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Best practices / filters / eyepieces for using large dobs (bought the Explore Scientific 16 inch)

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#1 David Lo Pan

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Posted 09 October 2019 - 10:23 AM

Hi all,

 

I had been reading up on the Explore Scientific 16" truss dob, with the most recent questions in https://www.cloudyni...d/#entry9678584

 

Last Monday I decided to dive in, added the scope to my cart on the ES site, then cancelled and  thought I should try Astronomics first after I deposited a check in the bank. When I got home the price of the scope was $800 higher! I called Astronomics and they said a new price list had come out from ES, I called ES and explained that I had meant to buy one that day after depositing a check. The ES folks let me use the old price ($2400 vs $3200 USD) so I checked out.

 

Scope arrived yesterday. Box looked very roughed up by FedEx, torn and scratched and dented in several places. Thankfully the whole thing had been packed in a second outer box, and inside it the two large pieces were in their own boxes and styrofoam. ES had done a great job packing, and no pieces were damaged.

 

Weirdly, the manual that had been packed inside was for ES refractors. Called ES, they emailed me the right one and said they would mail a hard copy as well. I used the youtube video to get it together, and I agree with the comments of other users. For the money the workmanship and part quality seems good. The finder is garbage, and the hockey puck brakes don't seem to do anything. I had got the extra weight so no issues balancing at different altitude.

 

I couldn't get the shroud to fit properly, so called ES again and they sent me a return postage label to send it back. They said that the way I described the shroud sounded like the original version, not the updated one. Despite the small issues noted above, I think the scope looks great. Just waiting for the clouds to go away now.

 

So I have a few questions for you big dob / truss dob users :

 

1. The primary rests in the bottom of the box, and ES suggested keeping the tissue paper that covered it in shipment as a permanent cover during storage. I worry about scratching the mirror, is there some other way to keep it safe in storage and transit? Or do you guys leave the primary uncovered (but enclosed) in the box?

 

2. I have a 17mm Baader Hyperion, a couple of 2x barlows, some 1.25" Plossls (10mm, 26mm), and an Orion Q70 32mm 2" eyepiece. For filters I have the set of Orion 1.25" planetary filters (orange, blue, etc), an Orion 1.25" Ultrablock filter, and an Orion 2" UV/IR cutoff filter (I used for DSLR photography). With the shroud return I will have a credit at ES and they also carry filters and eyepieces.  With a 16" scope, would a Hydrogen Beta filter be better, or an OIII filter, or maybe a different eyepiece. My primary usage for the scope will be hunting the Herschel 400, Caldwell Catalogue, and Globulars.How can I best supplement my above accessories in a way that most improves my viewing of these targets with a 16" scope?

 

(I will have a shroud so that is not needed, and I hope to replace the finder with an old 9x50 RA finder I already own).

 

Thanks so much for the input and help!



#2 David Lo Pan

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Posted 09 October 2019 - 10:27 AM

Forgot to add, but the customer service at ES has been fantastic. Made it a lot easier to overlook the issue with the manual and shroud, really great service.



#3 macdonjh

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Posted 09 October 2019 - 10:32 AM

When I got home the price of the scope was $800 higher! I called Astronomics and they said a new price list had come out from ES...

 

"It's all in the reflexes."  

 

Congratulations for your new scope.  I hope you get everything sorted out before the sky clears so you'll be ready.


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#4 David Lo Pan

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Posted 09 October 2019 - 10:39 AM

When I got home the price of the scope was $800 higher! I called Astronomics and they said a new price list had come out from ES...

 

"It's all in the reflexes."  

 

Congratulations for your new scope.  I hope you get everything sorted out before the sky clears so you'll be ready.

This literally made me laugh out loud. Thanks!



#5 macdonjh

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Posted 09 October 2019 - 10:48 AM

No worries, making allusions is no fun at all if nobody gets them.

 

Oh, and to your actual astronomy question: is the scope you bought manual or go-to?  If manual, and you'll be looking for those faint Herschel objects by star hopping, you might get a really wide, long focal length eye piece.  I'm a bit skeptical you'll like the view through your Q70 with your fast Newtonian.  Of course, try that Q70 first, the view you get may prove me wrong.

