I am new to both the forum and photometry and am looking for some advice on which type of mount is best for photometry, Wedge or GEM. I am planning to purchase a new CCD camera & potentially a mount for imaging and while I do plan on using the new camers & mount for astrophotography I am more so intersested in the scientific side of things and the opportunity there is with photometry to make a discovery about the universe we live within. I am a mechanical engineer by profession which has given me the insight to judge what is and isn't possible with a paticular piece of equipment. The only down side to this knowledge is that I have the mindset that with enough persistance nothing is impossible. With that being said I am hoping everyone can help me in decide which type of telescope mount, GEM or Wedge mounted SCT, is best. My current telescope is a CPC800 Edge HD which is a fork mounted SCT and in doing research into which type of CCD to purchase have come across both positive and negatives views on which type of mount is best. As a result I can't decide wether to purchase a wedge for my current scope or replace it with a good GEM ($3,000 range) and OTA. If I had a permanent observatory installing it on a wedge would be the way to go because of the fact it eliminates the need for a meridian flip but since I do not I am worried I may spend more time trying to obtain a good polar alignment with the wedge than I would imaging. As a result of the need for potability, do the benfits from installing a fork mounted sct on a wedge outweigh the negatives with using a GEM for photometry. I would really like to hear what everyones thoughts are on the subject because I am at a loss as to what to do.
I was new to Astroimaging back in the 1980s, but the common refrain from experianced users (kinda like Real Estate's, location, location, location) was get as large and stable a mount as possible. Going from a fork mounted Meade LX200 to a german equatorial with a C-11 made a quantum difference in quality of tracking and image stability. Nowadays, I could not comprehend conducting astroimaging without computerized/rugged mounts (I have 2 X AP1200s and 1 X Ioptron CEM60). My base photometry instrument is a C-11 at F/6.3 on an iOptron CEM60 with a SBIG ST-10xme CCD with BVI photometric filters guided by a SBIG RGH/100mm guidescope. This system runs autonamously all night long driven by CCD Autopilot; it even takes darks, flats, and parks the mount and turns off the CCD cooling at the end of the run! My exposures range from 5 to 90 seconds and are guided near perfectly. This set-up has enabled me to amass more than 110K submited photometric measurements over the past 15 year, with the majority (75%) done in the last 5 years. Of course you dont need to break the bank to do routine photometry, but getting a quality tracking system that can zero onto a target field repeatively is very desirable for high quality photometry.
Before you buy a new camera I suggest you read the first few chapters of the AAVSO Guide to CCD Photometry. It will give you important information about picking a camera which is consistent with your telescope and seeing. You should talk to experienced imagers in your area about the typical seeing (scintillation) where you live.
GEM vs fork mount: It seems to me that that most amateurs now doing photometry use GEM type mounts. I'm not really sure why this is the case. Perhaps it is easier (less expensive) to build a really accurate, stable GEM than a fork mount. I use a GEM, and I don't find the "meridian flip" to be a problem for doing photometry. I wouldn't let that be a significant factor in picking a mount. More important is to get the best (stable, good pointing and tracking) mount that you can fit into your budget. When I look at my system, and systems used by other amateurs doing photometry regularly, it seems that the mount accounts for about 50% or more of the total cost of the system (mount, telescope, camera, filters, focuser).
I would also suggest that you consider some type of permanent setup. There are many way to protect a permanently mounted scope without building a full-fledged observatory. I have a simple roll away "out house" for one of my telescope that works well. I've found the all season "365" covers to be very effective (but I live in a fairly mild climate). I've seen some nice designs for light weight removeable (or tip off) boxes that are mounted on the pier. This might be just the ticket for an 8 inch SCT. For scientific imaging, I think setting up and polar aligning for each observing session will get very old, very fast.
