Edge HD 14 - Shooting at 3910mm Celestron EdgeHD 14" · MeridianSkyAstronomy · ... · 38 · 1604 · 3

Benoit Schillings:
this brings a good question... once  you lock the primary, what is actually moving that makes guidescope works so poorly ? 

I am actually doing a few test in the next few night, the measure the motion between the main scope and a guide scope... hopefully will get some clues.


-  benoit

There are a couple of important issues.  First, the entire assembly of the C14 is not very mechanically rigid; especially the mechanism that mounts the primary mirror.  When you engage the mirror locks, you are constraining multiple small diameter rods that connect to the rear of plate bonded to the primary.  Those rods do a pretty good job of keeping the mirror from tipping but they are no so rigid against side loads and there is a certain amount of mechanical side-to-side slop in the focusing mechanism.  The entire central baffle assembly that the mirror rides on is “pretty good” but not when measured at arc-second levels.  Celestron did a good job of designing a low cost, manufacturable telescope but if you compare it to professional level optical equipment, it’s pretty flimsy.  And I want to be careful that no one takes that as an insult on their equipment.  The Celestrons can produce beautiful images but you have to appreciate their optical and mechanical limitations.

The second thing to realize is that in order to achieve the tiniest star images under the best conditions, the error budget for rms guide errors is below about 0.2”.   An angle of one arc-second is roughly a dime at two miles.  That is a VERY small angle.  Remember this:  At one arc-second, everything is made of rubber!  And in this case, we are trying to achieve an rms pointing accuracy roughly 5 times better than that.

If you are careful, you can get images with round stars using a guide scope on a C14; however, you’ll find that you’ll generally get larger FWHM values then you do using any through the scope guiding method.  That’s not to say that it will always be poor, but your image yield will go way down.  An additional ~ 1”- 4” of rms pointing error introduced by mechanical flexure does nothing good for producing sharp images.

John

Hi.

C14 XLT user for almost 10 years here. It doesn't matter if you have all the fancy staff everybody is suggesting here. If seeing is not as good as for planetary imaging, you are losing your time shooting at f/11 and 3910 mm. Seeing is everything when you work at a high focal length.

Also the internal temperature of the tube is very important. I use 3 different systems to cool down the tube: Lymax ( you can't use it due to internal lens in the Edge model ), Starizona Cooler ( risky, as you have to remove the secondary, but quite effective and fast ) and TEMPest ( always running both in deep sky or planetary imaging to keep the temperature down ). Unfortunately, Celestron never have added fans to their telescopes as Meade and others have done in the past. So you always have to spend a good amount of money after purchase to have all you need for astrophoto. 

Without a reducer ( currently I'm using the Starizona LF for the XLT with great results ), the stars in the corners are going to be distorted, even using the Edge HD that's suppose to correct them. During the post-processing,  you can use RC Astro BlurXterminator to correct them. These days it's a "must have" tool for all of us.

You can use an OAG or an ONAG ( a luxury you have to pay because they haven't released the patent as far as I know ). I personally use the ZWO OAG-L that can't be rotated just like the Celestron one. But honestly I just have struggled with a few objects to find stars and sometimes just change the framing of the whole camera system. Yes, sometimes is a shame no to have the "perfect framing" you have been planning for, but I have find that sometimes is even better as you are shooting with a different framing than others have used in their pictures and that's always refreshing. Without the OAG/ONAG/Dual Camera from ZWO/SBIG, you are never going to get round stars ( have lot of experience about it with guiding telescopes until I was able to buy the OAG and my life changed after that :-P LOL ).

For good tracking ( your really need it for a LFL ), there are some options out there: the MESU, the Paramounts, the 10micron, etc.. I use the SkyWatcher EQ8-R Pro that can handle up to 50 kg ( around 30-35 kg for astrophotography ) and it works great out of the box. There are some people struggling with it, but with these massive products build at SYNTA ( or Sunny or whatever name the company has these days, same maker of the Edge HD ) it's a lottery. But it cost less than half the Paramounts or 10micron and works perfectly for most of us. The MESU it's too industrial for me, I prefer something with local support and not to have to send it back to The Nederlands every time I have a problem. 

