EdgeHD optics and Imaging at f7

[pmrid]

Has anyone figured out whether it is possible to use a focal reducer with the new EdgeHD optics. With the built-in field flattener, is it possible to use a standard Celestron .63 reducer as well or are you stuck to imaging at f11 or f2 with a Hyperstar?
Peter.

[rat156]

Without having one to try, I'd be tempted to try the AP 0.75 reducer. That worked well with my RCX.

Cheers
Stuart

[pmrid]

I may have answered my own question. It's called the Optec Lepus .62 reducer. US199 but needs a camera adapter to suit at another uS$60 or so.
http://www.astromart.com/classifieds/details.asp?classified_id=696455

Peter.

[terrynz]

Celestron are about to release the EdgeHD focal reducer to their beta testers November/December this year

Lepus claim and market this for Meade and EdgeHD optics, however they're not 100% compatible based on testing with my HD14.

The EdgeHD's are designed to have a very flat field with very LOW vignetting at f/10 (HD8 and HD11) and F11 for the HD14. This is where the Lepus reducer falls down, the field isn't as flat as at F11, and vignetting was increased. using an SXVR-M25C and Apogee U16.

I'll keep you posted.

[pmrid]

Thanks Terrence. Will you be a beta tester for the Celestron reducer?
And how are you enjoying your HD 1400 anyway? Are you doing any f2 imaging?
Peter

[terrynz]

I can't say too much, but progress being made.

I love this scope and as a result have 2 very nice OTA's in the corner of the observatory kept on ice, so to speak.

The EdgeHD is a very fine scope and I'd highly recommend it to anyone wanting a high end OTA without the high end price. It compared favourably against the Planewave CDK for flat field images (not as fast though) and is great visually.

Yes I image at F1.9 using the hyperstar 3. Hyperstar is great, but my main use for it is wide field sky surveys on the Paramount ME as I can do a lot of 90sec exposures, and repeat the sequence multiple times in at evening.

I've been using Hyperstar for about 3-4 years now and it's great for wide field, Ultra deep imaging. the C11 gets to Mag 20 in 120 sec, the HD14 in about 90-100sec, so it has it uses.

cheers
T.

[marki]

I am a little confused about these scopes. All the advertising I have read on these edge scopes as well as many user posts claim a flat field but when I looked them up on the Celestron website their claim is:

The EdgeHD optics produce a focal plane more than 3 times flatter than the standard equivalent SCT telescope with diffraction limited stars across the entire Nikon D3 or Canon 5D field of view!

I guess what they are saying here is the edge series has 1/3 the curvature of a standard SCT (i.e. not flat at all). Has anyone put a big chip to the test on one of these yet???

Mark

[terrynz]

Very good question and yes that's correct and true be it SCT or CDK, or modified SCT such as the EdgeHD or the RCX/LXR... and is the apogee U16M not big enough for you :) ?

The reason you and others are confused is - there's a lot of confusion about what exactly "flat field" means.

In short, it means "The surface of best focus across the image lies in a plane."


For the EdgeHD, "flat field" means.... "small, round stars across a large, flat ccd". The EdgeHD has been design with this in mind.

This is important way? Because CCD's are getting bigger....

Remember NO optical system is perfect. Not even my Planewave CDK. It just does a better job than most; likewise the EdgeHD when using a large flat CCD such as a Canon 5D or Apogee U16M.

For example, you can still have a flat field that still has "coma" and "astigmatism". - In this case the stars will look terrible as they went out from the centre of field but it would still be a flat field.

Like wise, you can also have what "appears" to be a flat field with round stars all the way across it - However, it is in fact a "curved field". And when you change focus the stars will get smaller in one area and bigger in another.

Checkout Celestron on Flicker or celestroniages.com or some of results with the HD11 and ED14 Apogee U16M CDD.

cheers
T.

[marki]

So the real answer here would be to have a curved CCD that matched the error present in the optics yes, just like they used to do with the old film schmidt camera's. Obviously this would be too hard to do economically and possibly beyond any current technology. My previous comment was aimed at the common use of the term "flat field" relating to many scope types and their various flatteners as this is clearly misleading, there will always be some curvature present. My problem with this is while the hype remains manufacturers can charge excessive amounts for scopes as is evident by celestrons pricing for these scopes in OZ. The edge HD optics are not flat but are better (have 1/3 the curvature) then an SCT without a corrector. That said it is good to hear that yours can provide a reasonably flat and evenly illuminated field across the entire 16000 chip. Yes I have had a look at some of the images and they appear promising with nice round stars out to the edge but I think they lack the sharpness and fine resolution I have seen in similar images from the CDK scopes. This may very well be in the processing applied by the users?????/

Mark

[terrynz]

The old astrographs (typically f2~2.8) cetainly had large 72mm film holders that were curved, - in one plane only... so even they had coma present.

