece with a known focal plane location. Then I put the top of the focuser at the focal plane and put a pinhole in the focuser at the focal plane...

Then I looked to see if I could see the edge of the mirror. As I recall, I could see it. I also believe I measured it and checked it with Newt and all was well. Unfortunately I gave that scope away.


If the Moon is out I find the focal plane of a telescope by placing a translucent piece of paper or plastic over the end of the focuser and moving the focuser in and out until the image of the Moon is sharpest. I did not bother doing this with the FirstScope I had. Instead, I put a collimating cap in the focuser and moved the focuser in all the way and out all the way. In no position could I see the entire face of the primary.

Mike

#16 seryddwr

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Posted 13 October 2010 - 08:57 PM

I'm almost finished with my rebuild. I'll do a star test, when I'm done.






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