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Gear acquisition - planning ahead

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#1 Eddie_42

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Posted 25 April 2019 - 10:44 PM

Story time.... with a request for advice.

My current gear is an ES 80mm f/6 apo triplet, EQ6R PRO,  and Nikon d5600 dslr (no-mod). It is a basic setup, but from all my reading and sage advice around the community,  it got me started on the right foot. Short FL, solid mount.

Now comes the part where I accessorize, add parts and crawl further down the rabbit hole. I have the itch to get a scope with a longer FL to get deeper DSOs (galaxies, planetary nebula). My effective FOV with current gear is about 3x2 degrees, and chasing objects that are 10 arc minutes wide, just doesn't have the "wow" factor.

I have been trying to formulate a plan to round out my current gear, and grow into something in the next tier.  There are a great many options out there. I have tried to optimize budget, function, cross platform usability. Let me know your thoughts on my growth/expansion plan.

First on the list...a field flattener.  The ES specific one is now DSLR friendly, seems like a solid win.  $180

Next is guidance. I am looking at a 60mm guide scope, and either the ASI120  or ASI290 cameras to pair with it.  And because ES uses the Meade T type finder bracket, I either need a shoe adapter or clamp rings. Im leaning towards the adapter from scopestuff.  All told $350-500 depending on the camera choice.

Now the big choice. Which scope to choose for the expanded FL?  An SCT would be nice for its all around performance. I have two boys, both under 10, they would benefit from its visual capacity far more than my 80mm. With the right set of gear (.7FR, OAG) it transforms into a widely used AP tool.  My hesitation here is price.  $1200 for EdgeHD 8, $300 for the reducer, plus the OAG, an assortment of adapters, and the unknown of having the right back focus for the DSLR (read a bunch of threads where this is closer than not).

Alternative scope (and the one im leaning toward) would be a 6" RC. $400. The scope is friendly at its native length, seems a flattener or reducer is not required.  With my Dslr the image scale would be around 0.5 "/px. This is pretty oversampled, is that cumbersome? I don't believe i can bin pixels here, and a dedicated astro cam is a whole different can or worms (not in the cards at this point).

I could pair the aforementioned 60mm guide scope on this, since mirror flex is less of a factor.  I think this choice would want the asi290 give cam over the 120, simply for image scale.  This would put my ratio of image scale around 4.5:1. With the 120 its closer to 5.5:1. Cost savings here are huge. Around $1000. While not the best visual scope, the RC should have enough pop to please my 10yr old.

The number of objects the 6" opens up is about the same as the reduced 8" sct. The focal length difference is around 200mm.

Is the 8" rc as friendly as the 6 in terms of flat field and being able to guide piggy back?  The cost is double, and only adds 300mm of FL. It is an f/8 vs f/9 on the 6".

I image mostly from my porch in suburbia, sometimes from the neighborhood park.  I can get to decently dark sites in 20-30min, and have an SUV for transport.  Ultimately my rig has to be assembled and torn down each session. Im also trying to keep thing compact for summer nights camping, which means it has to fit in the car with that gear too.

Collomation is something I know will need doing with either scope. I have no serious experience here, but the task doesn't daunt me.

Thoughts on the overall plan? Did i miss a big chunk somewhere? Alternatives i haven't considered?  What's behind door #2? I'm sure the are some unexpected cost the come with the $400 scope, nothing is that easy.

Thanks

#2 bobzeq25

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Posted 25 April 2019 - 11:50 PM

Thoughts on the overall plan? Did i miss a big chunk somewhere? Alternatives i haven't considered?  What's behind door #2? I'm sure the are some unexpected cost the come with the $400 scope, nothing is that easy.

"unexpected costs" is right.  I now have $1200 in that "$400" scope.

 

The field is naturally curved, I don't see where you got the idea a flattener is not required.  Maybe with a small enough chip.  I got a CCDT67 reducer that flattens some (enough for me and an APS-C chip).  Electric focuser which bandaids the mediocre stock one.  Some people get a Moonlite, that costs more than the scope.  Dovetail for the top of the tube to stabilize it.  Various collimation tools. 