About the focusing, I don't know the C14 Edge HD too much ( just have used it a couple times, had a C8 Edge HD but sold it ), but my XLT with the EAF keeps the focusing really stable for long periods of time. I don't block the mirror and never have found the focus to drift constantly as others point here. As I said in the beginning, it's all about seeing. If seeing is average, just forget shooting at almost 4000 mm, try to do it with the reducer or better, the Hyperstar or have a second rig as I have with a short focal length or your session will go to the trashcan. 

About the objects, just focus in planetary nebulas or small galaxies and take at least 8-16 hours to get a decent result ( more the better ). For planetary nebulas you also might try with the same setup for planets ( planetary camera doing "lucky imaging", that means, recording video instead of pictures ). It's challenging but for some small but bright planetary nebulas it can work.

But I repeat what I said first: SEEING IS EVERYTHING AT LONG FOCAL LENGTH!!

John Hayes:

Oh, and I suggest the Celestron metalic due sheild, and although I also have the dew ring installed, I suggest wrapping a few dew bands around the sheild instead;

Pete,
I'm really glad to hear that your system is working well.  Your last bit of advice is absolutely spot on!   I'm working on a new presentation about the physics of dew prevention and in it, I show why the approach that you've recommended is so good.  With a dew shield about 1.5 times as long as it is around, you only need to uniformly raise its temperature by about 3C to prevent dew--and I'll show that calculation.

John

Hi John,
I'm looking forward to that presentation!
I never thought that the collector plate dew ring was doing a good job (dew would still form near the centre) and I suspect that it might have also caused some strangeness with the corrector plate and then it eventually failed. I switched to dual wrap aound straps on the *shield* (spelled it right this time!) and haven't seen a spot of dew ever since; even when every thing else outside is dripping wet.
Pete

Hi group,

I am preparing to set up my new Edge HD14 in my observatory in Italy, and am trying to gather as much info as possible to do it as right as possible as from the first try. My mount is a CEM120, which already does a great job in serving my TAK106 (530mm), however the long focal length of the HD14, scares me a bit. I will probably want to buy the IF ONAG XM, but have a few questions I hope someone in this forum can help me out with :

1. since the imager (I use a ZWO 6200mm pro) is at 90° from the axis, and the HD14 is pretty bulky, does anyone have a similar setup but with a filter wheel installed ?
2. what is the best way to combine the ONAG with a focuser ? is that the optic SMFS, or are there better alternatives now ?
3. how do you manage the imaging train combo of focusing and guiding ? I am planning to use Nina from here on out (now have asiair, but is somewhat limited to also control dome shutters etc…) 

Any suggestion, reflection, advice, critique is greatly appreciated.

Thanks
Hans

Hans C:
1. since the imager (I use a ZWO 6200mm pro) is at 90° from the axis, and the HD14 is pretty bulky, does anyone have a similar setup but with a filter wheel installed ?
2. what is the best way to combine the ONAG with a focuser ? is that the optic SMFS, or are there better alternatives now ?
3. how do you manage the imaging train combo of focusing and guiding ? I am planning to use Nina from here on out (now have asiair, but is somewhat limited to also control dome shutters etc...)

Hi Hans,

I have the exact same set up that you have described: Edge 14, CEM 120, ASI 6200 MM, with an IF On Axis Guider (XM version) and Optec SF; I am using a 7P Filter Wheel (ZWO) as well which works well with the On Axis Guider configuration. I'm using an ASI 0171 as the guide camera which has a large APS chip format and most importantly does NOT have a built in window IR filter so that the required ONAG IR signal passes through nicely and provides plenty of guide stars.