There are a hundred combinations of hardware and software that will work. Problem for the new guy is that few folks publish details. So here is one more example;
I use a Losmandy G11 with Gemini with MaxIm 6 Pro software on a Win7 Pro PC and an ST8XME camera to an 8" SCT on a pier in a $200 metal garden shed (modified so that the roof flops open). Walmart heating pad bungeed around the scope to keep dew and frost of the optic. I added an Icron Ranger 2304 USB extender this year. The combination is useable and in your price range and the G11 will handle your scope. I am pretty careless about polar alignment, as it seems hard to keep the thing aligned and working for more than 1 - 6 months. The software solves the images and corrects tracking well enough to do time series. I can time series and sleep. Point and shoot is always operator assisted. Takes about 20 minutes to slew, correct (PinPoint), and do 6 each BVI. I keep wanting to buy one of those wonderful Paramounts or AP mounts so I can schedual, point and sleep. However the Losmandy gives me more data than I can shake a stick at. Always weeks behind on image processing. My experience is that the mount and pier is most important, software next, camera next, telescope least important.
Don't forget the time. My PC is set for GMT and runs time syc sofware. Good to about 400ms.
My two cents worth...
The main problem I have with my fork mount is it reduces the maximum declination I can observe with my camera. Anything above about +60 degrees will result in the camera hitting the mount. Ungood.
I have never used a GEM for photometry, so I have no experience with the meridian flip.
I haven't figured out where I can put an observatory in my yard, so at present I have the scope on JMI Wheelie Bars and I roll it out to the driveway to observe. I have two locator spots painted on the driveway which makes the alignment a lot easier. It's not ideal, but it will have to do for now.
As James wrote, first of all you need is a stable mount. The best I know are the models of ASA direct drive mounts. In our IRIDA observatory we use ASA DDM85 mounts in remote setup, controlled via internet connection. The mounts are with absolute encoders, very stable, without any periodic tracking errors. They are extremely precise and could be used without guiding. The technical parameters of our equipment (mounts, CCD, optics, filters etc.) you can see here:
We are about to start a project which will be a university observatory for scientific observations and public outreach purposes. I am doing photometry for 10 years now. I have been using my ST-8XME for the last 7 years and very happy with the results. We are planning to buy 20" RC telescope if we can afford, and if cannot, we will go for 16" Meade LX200. What I need to ask you is about what CCD camera we should choose for nice photometric results? What I know is, for good quality photometric data, we should choose non-antiblooming camera (considering better QE). However, when I look at various brands' latest releases great majority are antiblooming. Does the CCD technology changed over time and now antibloomings provide better results then it was before? Or what? What camera would you suggest for us? We can afford about $10,000 for the CCD camera.
Thanks in advance.
I would go for a KAF9000 or KAF16803 based CCD. You find them in Europe with Moravian or in the US with several brands. I have an FLI ML16803 with the 7 slot filterwheel and the performance and reliability is just extraordinary. I am using it since Aug. 2011 at my remote site in Chile. Average clear nights are 320 and average exposures are aboiut 700 a night. You can calculate how many images I did already. Way above the expected MTBF of e.g. the shutter.
Most observers find that the mount is the most important thing to spend extra on to get a good one. You did not mention what mount you are considering. Also you can calculate the inprovement in performance from a 16 inch to a 20 inch for photometry. Its not a much as the increase in cost. Something to consider when fitting into any budget.
could you please elaborate on your statement concerning the scope size?
I am also thinking of getting a 20" to replace my 16" and concerning mirros surface the 20" is 56% larger.
Buy a used CCD on one of the sites that sells used equipment. (Can I mention names? Cloudy Nights, Astromart) You need to think about FOV. Phil has good advice, read up on this in the CCD manual. Something like a used SBIG ST8XE might work well. Non-antiblooming if you can find one but I use an antiblooming camera. If you are using more than one filter (V to start) you need a matching filter wheel and photometric filters. The V will get you started and that is all many use. Note: larger sensors may require larger photometric filters that can be much more expensive, so check to see if the camera takes 1.25" filters if you want to save some money.
About the mount: A C8 on a Losmandy G11 would allow you to easily add stuff like a guide scope if needed. A G8-class would be limiting in terms of adding things like guide scopes, cameras, filter wheels, etc. to the OTA. I have zero experience with forks, so I cannot advise on wedges and thing like tracking accuracy, etc. The good news is that used G11s pop up all the time on the used market and the going price is usually around $1500 without a Gemini go-to unit. I used mine with digital setting circles. Of course, if you want to and can afford it, there are much more expensive mount options that are well worth the money.