 

If you really like nebulae, I don't think you can beat a Lumicon UHC filter.  It passes both O-III and hydrogen-B.  I've never been tempted enough to buy either a separate O-III or hydrogen-B filter as long as I have the UHC.  There are alternatives to the Lumicon offering.


Edited by macdonjh, 09 October 2019 - 10:55 AM.


#6 Starman1

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Posted 09 October 2019 - 12:14 PM

A) I suggest the immediate addition of a coma corrector--either a TeleVue Paracorr II or an ES HRCC.

 

B) some suggestions for eyepieces may come from others here on CN, but some specs:

The coma corrector will result in a focal length of 1939mm if the HRCC is used, or 2103mm if the Paracorr is used.

Resultant f/ratios are 4.77 or 5.18.  Either way, a decent set of eyepieces for a 16" could include:

25-28mm low power.

13-14mm medium power

8-9mm for high medium power

6-7mm for low end of high power

5-5.5mm for high power

and maybe a shorter focal length for great seeing, and a focal length in between one of the focal lengths above for personal preference.

This would be a great scope for 100° eyepieces, especially because you are looking for deep-sky usage.

 

C) Filter use would be desirable.

Choice 1 is a narrowband: TeleVue Nebustar, Lumicon UHC, Astronomik UHC

Choice 2 is a line filter: O-III: Same 3 makers preferred

Choice 3 is a line filter: H-ß: TeleVue, Astronomik, Orion (Lumicon doesn't have one yet in their new series)

I suggest 2" because they thread directly to the coma corrector.  Your lowest power eyepiece will be used a lot on nebulae, so 2" is even desirable if the eyepiece is used

without a coma corrector, though I STRONGLY suggest a coma corrector for that scope.

The ES filters are a lot less expensive, but they have very wide bandwidths and much lower degrees of visual enhancement.

 

D) You will be disappointed with the performance of your current eyepieces in this shorter f/ratio scope.  Don't worry about that.  You can replace them one at a time.

A good friend owns one of these scopes.  You'll enjoy it.  I suggest mounting the focuser exactly on the side of the scope and on the right side of the scope as you stand behind

the scope and look toward the top.  You can use the scope seated much of the time with this focuser position, and it is an easy place to put the focuser and steer the scope

with your right hand (I presume you are right-handed).


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#7 David Lo Pan

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Posted 09 October 2019 - 01:22 PM

Don, is this the ES HRCC you are referencing?

 

HR Variable Coma Corrector (2-inch) - HRCC02-00

https://explorescien.../coma-corrector

 

The description sounds like it is a camera accessory. (I've never had a coma corrector)

 

I looked at the TeleVue one as well here: https://optcorp.com/...paracorr-type-2

 

Mercy! That is not cheap!

 

I think I may be able to try out the scope tonight for the first time, I guess based on how egregious the coma is will determine an order of priority for the suggestions. Thanks!



#8 Starman1

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Posted 09 October 2019 - 02:34 PM

Yes, that's the corrector I mentioned.

It, of course, like all coma correctors, can be used with cameras as well, and to that end, the HRCC even comes with camera adapters.

But, as set up out of the box, it is for visual correction of coma.

 

When you get set up tonight, and the scope is collimated, take a bright star from the center toward the edge and watch what happens to it.

Then move it back in toward the center and note where in the field you can no longer see any deformation of the star.

That is the point where visible coma begins, though it can damage the image of a planet or lunar crater even closer to center.

 

If you use the Q70 or the 17 Hyperion, you'll see the stars at the edge may also exhibit slight defocus due to field curvature, and astigmatism.

Astigmatism is easily identified if the star elongates radially on one side of focus, and 90° to a radial line on the other side of focus.

If you have coma, astigmatism and field curvature all rolled up together, the stars at the edge of the field might resemble bats or seagulls.

A coma corrector will eliminate the coma, and slightly flatten the field, but it won't correct the astigmatism--only better eyepieces will do that.




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