 

Some people get one with good collimation.  I didn't, the diffraction spikes were doubled.  This is bad.  Took me many hours to research collimation methods (people use a variety) and to do it.  It does seem to hold it well.

 

The mirrors are excellent.

 

Bottom line, some people love them, some hate them.

 

I replaced mine with a TS refractor, 130mm, F7, 3.7 focuser.  Pricy.

 

The Edge8 is a less risky purchase.


Edited by bobzeq25, 25 April 2019 - 11:51 PM.


#3 pinzmann

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Posted 26 April 2019 - 12:22 AM

I started out just like you only with a Sirius mount. When I started guiding I just mounted a SVbony dovetail clamp on the carrying handle of the scope rings. Then I got a generic 60mm guide scope with the helical focused and attached that to the mounting plate that came with the clamp. Worked great. My guide scope is the ASI120mm. I would not try to attach the guide scope to the finder bracket.
As far as more focal length, I first went with a APO 102mm which gave me 714mm and I did get some good galaxy images but did have to crop them a lot. Still was happy with the results. My biggest improvement was getting a C8 Edge with the .7 reducer. Now this was done after a year or so of experience with the 80mm and 102mm plus I had used a C8 in my fledgling Astro days in the 70-80's. I also am guiding with the Celestron OAG and my D5300. Had no problem getting the correct backfocus. I just had to get a Blue Fireball adapter to replaces the larger adapter between the reducer and OAG body. If you are thinking about this route get the ASI290mm mini, a better guide scope for the OAG.
I posted a question here on chosing between the C8 and an RC. It was recommended I get the C8. Very happy with that decision. Off course after I got mine I saw a few here in classified for a pretty decent price. They do seem to come up regular. Anyway hope some of this helps.

#4 Eddie_42

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Posted 26 April 2019 - 07:06 PM


The field is naturally curved, I don't see where you got the idea a flattener is not required.

 

Bottom line, some people love them, some hate them.

 

The Edge8 is a less risky purchase.

Flat I suppose is relative - Compared to SCT, I believe I have read that the RC is flatter.  I have read threads about people able to image on the 6" RC without a _NEED_ for a corrector. If 60-70% of the Field is flat, I think i would be happy. I am currently uncorrected on my 80mm f/6, so I might be already in the camp of "some curve is ok".  I have found very few with a 8" sct at native FL able to image without a flattener. Almost everyone uses  the .7 or 0.63 FR/FF.   No science to back it up, just what the "vibe" is around the inter-webs.

 

I have no doubt the Edge HD8 is a fantastic unit.  My contention is the start up cost relative to the RC.  If money were not a factor, Id chose the HD8.  The HD8 also doesnt have a lot of overlap on pieces and parts from the 80mm, which I have a delusion that i could re-purpose something, thus saving some cash.


Edited by Eddie_42, 26 April 2019 - 07:08 PM.


#5 Stephen Kennedy

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Posted 26 April 2019 - 07:32 PM

I have recently purchased a GSO RC6 which I paid $400 for and put it on a 30 year old Vixen Super Polaris GEM with an RA motor only that I bought while stationed in Japan. I really like the GSO RC6 which arrived in perfect collimation and has stayed that way. Here is an image I took of the third quarter Moon at 2:30 AM today with the RC6 which shows that it can also be used for visual observation of Solar System objects.

IMG_3509 (3).JPG

#6 WadeH237

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Posted 27 April 2019 - 09:00 AM

I own an RC 6, and I would not recommend it in this case.

 

The good news is that, for what you get, it's a great bargain.  The bad news is that it is finicky about collimation (and if you have a heavy imaging package, the collimation can change in different parts of the sky).  The field on the scope is not flat.  It works fine with my ST-10, which has a small sensor by today's standards.  But a DSLR size sensor would show plenty of curvature - and it would get worse with a reducer like the CCDT67 (technically, sensor size and reducers don't actually change the curvature, which is an artifact of the scope itself; they they just show more of the field, and the further off axis you get, the more you notice the curvature).