You will need a custom connector from Precise Parts in order to attach the ONAG to the Edge's visual back but their web page has a design tool which makes it fairly easy to customize and order etc. I suggest the Celestron Metalic Dew Sheild but don't bother with the corrector plate dew ring that Celestron pushes; it's simply a BAD design principle! Get two large dew heater straps equally spaced on the dew shield and a suitable power supply instead. I think that you already know about staying away from the Celestron reducer?

When seeing is good, the CEM 120 provides exceptional guiding providing that you have a very good PA and rig balance etc. However, it can be frustrating and unusable during bad seeing conditions which may otherwise look ok when just gazing upwards. 

You'll face some fun learning curves but it's very rewarding working at 3910mm and there are an unlimited number of targets !

Looking forward to your results!

Pete

Hans C:
1. since the imager (I use a ZWO 6200mm pro) is at 90° from the axis, and the HD14 is pretty bulky, does anyone have a similar setup but with a filter wheel installed ?
2. what is the best way to combine the ONAG with a focuser ? is that the optic SMFS, or are there better alternatives now ?
3. how do you manage the imaging train combo of focusing and guiding ? I am planning to use Nina from here on out (now have asiair, but is somewhat limited to also control dome shutters etc...)

ad 1): yes I use the C14 with the same camera, a filter wheel and an OAG (and there is plenty of room to even still install an Backfocus solution like the one from PrimaLuce (Esatto) - not sure why there is so much discussion around "not enough backfocus length" - in my experience there is plenty of back focus for a lot of setups

ad 2) sorry I a am not using the ONAG but eying to get the Esatto from PrimaLuce (since I also have a Hyperstar and want to use that quite often it just seems better if no other stuff is installed at my secondary...)

ad 3) I do have an asiAir but switched to NINA. works super fine. there are some advanced features I am missing (eg: conditional re-doing of autofocus - standard only enables you to run it and if it fails to abort the sequence instead of keep on trying until it gets you where you want it). NINA allwos you to ocntrol nearly all aspects and you will get used to it quite fast. there are some good starting "advanced sequence setups" freely available on youtube which makes the intro easy. Again: OAG is a must for a C14 I would not even try anything else (I do have the Celestron Autostar(?) or Autoguide(?) system - but use it only for star parties to find lots of different targets quickly using only the hand control for visual stuff of my guests

have fun!

Interesting post & topic.

John knows my name I hope from the Cloudy Nights forum (Pyrasanth)

I use the C14 edge at F7.7 using the Celestron reducer. The ONAG is essential along with the Optec SMFS & Optec focus locks- just remove the Celestron focus locks & replace with the low profile Optec locks (they do a much better job of locking the primary mirror).

I've just redesigned the adaptors to squeeze in a Falcon V2 rotator for the camera. I'm awaiting the arrival  new adaptors from precise parts for this configuration.

I use the Celestron Aluminium dew shield with a fabricated internal cable mount to loop the SMFS ethernet cable thus avoiding diffraction spikes. The Celestron dew rings are simply bad ideas especially coupled with the ludicrously expensive Celestron dew controllers. I use a dew band wrapped around the dew shield just in front of the corrector and I never get dew. The Celestron dew shield, whilst expensive, is the best I've used so no complaints there.

Balance can be an issue. I've got the OTA as far back in the rings as I can but can't achieve balance this way. The image shows a 7KG counterweight on top of the OTA to achieve balance however I've addressed this in the rebuild by extending a front ADM rail & attaching a weight assembly there thus saving about 5.5 kg in balance weight. I guess the paramount in this setup is carrying around 35 kilos. The setup has now been modified slightly but the picture gives the gist of the configuration.

My primary mirror is locked, I focus with the SMFS and the C14 is held on the Paramount MX+ using Bisque rings to get a great pointing model and improves the rigidity of the OTA vs the flimsy stock Celestron knock off rails.  Whilst I could use Skyguard I don't like its reliability so I use PHD2 and NINA handle the periodic focusing using Hocus Focus.