A Curved CCD matched to each optical system would be almost impossible to make. Hence it's easier to project the image onto a plane.

[gregbradley]

These Celestron HD scopes sound very good and a big step forward for Celestron.

As far as flat field goes there is usually some curvature present even with a flattener.

For example my TEC180 with flattener is quite flat and so is an FSQ but if you use CCD inspector it will report some curvature. Take the flattener off and redo the test and that curvature will be far greater.

Same with CDK. It is quite flat. The term flat is relative and some are more "flat" than others. Nuch like the term "APO" for refractors. Some are clearly not APO but for marketing reasons they are termed APO as that is a desirable quality helping sales.

But having said that you want to see an image from a large chip camera that shows round stars to the corners.

How is the Celestron HD for mirror flop and dewing? I believe it has a mirror lock but how well does it work?

These seem to be the 2 problems associated with the SCT design - a dewing up of the corrector, mirror flop and tube currents/thermal imbalance between mirror and ambient. To a lesser degree may be the chromatic aberration a corrector plate adds. That does not seem to be too bad from images I have seen.

CDK17 for example you cannot get a precise focus if the mirror temp is more than about .8C different to ambient. This highlighted for me how crucial that is and how many scopes out there are not performing to what they could be if the thermal issues were properly sorted out.

The main adavantage as I see it of this corrector is typical SCT images show bloated stars away from the centre of the image. If it handles that then they have just given SCTs a new lease of life as they are very good in many other ways.

Their mirrors don't get dirty, they are compact and relatively light for their size. If you put a reducer and get below F7 then their imaging speed would be good as well. F10 is a pain. They also have long focal length for their size. So a HD could well be an awesome scope especially if you modified it to blow cool air through it to cool the mirror 3 degrees below ambient and have a flow to get rid of tube currents.

Greg.

[marki]

Yes I know mate, they have enough trouble getting them flat but one can dream yes? Roll on nano technology I say.

Mark

[terrynz]

Greg, as I said. There is no perfect optical system.

Temperature and thermal expansion of components is an issue and depends on the coefficient of thermal expansion for those materials.
This goes for all OTA's - always will.

Yes the CDK can be difficult to keep in focus, however the Kendrick focuser matched to "trained" electronic focuser, resolves this. But you need to train the software.

The HD Mirror locks work well. Cooling like all SCT's it needs a good 60mins min to cool to ambient if not kept in a well vented observatory - same goes for the CDK. Internal thermal currents - non issue once reached ambient. Again care is needed not to over do the dew heaters.

As for compact... yes SCT's are that. But a HD14 is not small and getting onto the Paramount ME was a 2 man job.

Imaging at F11 is slow and a challenge. However, you soon learn how good or bad your mount is!

[gregbradley]

Thanks for the tip about training the focuser for the CDK. I'll have to do that.

Cheers,

Greg.

[terrynz]

Hey Greg,

where's your remote observatory? I'm seriously considering moving most of my main imaging equipment out of New Zealand. It just needs a better home as the weather here has been too fickle the last few years.

cheers
T.

[Kal]

It has been put to the test, and you can compare it to a stock C8 in these pics up to the original 4096x4096 resolution

link (http://www.flickr.com/photos/26671216@N02/sets/72157618792310067/detail/)

[terrynz]

These are some of the beta testing images. As I said before, using the Apogee U16M CCD. These one are posted by Andre. There are more of www.celestronimages.com (http://www.celestronimages.com) and flicker. You just need to have a hunt about; some are from Southern Hemisphere too.

[gregbradley]

When I say I have a remote dark site, I meant remote as is in distant and not many people around. I have a weekender I built and an observatory but no electricity. I use generators. It is between Goulburn and Crookwell in NSW and about 740 metres altitude. Often the skies there are clear and stay clear all night. There is virtually no light pollution. It can some years be quite windy but that doesn't affect a refractor but it did a closed tube RCOS.

I could get the power on but it would cost $30,000 +. I have internet access via 3G wireless modem.

Pingelly near Perth may be a good site. That's where Rob Gendler took a lot of shots from with a 14.5 inch RCOS.

Greg.