 

Also, since you mentioned that you might use it for visual with your sons, I would not recommend the RC6 for visual use.  I have used mine visually when I needed to do a quick mount test with a scope that doesn't dew up.  The issue is that the large central obstruction results in obvious diffraction rings around the stars when used with high magnification.  This also means that there is less contrast, which will affect viewing the planets or the moon.

 

That said, I think that the jump from a 480mm focal length with your current refractor, to the 1450mm focal length of a reduced EdgeHD 8, is a huge jump.  Your mount can handle it, but you're learning curve will get steep again.  If this is OK, the Edge is a very good telescope for the money, and it's an excellent visual scope.  The downside, beyond the focal length, is that the reducer is expensive and the mirror has a tendency to shift and flop.  You absolutely will need to use an OAG with it.

 

An alternative that you may not have considered is another refractor.  One with 800mm to 900mm focal length would be a good option.  It won't be a great galaxy imaging machine, except for some of the larger Messier galaxies (you need yet more aperture and focal length for that; the EdgeHD 8 is probably the bottom end of good galaxy imaging scopes).  But that focal length would be a nice step up from where you are at, and it would be very well matches to the smallish pixel sizes of many of today's common cameras.  I think that my next imaging telescope will be a refractor in this general focal length range.

 

I hope that this helps,

-Wade



#7 Eddie_42

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Posted 27 April 2019 - 05:09 PM

man....you guys and gals arent very good at this.  The proper response is "omg what a pefect setup. Im a bit jealous"   bow.gif

 

But....since you insist on give proper advise - I suppose the prudent choice is saving some pennies for a while and go for the Edge 8", and all the toys needed for that. Until then...its the 80mm widefield for me



#8 bobzeq25

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Posted 27 April 2019 - 06:56 PM

(curvature) would get worse with a reducer like the CCDT67 (technically, sensor size and reducers don't actually change the curvature, which is an artifact of the scope itself; they they just show more of the field, and the further off axis you get, the more you notice the curvature).

The CCDT67 (which I have) has a mild flattening effect.  No doubt something in the design.  Not perfect, but definitely going in the right direction.  You need to space it properly of course.  It is listed for a .67X reduction (hence the model number), I (and others) found it flattened best around .75X.

 

Madratter's experience, with an 8RC.

 

"That led to a compression ratio of about .73x. It provided an image scale of about .94 arc-seconds/pixel when used with a 5.4 micron pitch camera. And the corners were very good with this combination. The focal ratio worked out to about f/5.86.

 

As can be seen from the graph above my spacing was about 67mm from the rear flange. Achieving a spacing close to that is crucial for this to work well.

 

I have tried further spacings for more reduction and things get real ugly in the corners around the rated .67 compression."

 

http://astroimages.w...-my-system.html


Edited by bobzeq25, 27 April 2019 - 07:01 PM.


#9 WadeH237

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Posted 27 April 2019 - 08:08 PM

I've had a CCDT67 for a long time.  I bought it for use with my ST-10 on a Meade 8" ACF optical tube.  I specifically wanted a reducer that didn't introduce any other effects, and I've never noticed a flattening effect from it.  Also, Astro-Physics does not claim that it flattens.  Actually, they specifically claim that it *can* be used with flat field telescopes.  If it did any flattening of its own, they probably would not recommend it for use with flat field scopes.  Their description can be found here.

 

That said, it is a very nice reducer and does its job well, as long as your sensor is not too large or the focal ratio too fast.



#10 rhcrooks

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Posted 27 April 2019 - 11:46 PM

The RC's are tough to collimate because they are more sensitive to the primary-secondary distance as well. Also - they are F/9 and the additional resolution is meaningless when the seeing is 1" or so over a much easier to deal with f/9 refractor. Or a F/6 refractor.


Edited by rhcrooks, 27 April 2019 - 11:58 PM.



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